The lambing period of mountain sheep: synthesis

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The lambing period of mountain sheep: synthesis, hypotheses, and tests FREDL. BUNNELL Faculty of Forestry, University of British Columbia, B.C., Canada V6T 1W5 Received April 7, 1981

F. L. 1982. The lambing period of mountain sheep: synthesis, hypotheses, and tests. Can. J . Zool. 60: 1-14. BUNNELL, Data on lambing periods from 30 populations of North American mountain sheep are reviewed. Among all populations lambing begins later and duration is shorter at more northern altitudes (p < 0.00006). Correlations are enhanced (p < 0.00003) when latitude is replaced by a phenological index incorporating altitude. Termination of lambing is not correlated with latitude or with phenological index.Two broad patterns are evident. Populations feeding on vegetation of less predictable growth patterns ("desert type") have lengthy lambing seasons; populations feeding on vegetation exhibiting more predictable growth patterns ("alpine type") have shorter lambing seasons, typically two oestrous cycles in length. Definition of "types" by latitude or sheep taxonomy reveals significant differences in lambing periods, but correlations are enhanced when "types" are defined on the basis of habitat. Among taxa, birth weights are correlated with female body weight (r, = 0.87). Birth weights are heavier in extreme environments, seasonal growth patterns are expressed better in the alpine type, and early weight gain is most rapid in the northernmost subspecies. All populations show a strong central tendency with regard to peak lambing (17 May + 6.8 days). Departures from that tendency respond more to predictability of vegetation than to thermal stress or predation pressure. BUNNELL, F. L. 1982. The lambing period of mountain sheep: synthesis, hypotheses, and tests. Can. J. Zool. 60: 1- 14. On trouvera ici une rCvision des donnCes sur les pCriodes de reproduction chez 30 populations de mouflons d'AmCrique du Nord. Chez toutes les populations, la pCriode de mise-bas commence plus tard et dure moins longtemps aux latitudes les plus borkales ( p < 0,00006). Les corrClations sont encore plus fortes ( p < 0,00003) lorsque la latitude est remplacCe par un indice phknologique qui tient compte de l'altitude. La fin de la pCriode de mise-bas n'est reliCe ni a la latitude ni a l'indice phCnologique. Deux patterns bien dCfinis sont mis en lumiere. Les populations qui se nourrissent de vCgCtation a croissance peu prkvisible ("type dksertique") ont de longues saisons de reproduction; les populations qui se nourrissent de vCgCtation a croissance plus prCvisible ("type alpin") ont des saisons de mise-bas plus courtes, gCnCralement la durCe de deux cycles oestraux.La caractCrisation des "types" en fonction de la latitude ou de la taxonomie des mouflons Ctablit des diffdrences significatives de pCriodes de mise-bas, mais les corrClations sont accentukes si les "types" sont dCfinis en fonction de l'habitat. Chez les diffkrents taxons, la masse a la naikance est relike a la masse totale de la femelle (r, = 0,87). La masse la naissance est plus importante dans les milieux extremes, les patterns de croissance saisonniere sont mieux dCfinis chez les animaux de type alpin et l'augmentation de la masse est plus hstive chez les sous-especes les plus borkales. La pCriode de mise-bas tres active suit une tendance centrale chez toutes les populations (17 mai + 6,8 jours). L'Cloignement de cette tendance reflete plus la prkvisibilitk de la vCgCtation que le stress thermique ou la pression de la prCdation. [Traduit par le journal]

Introduction Ungulates in northern latitudes breed seasonally (Schaller 1977, Table 14), presumably in response to a well-defined periodicity in available food (e.g. , Geist 1974). In north temperate regions domestic sheep are seasonally pol yoestrus, mate in the autumn, and exhibit a marked anoestrous period in summer (Asdell 1964). Although cold temperatures (Dutt and Bush 1955) and stin~ulationby males (Fraser 1968 and references cited therein) hasten onset of reproductive activity, the major proximate stimulus to the initiation of oestrus in sheep is decreasing day length (Hammond 1944; Yeates 1949; Hafez 1952; Amoroso and Marshall 1960). Marshall (1910) appears the first to suggest that the ultimate

factors governing periodicity in breeding are requirements of the young. Sugden (1961) observed that among wild Ovis canadensis length of the breeding season was shortened in harsher climates resulting in a correspondingly shorter lambing period. He also noted that in colder climates the time a lamb was born could affect its survival. Geist (1971, p. 239) extended Sugden's observation and provided the most recent summary statement of lambing seasons among North American Ovis: "The lambing season varies in date and duration according to the severity of the climate. In the northern latitudes and high altitudes lambing seasons are short and come later in the spring than in the warm uplands. In the hot deserts of

