Oecologia (2007) 151:605–615 DOI 10.1007/s00442-006-0612-z
PO PU L AT I ON EC OL O G Y
Stable carbon and nitrogen isotope ratio proWling of sperm whale teeth reveals ontogenetic movements and trophic ecology Sónia Mendes · Jason Newton · Robert J. Reid · Alain F. Zuur · Graham J. Pierce
Received: 22 May 2006 / Accepted: 2 November 2006 / Published online: 24 November 2006 © Springer-Verlag 2006
Abstract Teeth from male sperm whales (Physeter macrocephalus) stranded in the North-eastern Atlantic were used to determine whether chronological proWles of stable isotope ratios of C (13C) and N (15N) across dentine growth layers could be used to detect known ontogenetic benchmarks in movements and trophic ecology. ProWles showed a general decrease in 13C (median = 1.91‰) and an increase in 15N (median = 2.42‰) with age. A marked decline in 13C occurred for all 11 teeth around 9–10 years and again for six individuals around 20 years. After the early twenties the 13C continued to decline with age for all teeth. These results are consistent with males segregating from natal groups in low latitudes with the onset of puberty between 4 and 15 years and gradually dispersing pole-ward into 13C-depleted temperate waters. Penetration into further depleted, productive high latitudes
Communicated by Roland Brandl. S. Mendes (&) · G. J. Pierce School of Biological Sciences (Zoology), University of Aberdeen, Tillydrone Avenue, Aberdeen, AB24 2TZ, UK e-mail:
[email protected] J. Newton NERC Life Sciences Mass Spectrometry Facility, SUERC, East Kilbride, G75 0QF, UK R. J. Reid Wildlife Unit, SAC Veterinary Services, Drummondhill, Stratherrick Road, Inverness, IV2 4JZ, UK A. F. Zuur Highland Statistics Limited, 6 Laverock Road, Newburgh, AB41 6FN, UK
after the age of 20 might facilitate the spurt of accelerated growth rate observed around this age. Breeding migrations back to lower latitudes were not reXected in the 13C proWles possibly due to being short compared to the time spent feeding in high latitudes. The timings of marked isotopic change in the 15N proWles reXect those of the 13C proWles, suggesting a link between dietary changes and movements. The observed increase in 15N with age is likely to be caused by a trophic level increase as males grow in size, probably feeding on larger prey. An additional explanation could be that, in the higher latitudes of the North Atlantic, the main prey source is the high trophic level squid Gonatus fabricii. Also, the lower latitudes from where males disperse are depleted in basal 15N. ProWles of 13C and 15N in sperm whale teeth gathered from diVerent regions, sexes, and periods in time, could provide a unique way to understand the ecology of this species across diVerent oceans. Keywords Dentine · Dietary history · Migration · Physeter macrocephalus · Stable isotopes
Introduction Ontogenetic movements and trophic ecology of maturing and mature male sperm whales, Physeter macrocephalus, remain relatively obscure aspects of the biology of one of the best studied cetacean species. What is known has been inferred from catch data (e.g. Best 1979, 1999; Clarke et al. 1993) and stranding events (e.g. Santos et al. 1999), which suggest a complex and variable pattern. Males disperse from their natal groups in lower latitudes as juveniles, in what
123
606
seems to become a lasting segregation up to breeding age (Best 1979). They begin to expand their movement range into higher latitudes which results in a striking geographical segregation of the sexes and age-classes, for both the northern and southern hemisphere stocks, with mixed groups of female and young animals inhabiting lower latitudes and juvenile and adult males ranging up to the edge of the pack ice (Rice 1989). Such segregation increases dispersion and so lowers intraspeciWc competition for resources (Best 1979). The age at which males start segregating is unclear and estimates vary between studies. Rice (1989) estimated, based on data from the North PaciWc, that males attained puberty at an age of 7–11 years, and segregated from their natal groups between the ages of 15 and 21, just prior to attaining sexual maturity (18–21 years). In contrast, Best (1979) suggested, based on length, age, maturity and cyamid infestation data for the south Atlantic, that males started segregation to join other males in “bachelor schools” before the onset of puberty, which could be as young as 4–5 years, or as old as 15 years. This could take place shortly after weaning, which occurs at any time between the ages of 2 and 13. Using information on the sex ratio of mixed groups oV Ecuador, Richard et al. (1996) estimated that segregation occurs at around 6 years of age. Male-only groupings are usually found separated into size and age classes: the bigger and older the animals, the smaller the groupings and the higher the latitudes at which they occurred (Rice 1989). Adult males start undertaking large-scale latitudinal migrations (between high latitudes and the breeding grounds) when reaching social and sexual maturity at around 25–27 years of age (13.7 m length) (Best 1979). The extent, timing, frequency and duration of these migrations are not entirely understood. Sperm whales have perhaps the lowest reproductive rate of all marine mammals (Best 1979). Hence, the age at which males start going back to breeding latitudes and the frequency of those migrations will potentially inXuence breeding dynamics, gene Xow through populations, population structure and, ultimately, the maintenance of viable population levels. The ultimate controlling and structuring factor for sperm whale populations, apart from predation, may be food supply. Stomach content analyses have indicated that sperm whales are top predators of mesopelagic ecosystems and their diet composition may vary between sexes, age or body length classes and also between regions, seasons and years (Best et al. 1984; Clarke et al. 1993; Best 1999). Best (1979) stressed the importance of studying the trophic positions of the diVerent social groupings to ascertain if indeed males
123
Oecologia (2007) 151:605–615
