Investigating the Consistency of Mate-Locating ... - public.asu.edu

P1: VENDOR Journal of Insect Behavior [joib]

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Style file version Feb 08, 2000

Journal of Insect Behavior, Vol. 14, No. 1, 2001

Investigating the Consistency of Mate-Locating Behavior in the Territorial Butterfly Hypolimnas bolina (Lepidoptera: Nymphalidae) Darrell J. Kemp1 Accepted August 2, 2000; revised September 5, 2000

The study of butterfly behavior has afforded valuable insights into the evolution of alternative mating tactics. Two hypotheses derived from this area of research contend that (1) territoriality is only viable under low to moderate conspecific densities (due to the costs of site defence) and (2) perching may be employed only when thermal conditions constrain flight activity. These hypotheses were evaluated by investigating mate locating behavior in Hypolimnas bolina, a territorial species that is naturally subject to variation in population density and weather conditions. Male behavior was charted throughout the day during a period of high population density at an encounter site in tropical Australia. Perching was the primary tactic, although a small proportion of individuals patrolled nonaggressively in the afternoon. Population-level male behavior failed to support predictions drawn from either the “territory economics” or “thermal constraint” hypotheses. First, the proportion of perching males and the number of aggressive conspecific interactions (per male) increased with increasing male density at the site. Second, few males patrolled at the hottest, brightest time of day (approximately midday), and the diel distribution of perchers did not emulate the “U-shaped” distribution shown by the occurrence of dorsal basking behavior. These results show that perching in this species is not a suboptimal tactic employed when temperatures constrain flight activity but may represent the best method of locating receptive females. At this stage the reproductive significance of the observed patrolling behavior remains obscure. KEY WORDS: alternative mating tactics; sexual selection; intrasexual competition; behavioral plasticity; reproductive behavior; perching.

1School

of Tropical Biology, James Cook University, P.O. Box 6811, Cairns, Queensland 4870, Australia. e-mail: [email protected]. 129 C 0892-7553/01/0100-0129$19.50/0 ° 2001 Plenum Publishing Corporation


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INTRODUCTION Alternative male mating tactics are such a conspicuous component of animal mating systems (reviews by Arak, 1984; Austad, 1984; Dominey, 1984; Gross, 1996) that this situation is now considered as the rule rather than the exception (Waltz and Wolf, 1984). In its simplest form, plasticity in mating behavior is expressed as the cooccurrence of territorial (aggressive) and nonterritorial or sneaky tactics (e.g., Arak, 1984; Gross, 1991; Higashi and Nomakuchi, 1997). Individual males of these species either may be morphologically “locked in” to specific tactics at certain life stages (e.g., Sinervo and Lively, 1996; Cook et al., 1997; Emlen, 1997) or may possess the flexibility to adopt either tactic (e.g., Davies, 1978; Alcock and Houston, 1987; Alcock, 1997). In all cases, the maintenance of alternative mating tactics within a population is believed to reflect underlying variation in either environmental conditions or competitive abilities (Austad, 1984, Hazel et al., 1990; Greeff, 1998). However, although a large body of empirical work has accumulated, the types and nature of environmental variation responsible for the evolution of behavioral plasticity are not completely understood. The study of butterfly behavior has provided a useful tool for documenting the occurrence and ecological correlates of alternative mating tactics in animals (see Dennis, 1982; Wickman and Wiklund, 1983; Shreeve, 1984; Wickman, 1985, 1988; Alcock and O’Neill, 1986; Alcock, 1994; Hernandez ´ and Benson, 1998). Traditionally, the primary difference between individual mating tactics in this group has been related to mobility, that is, the extent to which males either perch or patrol to locate females. However, since perching behavior in butterflies is often accompanied by site defense (Dennis and Shreeve, 1988; Rutowski, 1991), species that exhibit behavioral plasticity invariably switch between territorial and nonterritorial behaviors (see examples as above). The correlates of these behavioral switches provide a framework for formulating adaptive hypotheses regarding the evolution of similar forms of plasticity in butterflies and animals generally. In most territorial systems, a proportion of males may be forced to adopt an alternative means of locating mates due to their inability to compete with superior males (Dawkins, 1980). This “best of a bad job” tactic has been observed in butterflies (e.g., Davies, 1978; Hernandez ´ and Benson, 1998), although the determinants of competitive superiority are not always clear (Austad et al., 1979; Stutt and Willmer, 1998). Outside of this prospect, the primary ecological correlates of conditional behavior of male butterflies are conspecific male density (Dennis, 1982; Alcock and O’Neill, 1986) and weather (Wickman, 1985, 1988). Males of several butterfly species freely switch between territorial perching and nonterritorial patrolling behavior