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dependence, the specific alternative hypotheses being tested are presented in Table 1. The probability of error in rejecting the null hypothesis, p, was derived using z = r,(n - 1)lI2, Bunnell(1980) discussed the lambing period of Dall's which is approximately a standardized normal variable when n sheep and suggested factors controlling its onset and L 10. The value of n is never less than 11 in the tests presented. duration. This paper reviews available data on lambing To evaluate the generality of controls on lambing period, I period of North American Ovis. The hypothesis that have collated studies reporting weights of mountain sheep at factors limiting the lambing period for Dall's sheep different ages. The summary statement of Geist (1971) was presented as a apply to other Ovis taxa is addressed, permitting an statement of fact. To apply statistical tests it must be rephrased extension of the summary by Geist (197 l), cited above. Specifically, two predictions are examined: ( I ) onset of as explicit alternative hypotheses: HI: lambing begins later at more northerly latitidues. lambing is determined primarily by forage quality and H2: lambing ends earlier at more northerly latitudes. quantity and not by avoidance of thermal stress, and (2) H3: lambing periods are shorter at more northerly latitudes. duration of lambing is limited by the need of lambs to These hypotheses are evaluated against the null hypothesis of attain adequate body size by winter. Subsidiary predic- no correlation between latitude and a given feature of lambing tions concerning birth weight and growth rates are period (e.g . , beginning, end, duration) using Spearman's r,. Plant phenology is influenced by both latitude and elevation. tested. For an alternative treatment of factors governing geographic variation in lambing seasons see Thompson Analyses of covariance are impossible given the nature of the data, but some of the potential confounding of latitude and (1981). elevation can be accommodated. Studies of plant phenology have reported relationships equating elevational increases with Methods specific horizontal distances northward (degrees latitude). The I have attempted to include all published studies of wild, first well-quantified relationship was the "bioclimatic law" of North American Ovis that discussed lambing dates. Very few Hopkins ( 1938) which equated an elevational change of 100 to studies provide detailed lambing chronologies. Where possi130 m with a horizontal distance of lo latitude. Other studies ble I have computed mean birth dates from the lamb ages or have modified Hopkins' law for particular regions (e.g., birth dates provided. In some instances authors did not present Caprio 1957; Jeffree 1960; Ricklefs 1973), but Hopkins' law chronologies, but did provide alternative, less rigorous estihas proven widely applicable in continental areas and is mates of mean birth date (e.g., "mean peak date of lambing ," employed here. Using Hopkins' law three further alternative Nichols 1978, p. 577). My calculations or author's statements hypotheses can be evaluated: were used to estimate the dates by which 50% of the lambs had H4: onset of lambing is positively correlated with phenolbeen born. OgY. Among Dall's sheep, lambs born early in the lambing period H5: termination of lambing is negatively correlated with experience greater mortality than those born later. The usual phenology . methods of reporting lamb crops, progressive counts of lambs H6: duration of lambing is negatively correlated with or 1amb:ewe ratios, tend to omit early mortality and thus shift phenology . the estimate of mean date of lambing to later in the year In each case the null hypothesis assumes no correlation (Bunnell 1980). Such bias may exist for several of the studies between phenological state and lambing, and phenological illustrated (Fig. I), particularly among northern populations. state is predicted for latitude and elevation using Hopkins' law Studies 8 through 11 (Fig. 1) examined 0 . c. nelsoni at the (1 15 m change in elevation equivalent to l o N latitude). same location in Nevada and are arranged in chronological Two studies (Wilson 1968; McQuivey 1978) that were order of study. More recent studies encompass more interan- retrieved too late to be included in Fig. 1 are included in the nual variation and report a wider distribution of birth dates. statistical analyses. Significant interannual variation in birth dates is also evident in northern populations (e .g., 18a and 18b and 28a and 28 b of Results and discussion Fig. 1 and Nichols (1978)). In statistical tests data for different years from the same population are treated as independent Lambing period When arranged along latitude the populations of Fig. samples. If broad environmental features exert strong controls 1 suggest two broad but largely discrete patterns of on lambing period these should dominate interannual variation lambing, termed "desert" and "alpine." If those same and be detectable statistically. Dates of onset and termination of lambing are not normally populations are characterised by predictability of the distributed (Fig. 1). Furthermore, some authors estimated habitat (precipitation and primary production) the two rather than quantified dates. The estimates, however, are groups differ slightly. I first treat the groupings on the ordinal and can be analysed with nonparametric statistics. basis of latitude, then on the basis of predictability of the Potential correlations between latitude and lambing dates are habitat. evahated by Spearman's rank correlation (r,); potential Latitude and elevation differences between "alpine" and "desert" types are evaluated Earlier summary statements implied a continuous, by the Mann-Whitney U-test. Formulations used are those of clinal variation in lambing evident at least among those Conover (197 1). Because Spearman's r, is insensitive to some types of forms outside desert regions. Initial tests examine