are exploiting diVerent ecological niches based on size/ age (and hence on their ontogenetic stage). Due to intra-speciWc competition for food resources, the trophic ecology of diVerent groups could inXuence the response to a change in the density of a component of the population (Best 1979), which was highly relevant during modern whaling times when size-based regulations on catch were being drafted. Sampling of growth increments in marine mammal teeth has demonstrated that dentine preserves a highresolution chronological isotopic record of physiological events, habitat use and dietary patterns (Hobson and Sease 1998; Walker and Macko 1999). This record is similar to that found, for example, in baleen plates of mysticetes (Best and Schell 1996). This is because oscillations in isotopic ratios can be linked to sources of variation in the environment and in food webs (Kelly 2000) through the biochemical assimilation of dietary components into consumers’ tissues (De Niro and Epstein 1978, 1981; Peterson and Fry 1987). Monophyodont vertebrates such as the sperm whale develop only one set of teeth, which grow throughout life in a series of layers. These are composed mainly of dentine and each alternating dark and light layer pair constitutes a growth layer group (GLG), assumed to represent 1 year’s growth (Evans et al. 2002). Once deposited, the organic (collagen) and mineral (hydroxyapatite) phases of dentine do not re-metabolise and so the isotopic signatures of the body during the animal’s lifetime become permanently and chronologically archived along the length of a tooth. Here, the stable C and N isotopic composition of dentine collagen was measured in the growth layers of teeth from 11 male sperm whales from the NE Atlantic, to investigate the existence, timing, rate and prevalence of dietary and/or foraging location shifts that might be indicative of ontogenetic benchmarks related to changes in schooling behaviour, movements, environmental conditions, foraging ecology and physiology. C isotope ratios are conservative from phytoplankton up to top consumers with less than 1‰ enrichment in whole body per trophic level (Peterson and Fry 1987; Post 2002) and hence are ideal to trace gradients in the marine environment, as the signature of a consumer should reXect the sources of C at the base of the food chain. The 13C values in the tissue of marine animals has been shown to vary with latitude (e.g. Kelly 2000; Takai et al. 2000), reXecting the depletion of 13C in phytoplankton towards higher latitudes (Rau et al. 1982; Goericke and Fry 1994). Thus, it is expected that this gradient would be reXected in the isotopic signature of male teeth proWles coincidental with a move to higher latitudes with age. The 13C proWles were also investigated to seek evidence
Oecologia (2007) 151:605–615
607
of large-scale movements to and from the breeding grounds once the animals reach the age of maturity. N isotope ratios can be indicative of trophic position, with enrichments of approximately 3.4‰ per trophic level (Minagawa and Wada 1984; Michener and Schell 1994; Post 2002), although more recent research is revealing that this may vary due to metabolism and speciWc amino acid composition of consumer tissues as well as diet quality, feeding level and growth rate (Gaye-Siessegger et al. 2003; Schmidt et al. 2004; Trueman et al. 2005). In marine communities, trophic level is expected to be higher for larger individuals of the same species (Jennings et al. 2002). Sperm whales are approximately 4 m long at birth, with males growing up to 18 m, while females reach a maximum size of 12 m (Best et al. 1984). The relative trophic position of the diVerent stages in the life of several male sperm whales was investigated through the use of N stable isotope ratios. It was expected that the trophic position would increase with age, as the animals become larger.
Materials and methods Teeth were collected from 11 male sperm whales stranded in Scotland or Ireland (Table 1) between 1993 and 1998. Tooth GLGs were sampled and analysed for 13C and 15N in order to obtain chronological proWles with age. Tooth preparation A single mandibular tooth located Wve to six teeth from the front (the least worn) was taken from each whale and mounted onto a wooden base using epoxy adhesive and cut in half along the longitudinal axis using a slowly rotating diamond saw, such that the resultant halves followed the midline of the tooth as closely as possible (Fig. 1). A thin section (1–3 mm) was sliced oV
one of the halves and further bisected with a band saw to yield two symmetrical portions. One portion was used for age estimation and was accordingly etched with formic acid (Evans and Robertson 2001). The other portion was used for stable isotope measurements, and was decalciWed to remove inorganic C using HCl (0.5 N) for at least a week, until it became Xexible. At that point it was considered that only the organic portion of the tooth remained, composed of collagen. Age estimation The age of each individual was estimated by counting GLGs in the etched half of the section. Three or more independent age estimates were carried out by the same reader, spaced at intervals of over a week between readings. The Wnal age estimate for each tooth (Table 2) was either the most repeated GLG count (six teeth) or the mean of the three most similar counts (Wve teeth) (following Evans et al. 2002). There was typically a diVerence of 1–2 years between successive individual tooth readings. Some tooth sections (three teeth) did not present the whole axis because the tooth was cut slightly oV-centre, hence losing some of the apex layers representing the earliest ages. By visually inspecting the apex of both the thin section and the other tooth half it was estimated that at least two layers had been missed out. Thus the age estimates of these sections consisted of the number of GLGs counted plus two. There is a degree of subjectivity in the interpretation of growth layers in teeth (Evans et al. 2002) due to poor deWnition of light and dark layers, and presence of accessory layers and anomalies (Pierce and Kajimura 1980; Lockyer 1995), which will inevitably result in variation of estimates between readers and between readings by the same reader. For the purpose of this study, any biases would have been consistent, as teeth were read by the same person and so we assume that the proWles will be directly comparable.
Table 1 Date, location and length of 11 stranded sperm whales Sperm whale/tooth ID
Date of stranding
Location of stranding
Length (m)
M2679/93 M2683/93 M2583/94(1) M2583/94(7) M2583/94(10) M546/95 M143/96D M143/96E Moby M447/98 I1/98
21 November 1993 24 November 1993 7 December 1994 7 December 1994 7 December 1994 23 March 1995 28 January 1996 28 January 1996 31 March 1997 2 March 1998 6 March 1998
Lochailort, Inverness-shire Loch Ainort, Isle of Skye Backaskail Bay, Sanday, Orkney Islands Backaskail Bay, Sanday, Orkney Islands Backaskail Bay, Sanday, Orkney Islands Carse of Ardersier, Inverness-shire Cruden Bay, Aberdeenshire Cruden Bay, Aberdeenshire Airth, Firth of Forth Yell, Shetland Islands Malin Head, County Donegal, Ireland
15.00 15.65 12.30 12.40 13.30 13.70 13.75 13.65 15.20 14.30 ?