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depending on the density of conspecific males at the encounter site (Dennis, 1982; Alcock and O’Neill, 1986). This type of behavioral switch has generally been interpreted in terms of the economics of territory defense, with rising cost of defense eventually outweighing the reproductive benefits of this activity (Parker, 1978; Thornhill and Alcock, 1983; Alcock and O’Neill, 1986; Rutowski, 1991). Alternatively, male butterflies may switch between perching and patrolling depending on the prevailing thermal conditions (e.g., Dennis, 1982, 1987; Wickman and Wiklund, 1983; Shreeve, 1984; Wickman, 1985, 1988). Wickman (1985) suggested that perching behavior is the most efficient method of mate location at suboptimal temperatures for flight, evolved by many temperate butterfly species due to the restrictions of low temperatures on extended flight activity. However, the generality of this hypothesis is unclear, and therefore, the role of thermal constraints as an influence on the evolution of butterfly mate locating tactics is not fully understood. Here I aim to broaden our general understanding of behavioral plasticity in butterflies by investigating the consistency of male mate-locating behavior in the tropical species Hypolimnas bolina (L.) (Nymphalidae). Males of this species are noted for their territorial male mate-locating behavior (McCubbin, 1971; Rutowski, 1992). This behavior (described in detail later) is comparatively very similar to that of other butterfly species that defend perching sites to maximize their encounters with receptive females (see Davies, 1978; Wickman and Wiklund, 1983; Rosenberg and Enquist, 1991; Lederhouse et al., 1992; Lederhouse, 1993; Hernandez ´ and Benson, 1998). Moreover, throughout tropical Australia, male H. bolina are potentially subject to regimes of ecological variation (outlined specifically below) similar to those of some territorial temperate species that exhibit alternative mating tactics [e.g., Coenonympha pamphilus (Wickman, 1985) and Strymon melinus (Alcock and O’Neill, 1986)]. This species therefore provides an ideal tropical candidate for testing predictions regarding the occurrence of behavioral plasticity in butterflies. Mate-locating male H. bolina are subject to considerable daily and seasonal variations in the two variables that correlate primarily with behavioral plasticity in temperate species—population density and weather conditions. First, although normally present at low densities throughout north Queensland (see Rutowski, 1992; Kemp, 1998), numbers of this species at particular sites can increase markedly at times during the wet season [immediately after extended rainy or overcast weather patterns (D. J. Kemp, unpublished data)]. This population variation is qualitatively similar to that described for S. melinus, a predominantly territorial species that possesses a density-related switch to patrolling behavior (Alcock and O’Neill, 1986). The “territory economics” hypothesis predicts that a greater proportion of male


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H. bolina should either patrol (Alcock and O’Neill, 1986; Rutowski, 1991) or continue to perch nonaggressively (Alcock, 1985) when the male density at the encounter site reaches extreme levels (Emlen and Oring, 1977). Second, because males are active at encounter sites for an extended daily period [from 0800 to 1700 h (Rutowski, 1992)], and temperature/solar radiation changes considerably during this time, mate-locating males are potentially subject to a broad range of thermal conditions. If territoriality in this species is adopted at suboptimal temperatures, then this behavior should be limited to those times of day when males are thermally constrained. The “thermal constraint” hypothesis predicts that a greater proportion of males should patrol in the middle of the day when temperatures are least limiting and that the diel distribution of perching behavior should follow a “U” shape (as demonstrated by Wickman, 1985). This prediction is perhaps most likely to be fulfilled in summer months, since daily ambient temperatures at these times are certain to reach levels that permit extended patrolling behavior. However, in the tropics, the possibility that midday temperatures may reach levels that are too high for flight activity also needs to be considered. Under this scenario, males would again become inactive or perch in the middle of the day, and the predicted diel distribution of perchers would therefore approximate a “W” shape (Wickman, 1985). In this paper I closely examine the behavioral consistency of matelocating male H. bolina at a single site in tropical Australia, to evaluate the above predictions of the territory economics and thermal constraints hypotheses. This investigation encompasses two elements: (1) a seasonal-scale investigation, in which male density and behavior are observed over a 2-year period, and (2) a diel investigation, in which specific predictions of each hypothesis (outlined above) are evaluated, using changes in male density, behavior, and thermal conditions throughout the day. To maximize potential variation in thermal conditions and male density in diel censuses, males were observed throughout the entire day (0700–1800 h) over a several-week period when the male density at the site reached unusually high levels (indicated by seasonal censuses). Additional aspects of the mate-location behavior of H. bolina at this site, such as temporal activity patterns and favored perching sites, are dealt with in detail by Kemp and Rutowski (2001).