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RG.1. Lambing periods of North American Mountain sheep versus latitude. Solid lines represent the major lambing seasons; broken lines represent infrequent births; solid circles represent the date by which 50% of lambs were born. Ovis canadensis mexicana: (1) Hailey (197 1); (2) Davis and Taylor (1939); (3) Lenarz (1979); Ovis c. nelsoni: (4) Russo (1956); (5) Robinson and Cronedler (1954); (6) Leslie and Douglas (1979); (7) Welles and Welles (1961); (8) Allen (1939); (9) Pulling (1945); (10) Aldous et al. (1968); (1 1) Wansen (1965). Ovis c. canadensis:(12) Moser (1962); (13) Spencer (1943); (14) Packard (1946); (15) Honess and Frost (1942); (16) Davis (1938); (17) Smith (1954); (18) Ogren (1954), a = 1952, b = 1953; (20) Geist (1971) and Stelfox (1976). Ovis c . californiana:(19) Blood (1963); (21) Sugden (1961), Riske Creek; (22) Sugden (1961), Churn Creek. Ovis h l l i stonei: (23) Geist (197 1); (24) Seip and Bunnell (unpublished data). Ovis d. h l l i : (25) Nichols (1978), mean of all populations in all year%;(26) Pitzman (1970); (27) Bunnell(1980); (28) Murie (1944), a = 1932 and 1940, b = 1939 and 1941. TABLE1. Tests of hypotheses regarding lambing period in North American mountain sheep (&: no difference; p for one-tailed tests) Test g ~ w p All populations "alpine" type All populations "alpine" type All populations "alpine" type All populations "alpine" type All populations "alpine'?ype All populations "alpine" type

Alternative hypotheses

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HI: lambing begins later at more northern latitudes HZ:lambing ends earlier at more northern latitudes H3:lambing period is shorter at more northern latitudes I&: onset of lambing is positively correlated with phenology H5:termination of lambing is negatively correlated with phenology Hg: duration of lambing is negatively correlated with phenology

whether such continuous variation is evident within available data. Tests are applied first to all populations, then to those termed ""alpine" (Fig. 1). When d l populations are considered, onset of lambing is positively correlated with latitude (lambing begins later at more northerly latitudes, r, = 0.69; Table 1). The tendency is nonsignificant when only alpine populations are considered. Among all populations there is

no tendency for earlier dates of termination of lambing to be associated with more northerly latitudes (positive values of r, for PI2, Table 1);within alpine populations a weak tendency is evident (p < 0.88). When all populations are considered, shorter lambing periods are associated with more northerly latitudes (r, = -0.73; p I8.0001). When only alpine populations are considered the trend is less evident (r, = -0.27).