Notes
Mass stranding Mass stranding Mass stranding Mass stranding Mass stranding
123
608
Oecologia (2007) 151:605–615
Fig. 1 a–c Diagram illustrating how teeth were cut and sampled. a Whole tooth showing where a thin (1–3 mm) slice was cut from the centre, along the bucco-lingual plane. b Thin slice cut in half to yield two symmetrical portions [cement (c), pulp cavity (p)]. c Two half-portions. The left one was etched and used for age estimation and the right one decalciWed and sampled to obtain one sample per growth layer group (g)
Tooth sampling and lipid extraction The dentinal collagen in the decalciWed half-section of each tooth was cut from the apex to the pulp cavity with a scalpel, to obtain one sample per GLG (Fig. 1). For the decalciWed half-sections where we could not distinguish the GLGs clearly, we used the etched half-section as a guide to where to cut the samples. Because it is diYcult to obtain slices thinner than 1 mm and due to the way the dentine is laid down (in a succession of stacked cones), it is likely that, when sampling a GLG, we obtained a small proportion of material that belongs to the previous or next GLG, making the sampling slightly coarse. All teeth were sampled based on a single age reading, i.e. the number of GLGs counted at the time of sampling and prior to all subsequent readings and Wnal age estimate, which resulted, for some animals, in a small discrepancy Table 2 Age estimates and number of samples of dentinal collagen taken for each sperm whale tooth
123
between the number of samples and the age estimated. If the Wnal estimate was assumed to be closer to the true age of the animal, then some GLGs along the tooth proWle would have been either sampled together or sampled twice. In addition, the Wrst sample taken from the three sections that did not present the whole tooth axis was considered to have been deposited at 3 years of age. Although teeth have low fat content (Hess et al. 1956) samples were delipidiWed in order to reduce any error in 13C due to potential diVerences in lipid content along the tooth. The samples were placed in glass vials and washed in successive rinses in a 1.0:2.0:0.8 ratio solution of chloroform, methanol and water, following the method of Bligh and Dyer (1959), until the chloroform phase was clear. The samples were then rinsed and sonicated several times with deionised water and freeze dried. Stable isotope analysis Approximately 0.8 mg of each sample was placed into a tin capsule and the C and N isotope analyses performed simultaneously using continuous-Xow isotope ratio mass spectrometry. All stable isotope ratios are expressed in permil (‰) deviations from primary international standards, using the delta () notation. Replicate measurements of internal laboratory standards (gelatine) indicate a precision of 0.1 and 0.2‰ for 13C and 15N, respectively. The C:N ratio for all samples varied between 2.96 and 3.34—well within the acceptable values that guarantee the purity of collagen and allow for comparison between samples (De Niro 1985; Ambrose 1990). Data exploration The stable isotope proWles were standardised (normalised) and two statistical methods used to describe
Sperm whale/tooth ID
Age estimates (years)
M2679/93 M2683/93
37 20
39 23
36 28
M2583/94(1) M2583/94(7) M2583/94(10) M546/95 M143/96D M143/96E Moby M447/98 I1/98
25 19 26 23 22 19 48 33 39
22 22 24 24 22 22 44 33 41
24 18 24 22 19 19 46 34 38
Final age estimate (years)
No. of samples
– 28
37 28
37 20 + 3 discarded
– – – – – – 46 – –
24 20 24 23 22 + 2 19 46 + 2 33 39 + 2
24 22 26 23 22 19 44 33 39
Notes
Broken, diYcult to read, discarded samples age 1, 2, 3
Section not central Section not central Section not central
Oecologia (2007) 151:605–615
temporal patterns in the data. Around half the animals were under 25 years of age. Therefore, to minimise the number of missing values in the dataset, only the values up to (and including) the age of 25 years were analysed. Principal components analysis (PCA) (JolliVe 2002) based on a correlation matrix was used to determine the general pattern in the proWles and how this varies between teeth, with each tooth being a variable and each age a sample. As PCA cannot cope with missing values, these (50 out of 550) were replaced by an average of the two values of the nearest ages for each tooth. Additionally, to conWrm any age groupings in the proWles suggested by the PCA, chronological clustering (CC) (Legendre et al. 1985) was used, which allows the identiWcation of the timings of change by organising the data into clusters of sequential years. This was used to detect points in the life of the animals where a marked change in their stable isotope signatures occurred. DiVerent values of alpha (clustering intensity parameter) were used, together with a 0.5 connectedness level and the Euclidean distance function to calculate the (dis)similarity between ages. Generalised additive models (GAMs; Hastie and Tibshirani 1990) were Wtted to C and N data to see if isotopic values diVered between animals born earlier and later, since all the whales investigated here stranded in the 1990s but diVered in age, and long-term trends of isotopic signatures at the base of foodwebs have been detected (Schell 2000). Because calendar year and age are collinear we were not able to include them both in the models but instead we included the residuals of the regression of year against age (with slope set to 1.0) as a factor in the GAM (“Birth date” eVect). To take autocorrelation between isotope values at consecutive ages into account, C and N values with a lag of 1 and 2 years (Ct¡1, Ct¡2 and Nt¡1, Nt¡2) were included as linear explanatory variables, analogous to Wtting an ARIMA (2,0,0) model (Box and Jenkins 1970). GAMs were Wtted, estimating df for the “Age” smoother by cross-validation and obtaining the optimal model (as identiWed by the Akaike's information criteria value) by forwards and backwards selection. Preliminary data exploration indicated that a Gaussian distribution could be assumed for the response variables. The statistical software Brodgar (www.brodgar.com) was used for all analyses.
Results Age estimates The estimated ages for the male sperm whales investigated here ranged between 19 and 48 years (Table 2).