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Northern populations give birth later and over a shorter period than populations at more southerly latitudes. The shortening of the lambing period is a product of its later commencement, for the termination of lambing changes little with latitude (Fig. 1 and Table 1). The highly significant correlations are products of the extremes. When only the alpine populations of Fig. 1 are considered, the values of r, generally decline (termination of lambing is an exception). Although the alpine type encompasses a range in latitude from about 39" N to 63" N, no feature of the lambing period shows a significant correlation with latitude (Table 1). Onset and duration of lambing are more highly correlated with the phenological index than with latitude alone, particularly among alpine populations (Table I). The correlation between onset of lambing and phenological index is highest (r, = 0.82, Table I). These observations are expected if new vegetative growth is important at the onset of lambing. Horejsi (1976) and Bunnell (1978) provided data, for populations of 0.c. canadensis and 0. dalli dalli respectively, that documented the importance of spring vegetative growth to lamb survival. The values of Table 1 indicate that the relationship between spring vegetative growth and lamb survival is general among mountain sheep, and is especially evident among alpine populations. Sheep taxonomy and habitat There are marked differences between the two broad types termed "desert" and "alpine" (Fig. 1). Populations in Fig. 1 were assigned to a type by reason of the latitude of their range. Although they capture one essential difference, the terms "desert" and "alpine" are imperfect. The desert type includes 0.c. nelsoni and 0. c. mexicana, both commonly termed desert bighorn and both occurring south of about 38" N latitude. These subspecies are not, however, consistently restricted to desert environments. Davis and Taylor ( 1939) described some 0.c. mexicana as living in mountains at elevations of 2100m or higher. Although including many plants typical of arid environments, their range also supported grasses, oaks, and pine (see Carter et al. 1934, p. 7). Similarly, the population of 0.c. nelsoni reported by Robinson and Cronemiller (1954) inhabited areas of deeper soils in conifer forests of the San Gabriel and San Bernardino mountains, California. The population reported by McQuivey (1978) also had access to "alpinelike" range. The alpine type of Fig. 1 has less clear taxonomic boundaries and includes 0.c. canadensis, 0. c. californiana, and the native thinhorns, 0. dalli dalli and 0. d. stonei. With the exception of relict populations of the former two taxa, populations termed alpine occur north of 38" N latitude. Precipitation on ranges occupied during lambing may in some years be less than 25 cm and thus similar to a desert environment. However, the

annual distribution of precipitation is much more predictable, and the range invariably supports some bunch grasses and conifers. The two broad patterns of lambing represented by these forms are associated more with the predictability of forage than with sheep taxonomy. With one exception all populations grouped as alpine exploit alpine or subalpine vegetation that exhibits a clear, predictable, seasonal periodicity in growth and abundance. The exception is a transplanted population of 0.c. canadensis now confined to Wild Horse Island, Montana ( 18 of Fig. I). All but three populations exhibiting the desert type of response inhabit the Chihuahuan, Mojave, or Sonoran deserts. Habitats of these populations (including 2 and 5 of Fig. 1) were noted above (Carter et al. 1934; Robinson and Cronemiller 1954; McQuivey 1978). Primary production of desert systems is a function of the amount and timing of precipitation (Beatley 1974). In desert systems the amount of precipitation necessary to stimulate plant productivity is low (near 2.5 cm) (Went 1955; Beatley 1974), but the probability that this threshold will be reached is also low relative to temperate or alpine systems (Conley et al. 1979). The populations grouped within the desert type by reason of latitude, but occupying subalpine ranges, experience a more predictable pattern of vegetative growth and show a more restricted lambing period than other desert forms. These populations could be considered desert on the basis of latitude or alpine on the basis of habitat. Applying the Mann-Whitney U-test after assigning types on the basis of environmental predictability (habitat) reveals better discrimination than that provided by latitude alone (Table 2). At the broadest level, distinguishing between lambing periods of the two "types," latitude of the range or taxonomic status are nearly as powerful a criterion as is habitat. Within "types," however, correlations between features of the lambing period and latitude or the phenological index are enhanced when the criterion determining "type" is habitat rather than latitude. Nine of the 12 correlation coefficients are improved when populations are assigned to type on the basis of habitat or environmental predictability (Table 3). No significant correlations are observed within the desert type determined on the basis of latitude; three are evident when the type is determined on the basis of habitat. Despite the less predictable habitat, lambing begins later and is of shorter duration in more northern desert populations (Table 3). The broad features of Geist's (1971) observation are correct, but are not expressed as a continuous gradient with either latitude or the phenological index, nor do desert forms lamb in any mon,thof the year. Instead, two broad patterns emerge. Mountain sheep taxa feeding on vegetation of less predictable growth pattern begin

TABLE2. Mann-Whitney tests of differences between lambing periods in desert and alpine types -

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Criterion to distinguish types Latitude

U

P