609
On four teeth, the number of samples cut diVered from the Wnal age estimate in years, i.e. an excess two samples for two of the teeth, two samples short for one tooth and Wve samples short for another tooth (Table 2), the latter therefore being left out of all further analysis. These diVerences were nevertheless within the limits of the variability inherent to sperm whale age estimation (Evans et al. 2002). Patterns in the stable isotope ratio proWles Figure 2a, b shows the unstandardised C and N isotope ratio proWles for the sperm whale teeth analysed. All followed approximately the same pattern with 13C remaining relatively constant for the Wrst decade of life and decreasing steadily for all individuals after that, and with 15N showing almost the opposite trend, becoming higher gradually with age after remaining constant or decreasing in the Wrst years of life. The median decline in 13C from the Wrst year of life to the time of death was 1.91‰ (interquartile range 1.39– 2.36‰) and median increase in 15N was 2.42‰ (interquartile range 0.87–3.53‰). Stable isotope ratios ranged between ¡14.39 to ¡11.21‰ for 13C and 10.49 to 17.16‰ for 15N. Animals that stranded together in the North Sea [M143/96D with M143/96E; M2583/ 94(1) with M2583/94(7) and M2583/94(10)] had very similar isotope ratios in their last GLGs (13C range ¡13.77 to ¡14.39‰; 15N range 14.28–15.77‰) and for the second group also similar values and trends in 13C throughout life. In the Wve individuals older than 25 years, we see a gradual depletion in 13C and a gradual or sharper enrichment in 15N, after the age of 25 years. In some of the teeth there is also some smaller scale variation, which apparently denotes some periodicity but is diYcult to quantify. The results of the PCA are shown in the biplot of Fig. 3. The eigenvalues (EV) of the two principal components (PC) combined explain 84 and 78% of the variation in the data for 13C and 15N, respectively. The 13C PC1 (EV = 72%) is related to the most prominent and general pattern of 13C depletion after around the age of 9–10 years, thus dividing the samples into two distinct age groups. All teeth were highly related to PC1 with loadings >|3|. The 13C PC2 (EV = 12%) shows three age groups: 1–7, 8–20, 21–25. The pattern on this axis was driven by the reversed proWles (up to the age of 25) of M2583/94(10) and M143/96D, both with loadings >|3|. The 15N PC1 (EV = 58%) is related to the general pattern of 15N enrichment after around the age of 9–10 years, thus dividing the samples into two age groups as for the carbon PCA. All teeth were highly related to PC1 as they presented loadings >|3|,
123
610
Oecologia (2007) 151:605–615
Fig. 2 ProWles of stable isotope ratios in teeth with age for each individual whale; a 13C, b 15N Fig. 3 Principal components analysis biplot of stable isotope ratio values with age for all teeth; a 13C, b 15N. Numbers refer to the age (samples) and lines to the teeth (variables). A-M2679/93, BM2683/93, C-M2583/94(1), D-M2583/94(7), E-M2583/ 94(10), F-M546/95, G-M143/ 96D, H-M143/96E, I-Moby, JM447/98, K-I1/98. Eigenvalue (EV) in parentheses next to corresponding principal component axis
with teeth I1/98, M143/96(D), M546/95 negatively related to this axis. These teeth were also the only ones with loadings >|2| for the second axis (EV = 20%), which divided the data into three age groups: 1–4, 5–20, 21–25 years. Timings of isotopic change The age clusters for the sperm whale teeth based on their 13C and 15N are shown in Fig. 4a, b. The most important cluster divisions in terms of 13C suggest that the main isotopic changes occur around the ages of 9, 13 and 22 years. For 15N the same major divisions are shown. For larger values of alpha, mostly the same divisions were obtained, with the further detection of smaller scale clustering depicting minor changes occurring around 7 and 20 years old for 13C, and 3, 7 and 15 for 15N. Long-term trends in the isotopic signatures For C, the Wnal GAM model, with “Age”, Ct¡1, Ct¡2 and “Birth date” included as signiWcant terms, explained 86.2% of deviance. The “Age” smoother
123
showed a prominent change (a slowing down of the decline in 13C) after 10 years and a less marked change (to a more rapid decline) around 20 years. The “Birth date” eVect resulted in overall higher values of 13C for the animals born earlier (late 1950s). For N, the Wnal model explained 87.4% of the deviance. Both “Age”, Nt¡1 and Nt¡2 eVects were signiWcant whereas the “Birth date” factor was excluded in the Wnal model. The “Age” smoother showed a linear positive eVect up to the age of 30 years with no isotopic change after that age. Residuals did not show any signiWcant auto-correlation, indicating a reasonable Wt for both models, and there was no strong correlation between the residuals of diVerent whales.
Discussion The main results of this study show a decrease in 13C and an increase in 15N with age as predicted, as the animals start segregating into 13C-depleted higher latitudes and possibly feeding at a higher trophic level as they grow in size. Stable isotope proWle age-group clusters seem to be linked to biological benchmarks that
Oecologia (2007) 151:605–615
611
Fig. 4 Chronological clustering for all teeth combined: a 13C, b 15N. Small values of alpha (0.01–0.1) show the most important points of distinction between clusters of sequential years (vertical lines), with larger values (01–0.3) showing the smaller scale divi-
sions. Each cluster is numbered, so that to each estimated age corresponds a cluster. Asterisks indicate “singletons”, ages that do not belong to the group, immediately before or after them and are thus omitted from the analysis
coincide with reported changes in schooling behaviour and latitudinal movement to higher latitudes (Best 1979), with the segregation from natal groups at around 9–10 years and the penetration into high latitudes after the age of 20.
trophic compartment (Jacob et al. 2005). Lipid extraction has also been shown to alter the 15N of Wsh tissues and it has been recommended that two separate batches of treated (13C) and untreated (15N) samples should be analysed (Sweeting et al. 2006). However, this increases the cost of a study which often makes it unfeasible. We expect that, if any, an eVect on dentinal 15N in this study would be small and consistent, and therefore would not signiWcantly inXuence our interpretation since the patterns observed are well marked and prevalent.
Potential biases related to methodology We assume that the dentine in a GLG is representative of the food metabolised in 1 year, corresponding to the age depicted by that layer. A dietary change will be reXected in the composition of the newly synthesised tissue after the transition period during which the turnover of the metabolic pool occurs, which for dentine is likely to be short, around 1–4 months (Balasse et al. 2001). Thus, the duration of isotopic changes found in sperm whale teeth will represent the duration of ontogenetic events rather than of the metabolic transition period. However, the estimated timing (age) of the isotopic changes might not be precisely determined (aVecting the nearness of the Wnal estimate to the actual age), mainly due to the subjectivity associated with the age estimates (Evans et al. 2002). In the present study, however, the consistency of the timings of isotopic change, together with the fact that teeth were aged and sampled in the same way and by the same reader give us conWdence in the accuracy of our estimates and the robustness of inter-individual comparisons. The sample treatment methods (acidiWcation and delipidiWcation) used here to address biases in 13C have previously been investigated for potential eVects on 15N. Fish tissues acidiWed to remove inorganic C have shown changes in 15N (e.g. Pinnegar and Polunin 1999), although these seem to be generally small, and in the same range of the variability as found within one
Stable isotope values in sperm whale teeth The absolute values of 13C and 15N in our study compared reasonably well with the only other published stable isotope analysis of an adult sperm whale tooth (¡12.1 to ¡13.8‰ and 18.1 to 16.1‰, for 13C and 15N, respectively) (Walker and Macko 1999). Although 13C showed a decreasing trend with age, 15N showed the opposite trend to that seen in the present study, together with slightly higher values. Also, the authors did not have any information on the sex of the animal or its capture location and did not aim to obtain a sample per year of age. Male segregation from natal groups The movement of males into 13C-depleted higher latitudes (Rau et al. 1982) is the most likely reason for the changes observed in the 13C proWles. This seems to be a gradual move, starting after around the age of 9 years, probably indicating males’ segregation from their natal groups. This pattern seemed reasonably consistent amongst the 11 individuals and lasts approximately 4 years. These results appear to corroborate
123
612
the idea that pubertal males start heading to higher latitudes progressively, possibly still visiting lower latitudes in the winter (Best 1979). This timing is suggested by the PCA results and supported by the CC. However, looking at the individual proWles, we can see that there is some inter-individual variation (6– 12 years of age for most teeth and 20 years for M143/ 96D). This age range compares well with that previously inferred by Best (1979) and implies that these animals start segregating at the onset of puberty and not just prior to attaining sexual maturity as Rice (1989) had suggested. After the age of 13, another isotopic change is detected, with 13C halting its decline and remaining within a restricted isotopic range (around ¡13‰), which is observed in most proWles (except Moby and M447/98) and lasts until the early twenties. This period might be indicative of another ontogenetic stage, possibly characterised by animals mostly remaining in a narrow latitudinal range between the tropical and subtropical habitats of mixed groups and the higher latitudes where adult males are found. Juvenile males appear to form groups more homogeneous for size than age, which led Best (1979) to classify groups caught oV Donkergat (33°S, 18°E) into three categories based on length: “small bachelors” (10.7–11.6 m); “medium-sized bachelors” (12.2–13.7) and “large bachelors” (>13.7 m). Looking at the age-composition tables provided by this author we can see that 88.5% of the animals found in “small bachelor” groups were estimated to be between 13–21 years old. As male sperm whales grow it might become advantageous to join other males of similar condition to pursue cooperative feeding (Best 1979) and so it is possible that, once leaving the mixed groups, they will move gradually into more temperate waters where they gather in these “small bachelor” groups. Penetration into high latitudes After the early twenties the 13C continued to decline with age. While for the four individuals born earlier this was a gradual decline, for the other six a steeper decrease (1–1.5‰) occurred after 20 years of age, detected in the CC. This might be indicative of a movement into even higher latitudes. Best et al. (1984) showed evidence that 50% of male sperm whales were fertile by the age of 20, the age that marks a bout of accelerated growth, and the penetration of male aggregations into higher latitudes in the summer (Best 1979). All the six animals for which this steep decrease in 13 C occurred died soon after, as part of two mass
123
Oecologia (2007) 151:605–615
strandings (apart from M546/95). These six animals were born in the 1970s as opposed to the ones presenting the gradual decline in 13C, born in the 1950s/1960s. It is possible other factors (e.g. diVerences in basal 13C in regions frequented by the older and younger animals) combined with the expected ontogenetic penetration into higher latitudes might have caused the observed diVerences. Additionally, the GAM analysis showed that animals born earlier had an overall higher 13C. Long-term trends in basal 13C can be related to variations in phytoplankton growth rate or species composition and in the isotopic composition of dissolved inorganic C and these have been detected for example in baleen gathered from 37 whales spanning four decades (Schell 2000). A larger sample size encompassing whales born across many decades could potentially elucidate the diVerence observed here. Animals that were part of the group that mass stranded in 1994 show a similar pattern of 13C throughout life, both in terms of absolute values and trends while the same is not true for the two animals that mass stranded in 1996. Genetic analyses carried out showed that the animals analysed here were not related, although there were three pairs in these two groups who were half-siblings (Engelhaupt 2004). Best (1979) had suggested that it is unlikely that individual bachelors remain in close association with each other for any long period of time. If we assume that animals associating for long periods of time would present similar isotopic proWles, then the three animals from the 1994 group might have been together for a long time while the other two animals might have been together only in the last few years. All of these six individuals had high percentages of Gonatus fabricii in their stomachs when they died (Santos et al. 1999) and so were assumed to be on their way south from feeding on the spawning aggregations of this squid oV the coast of Norway, building up energy reserves for breeding. This species of squid has been found to contain a high lipid percentage compared to other species (Hooker et al. 2001), and to be highly energy rich (Clarke et al. 1985). The 13C values of the GLG deposited most recently, representative of the last year in the life of the whale, averaged ¡13.84‰ (§0.38), higher than expected if animals were feeding on this squid species oV Norway, 13C values for which ranged between ¡20 and ¡19‰ (n = 5; Hooker et al. 2001). Isotopic enrichment of C between diet and collagen has been shown experimentally to vary in mammals and could be as high as 6‰ (Bocherens and Drucker 2003), which would mean a 13C value for sperm whale collagen of »14‰, closer to what was found here.
Oecologia (2007) 151:605–615
Breeding migrations The four “large bachelors” analysed here do not show an increase in 13C after the age of 25–27 years (age of sexual maturity), which could be indicative of trips to breeding latitudes. Indeed, 13C continues to decrease, indicating that animals might be frequenting even higher latitudes. Best (1979) noticed that the larger the males, the higher the latitude at which they occur and Rice (1989) suggested that mature males might spend most of their time foraging in high latitudes. Hence, it could be that migrations to breed are short and we are losing the isotopic detail by sampling entire GLGs, and also that they do not feed much (comparatively) while breeding and therefore do not pick up the isotopic signatures of lower latitudes. Indeed males seem to engage brieXy in the breeding activity for less time than the already concentrated breeding season (Best and Butterworth 1980; Coakes and Whitehead 2004). Large males may be at a nutritional disadvantage when joining mixed groups and so it might be beneWcial for them to minimise the period spent with the school. Large males seem to have lower feeding success than females and small males in some breeding areas, with the former feeding at perhaps 50–80% of the rate of the latter (Clarke et al. 1988; Whitehead 1996; Best 1999). Only 10–25% of large males might become involved in the breeding activity each year (Best 1979) and so we cannot discount the possibility that the four adult males analysed here had never travelled to and back from breeding latitudes. 15N increase with age–trophic level eVect or change in basal 15N? The median increase of 2.42‰ in dentinal 15N with age suggests that, at the time of death, the whales could be feeding at a higher trophic level than when they were born. Isotopic C and N data have provided evidence of an increase in trophic level as animals age and grow in size, potentially consuming larger prey at higher trophic levels (Minagawa and Wada 1984; Lesage et al. 2001; Jennings et al. 2002). Sperm whales, like most cetaceans, will feed on larger prey as they grow (Clarke 1996). Stomach contents analyses showed that adult males will take large prey as compared to females and young males (Clarke et al. 1993). It is also possible that the increase in 15N could be due not to consumption of larger prey but to consumption of the same sized prey, of a diVerent species or diVerent area, with a higher trophic position. Hooker et al. (2001) suggested that Gonatus sp. might belong to a higher trophic level than other squid genera and this is
613
the main prey of sperm whales in the high latitudes of the North Atlantic (Santos et al. 1999; Bjorke 2001). The timings of marked 15N enrichment reXect quite closely the C proWle patterns, suggesting a link between dietary changes and movements. For sperm whales, as suggested for northern elephant seals, segregation away from natal groups might be triggered by a change in the metabolic requirements associated with the accelerated growth rate of males during the onset of puberty, which is generally dependent on physical condition and size (Stewart 1997), and hence will likely be concurrent with a need for a change in diet. Furthermore, access to larger prey items and higher food availability as the animals penetrate high latitudes could facilitate the spurt of growth seen around the age of 20 (Best 1979). However, according to recent research, changes in growth rate might aVect the N isotopic fractionation between diet and consumer (Trueman et al. 2005), even though some studies found no diVerence in trophic isotope fractionation between young and adult animals fed on the same diet (Hobson and Clark 1992; Ponsard and Averbuch 1999). Also, diet quality and feeding level have been shown to aVect trophic isotope fractionation (Gaye-Siessegger et al. 2003). Although all these studies add to the uncertainty involved in using 15N values as a proxy for trophic level, the eVects reported seem to lead to variation in 15N values in the order of around 1‰, which is small enough not to aVect the semi-quantitative interpretation of our results. Finally, and potentially more signiWcant, the relationship of 15N with body size in animals that move across large stretches of ocean during their lives might not necessarily correspond to a relationship with trophic level, since the basal 15N of foodwebs may vary widely as a function of area and season (Owens 1987; Lesage et al. 2001; Post 2002). For example, oligotrophic tropical and subtropical waters of the North Atlantic, where sperm whale mixed groups occur, present relatively low values of 15N (Montoya et al. 2002). Takai et al. (2000) found, however, that 15N in squid did not correlate signiWcantly with latitude but reXected geographical diVerences in N uptake by phytoplankton and water-column denitriWcation, which are inXuenced by oceanographic parameters. In a survey across the North Atlantic, 15N in phytoplankton varied between 3.65 and 8.48‰ (Waser et al. 2000). We cannot therefore discount the possibility that the observed enrichment in 15N might be a combination of trophic level eVect and diVerent N isotopic signatures of the mesopelagic food webs exploited. The gradual depletion in 15N in the Wrst years of age depicted in teeth M546/95, M143/96D and I1/98, related
123
614
to the second axis in the PCA, ranged between 1.79 and 2.94‰ and might reXect protracted weaning (Richards et al. 2002). Nursing animals are in a consumer position relative to their mothers, hence tend to become enriched in 15N compared to them (Polischuk et al. 2001; Richards et al. 2002). Thus, when a calve starts to forage— and presumably adopts a diet close to that of its mother—the 15N in body tissues is expected to fall. Sperm whales are usually weaned onto solid food within the Wrst year of life but can continue nursing up to the age of 13 years, even if only occasionally (Best 1979). Further work This study conWrms the potential of using 13C and 15N proWles in chronological sections of dentine to investigate movements and dietary history of sperm whales. Here, we investigated only 11 whales and, although there are common patterns, there is still some variability left unexplained. For the future, oxygenand C isotopic composition of the inorganic fraction of dentine should also be investigated as it might provide complementary information about physiology, latitudinal movements and habitat preferences (Yoshida and Miyazaki 1991; Clementz et al. 2006). The assessment of how teeth isotopic proWles vary among individuals from diVerent regions, sexes, and periods in time, could provide a unique way to understand the ecology and ontogeny of this species across oceans, particularly for males, which move long distances between diVerent habitats and show distinct ontogenetic stages. Sampling of historical collections of teeth from whaling days might provide an important opportunity to study sperm whale ecology and their habitats in the Wrst half of the twentieth century or even earlier. Acknowledgements We thank Colin MacLeod, Jennifer Learmonth, Patricia Lastra, Gabriele Stowasser and David Mackenzie, at the University of Aberdeen, for help with sample preparation and Dave McNamara (Donegal County Council) for the provision of the Irish tooth. We thank Colin MacLeod and two anonymous reviewers for useful comments on the manuscript. S. M. was supported through a PhD studentship by the Portuguese Foundation for Science and Technology (grant SFRH/ BD/5466/2001), and the stable isotope analyses were carried out at the NERC Life Sciences Mass Spectrometry Facility (application no. EK74-11/04).
References Ambrose SH (1990) Preparation and characterization of bone and tooth collagen for isotopic analysis. J Archaeol Sci 17:431–451 Balasse M, Bocherens H, Mariotti A, Ambrose SH (2001) Detection of dietary changes by intra-tooth carbon and nitrogen
123
Oecologia (2007) 151:605–615 isotopic analysis: an experimental study of dentine collagen of cattle (Bos taurus). J Archaeol Sci 28:235–245 Best PB (1979) Social organization in sperm whales, Physeter macrocephalus. In: Winn HE, Olla BL (eds) Behaviour of marine animals, vol 3. Plenum Press, New York, pp 227–289 Best PB (1999) Food and feeding of sperm whales Physeter macrocephalus oV the west coast of South Africa. S Afr J Marine Sci 21:393–413 Best PB, Butterworth DS (1980) Timing of oestrus within sperm whale schools. Rep Int Whal Comm (Spec Issue) 2:137–140 Best PB, Schell DM (1996) Stable isotopes in southern right whale (Eubalaena australis) baleen as indicators of seasonal movements, feeding and growth. Mar Biol 124:483–494 Best PB, Canham PAS, MacLeod N (1984) Patterns of reproduction in sperm whales, Physeter macrocephalus. Rep Int Whal Comm (Spec Issue) 8:51–79 Bjorke H (2001) Predators of the squid Gonatus fabricii (Lichtenstein) in the Norwegian Sea. Fish Res 52:113–120 Bligh EG, Dyer WJ (1959) A rapid method for total lipid extraction and puriWcation. Can J Biochem Physiol 37:911–917 Bocherens H, Drucker D (2003) Trophic level isotopic enrichment of carbon and nitrogen in bone collagen: case studies from recent and ancient terrestrial ecosystems. Int J Osteoarchaeol 13:46–53 Box GEP, Jenkins GM (1970) Time series analysis: forecasting and control. Holden-Day, San Francisco, Calif. Clarke MR (1996) Cephalopods as prey. III. Cetaceans. Philos Trans R Soc B 351:1053–1065 Clarke A, Clarke MR, Holmes LJ, Waters TD (1985) CaloriWc values and elemental analysis of 11 species of oceanic squids (Mollusca, Cephalopoda). J Mar Biol Assoc UK 65:983–986 Clarke R, Paliza O, Aguayo A (1988) Sperm whales of the southeast PaciWc. Part IV. Fatness, food and feeding. Investig Cetacea 21:53–195 Clarke MR, Martins HR, Pascoe P (1993) The diet of sperm whales (Physeter Macrocephalus Linnaeus 1758) oV the Azores. Philos Trans R Soc B 339:67–82 Clementz MT, Goswami AJ, Gingerich PD, Koch PL (2006) Isotopic records from early whales and sea cows: contrasting patterns of ecological transition. J Vertebr Paleontol 26(2):355–370 Coakes AK, Whitehead H (2004) Social structure and mating system of sperm whales oV northern Chile. Can J Zool 82:1360– 1369 De Niro MJ (1985) Post-mortem preservation and alteration of in vivo bone collagen isotope ratios in relation to paleodietary reconstruction. Nature 317:806–809 De Niro MJ, Epstein S (1978) InXuence of the diet on the distribution of carbon isotopes in animals. Geochim Cosmochim Acta 42:495–506 De Niro MJ, Epstein S (1981) InXuence of the diet on the distribution of nitrogen isotopes in animals. Geochim Cosmochim Acta 45:341–351 Engelhaupt DT (2004) Phylogeography, kinship and molecular ecology of sperm whales (Physeter macrocephalus). PhD thesis. Department of Biological Sciences, University of Durham Evans K, Robertson K (2001) A note on the preparation of sperm whale (Physeter macrocephalus) teeth for age determination. J Cet Res Man 3:101–107 Evans K, Hindell M, Robertson K, Lockyer C, Rice D (2002) Factors aVecting the precision of age determination of sperm whales, Physeter macrocephalus. J Cet Res Man 4:193–202 Gaye-Siessegger J, Focken U, Abel Hj, Becker K (2003) Feeding level and diet quality inXuence trophic shift of C and N isotopes in Nile tilapia [Oreochromis niloticus (L.)]. Isotopes Environ Health Stud 39(2):125–134
Oecologia (2007) 151:605–615 Goericke R, Fry B (1994) Variations of marine plankton 13C with latitude, temperature, and dissolved CO2 in the world ocean. Global Biogeochem Cycles 8:85–90 Hastie TJ, Tibshirani RJ (1990) Generalised additive models. Chapman and Hall, New York Hess WC, Lee CY, Peckham SC (1956) The lipid content of enamel and dentin. J Dent Res 35(2):273–275 Hobson KA, Clark RG (1992) Assessing avian diets using stable isotopes. 2. Factors inXuencing diet-tissue fractionation. Condor 94:189–197 Hobson KA, Sease JL (1998) Stable isotope analyses of tooth annuli reveal temporal dietary records: an example using steller sea lions. Mar Mammal Sci 14:116–129 Hooker SK, Iverson SJ, Ostrom P, Smith SC (2001) Diet of northern bottlenose whales inferred from fatty-acid and stableisotope analyses of biopsy samples. Can J Zool 79:1442–1454 Jacob U, Mintenbeck K, Brey T, Knust R, Beyer K (2005) Stable isotope food web studies: a case for standardized sample treatment. Mar Ecol Prog Ser 287:251–253 Jennings S, Pinnegar JK, Polunin NVC, Warr KJ (2002) Linking size-based and trophic analyses of benthic community structure. Mar Ecol Prog Ser 226:77–85 JolliVe IT (2002) Principal component analyses, 2nd edn. Springer, Berlin Heidelberg New York Kelly JF (2000) Stable isotopes of carbon and nitrogen in the study of avian and mammalian trophic ecology. Can J Zool 78:1–27 Legendre P, Dallot S, Legendre L (1985) Succession of species within a community: chronological clustering, with application to marine and freshwater zooplankton. Am Nat 125:257–288 Lesage V, Hammill MO, Kovacs KM (2001) Marine mammals and the community structure of the Estuary and Gulf of St Lawrence, Canada: evidence from stable isotope analysis. Mar Ecol Prog Ser 210:203–221 Lockyer CH (1995) A review of factors involved in zonation in odontocete teeth, and an investigation of the likely impact of environmental factors and major life events on harbour porpoise tooth structure. Rep Int Whal Comm (Spec Issue) 16:511–529 Michener RH, Schell DM (1994) Stable isotope ratios as tracers in marine and aquatic foodwebs. In: Lajtha K, Michener RH (eds) Stable isotopes in ecology and environmental science. Blackwell, Oxford, pp 138–157 Minagawa M, Wada E (1984) Stepwise enrichment of 15N along food chains: further evidence and the relation between 15N and animal age. Geochim Cosmochim Acta 48:1135–1140 Montoya JP, Carpenter EJ, Capone DG (2002) Nitrogen Wxation and nitrogen isotope abundances in zooplankton of the oligotrophic North Atlantic. Limnol Oceanogr 47:1617–1628 Owens NJP (1987) Natural variations in 15N in the marine environment. Adv Mar Biol 24:389–451 Peterson BJ, Fry B (1987) Stable isotopes in ecosystem studies. Annu Rev Ecol Syst 18:293–320 Pierce KV, Kajimura H (1980) Acid etching and highlighting for deWning growth layers in cetacean teeth. Rep Int Whal Comm (Spec Issue) 3:99–103 Pinnegar JK, Polunin NVC (1999) DiVerential fractionation of 13C and 15N among Wsh tissues: implications for the study of trophic interactions. Funct Ecol 13:225–231 Polischuk SC, Hobson KA, Ramsay MA (2001) Use of stable-carbon and -nitrogen isotopes to assess weaning and fasting in female polar bears and their cubs. Can J Zool 79:499–511
615 Ponsard S, Averbuch P (1999) Should growing and adult animals fed on the same diet show diVerent 15N values? Rapid Commun Mass Spectrom 13:1305–1310 Post DM (2002) Using stable isotopes to estimate trophic position: models, methods and assumptions. Ecology 83:703–718 Rau GH, Sweeney RE, Kaplan IR (1982) Plankton 13C:12C ratio changes with latitude: diVerences between northern and southern oceans. Deep Sea Res 29:1035–1039 Rice D (1989) The sperm whale Physeter macrocephalus Linnaeus 1758. In: Ridgway SH, Harrison R (eds) Handbook of marine mammals, vol 4. Academic Press, London, pp 177– 233 Richard KR, Dillon MC, Whitehead H, Wright JM (1996) Patterns of kinship in groups of free-living sperm whales (Physeter macrocephalus) revealed by multiple molecular genetic analyses. Proc Natl Acad Sci USA 93:8792–8795 Richards MP, Mays S, Fuller BT (2002) Stable carbon and nitrogen isotope values of bone and teeth reXect weaning age at the Medieval Wharram Percy site, Yorkshire, UK. Am J Phys Anthropol 119:205–210 Santos MB, Pierce GJ, Boyle PR, Reid RJ, Ross HM, Patterson IAP, Kinze CC, Tougaard S, Lick R, Piatkowski U, Hernandez-Garcia V (1999) Stomach contents of sperm whales, Physeter macrocephalus, stranded in the North Sea 1990– 1996. Mar Ecol Prog Ser 183:281–294 Schell DM (2000) Declining carrying capacity in the Bering Sea: isotopic evidence from whale baleen. Limnol Oceanogr 45:459–462 Schmidt K, McClelland JW, Mente E, Montoya JP, Atkinson A, Voss M (2004) Trophic level interpretation based on values: the implications of tissue-speciWc fractionation and amino acid composition. Mar Ecol Prog Ser 266:43–58 Stewart BS (1997) Ontogeny of diVerential migration and sexual segregation in northern elephant seals. J Mammal 78:1101– 1116 Sweeting CJ, Polunin NVC, Jennings S (2006) EVects of chemical lipid extraction and arithmetic lipid correction on stable isotope ratios of Wsh tissues. Rapid Commun Mass Spectrom 20:595–601 Takai N, Onaka S, Ikeda Y, Yatsu A, Kidokoro H, Sakamoto W (2000) Geographical variations in carbon and nitrogen stable isotope ratios in squid. J Mar Biol Assoc UK 80:675–684 Trueman C, McGill R, Guyard P (2005) The eVect of growth rate on tissue-diet isotopic spacing in rapidly growing animals. An experimental study with Atlantic salmon (Salmo salar). Rapid Commun Mass Spectrom 19:3239–3247 Walker JL, Macko SA (1999) Dietary studies of marine mammals using stable carbon and nitrogen isotopic ratios of teeth. Mar Mammal Sci 15:314–334 Waser NAD, Harrison WG, Head EJH, Nielsen B, Lutz VA, Calvert SE (2000) Geographic variations in the nitrogen isotope composition of surface particulate nitrogen and new production across the North Atlantic Ocean. Deep Sea Res I 47:1207–1226 Whitehead H (1996) Variation in the feeding success of sperm whales: temporal scale, spatial scale and relationship to migrations. J Anim Ecol 65:429–438 Yoshida N, Miyazaki N (1991) Oxygen isotope correlation of cetacean bone phosphate with environmental water. J Geophys Res 96:815–820
123