METHODS Mate-Locating Behavior of Male H. bolina Hypolimnas bolina is conspicuous due to its apparently aggressive male behavior (Rutowski, 1992). Males engaged in this behavior station


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themselves on the outermost leaves of overhanging trees and shrubs, or similar vantage points, and launch themselves at flying objects that pass within their visual range. Males are more likely to investigate objects that visually resemble conspecific females, and if a female is located, she is courted (Rutowski, 1992; Kemp, 1998). When not interacting with other butterflies, territorial male H. bolina spend almost all [up to 95% (D. J. Kemp, unpublished data)] of their time perching. Individuals are faithful to specific sites in the environment, such as small forest clearings or open tracks through forest, and typically do not change sites within a day (Rutowski, 1992). Site fidelity in this species may range up to several weeks (Rutowski, 1992). While perching, males do not tolerate the presence of conspecific males within their visual range (5–10 m). Disputes between territorial males take the form of stereotyped spiraling maneuvers, similar to those described for other territorial nymphalid species [e.g., Pararge aegeria (Davies, 1978; Wickman and Wiklund, 1983), Limenitis weidemeyerii (Rosenberg and Enquist, 1991), and Heliconius sara (Hernandez ´ and Benson, 1998)]. In H. bolina, these encounters also encompass a chase component, whereby the losing male is pursued up to 100 m from the disputed perching site (Kemp, 2000). These male encounters are potentially costly (Rosenberg and Enquist, 1991) with respect to lost mate-locating time, energy expenditure (Wickman and Wiklund, 1983), and the opportunity for damage to wing membranes (Hernandez ´ and Benson, 1998). Like other territorial species, perching site defense in male H. bolina is believed to lead to a higher rate of encounter of receptive females (Rutowski, 1992) and, hence, increased reproductive success (see Lederhouse, 1982; Wickman, 1985; Alcock and O’Neill, 1987).

Study Site This study was conducted at a site adjacent to the Freshwater Creek, near Cairns in Queensland, Australia (16◦ 530 S, 145◦ 450 E). Butterflies were observed along a 770-m-long discontinuous transect, which was established along an existing mown track of 5- to 15-m width at the site. Like the territory locations described previously for this species (McCubbin, 1971; Rutowski, 1992), several aspects of this site suggest that it would be a good place to locate receptive females. First, several larval foodplants of H. bolina, including Synedrella nodiflora (Asteraceae), grew along the margins of the track, and late-instar larvae and pupae were discovered on these plants (D. J. Kemp, unpublished data). This suggests that females would be eclosing in the vicinity of the transect, and virgins are the primary source of receptive females in this species (Rutowski, 1992). Second, the surveyed track separates the


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edge of a cultivated sugarcane field from riparian rainforest vegetation and, therefore, represents an open, continuous corridor through relatively dense vegetation. These types of corridors may serve as flight paths for receptive female butterflies (Rutowski, 1991, 1992; Lederhouse, 1993). Finally, male H. bolina search for females visually (Rutowski, 1992), although their effective visual range is limited to less than about 10 m. Since the width of much of the censused corridor falls within this range (Kemp and Rutowski, 2001), males perching in these areas would stand a greater chance of detecting passing females. These features are relatively localized to the region of surveyed track, and individual H. bolina (males or females) are rarely seen outside the immediate vicinity of this census route (D. J. Kemp, unpublished data). Classification of Male Behavior Qualitative observations of male behavior were made along the census route throughout September and October 1997. This was done to investigate the feasibility of using instantaneous scan sampling (Martin and Bateson, 1993) to measure the behavior of a population of male H. bolina and to define the categories for behavioral classification. Males were seen to exhibit behavior highly consistent with their documented territorial tactic (as above), but other males were also seen to patrol in a manner which could represent an alternative mate-locating tactic. The behavior of most males segregated clearly into these two categories, with clear differences between them in the amount of time spent flying, flight speed and action, and the extent of aggression toward conspecifics (see below). These preliminary observations indicated that it was feasible, in almost all instances, to classify the behavior of individual males in less than 30–40 s. Similar sampling methodologies have been used to quantify the mate-locating behavior of male butterflies (e.g., see Douwes, 1975; Shreeve, 1984; Wickman, 1985, 1988; Rutowski et al., 1997; Stutt and Willmer, 1998). On the basis of preliminary observations, four discrete behavioral categories were defined—perching, patrolling, feeding, and indeterminate. 1. Perching—males (a) return on consecutive occasions to perch within a localized area (i.e., a section of the transect smaller than 20 m in length), (b) respond to flying intruders or pebbles tossed in their visual range by flying out to investigate, (c) perch facing outward on exposed, overhanging vegetation when not investigating or interacting with a passing object, (d) move around their site using fast, direct, or gliding (slow wingbeat) flight, and (e) always respond aggressively (with stereotyped dispute behavior—as described above) to the presence of conspecific males within their visual range.


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2. Patrolling—Males (a) remain in flight for most or all of their time, even when not interacting with flying objects, (b) fly with a distinctive slow, fluttering (fast wingbeat) flying action, (c) fly mostly in and around the foliage of fringing vegetation, (d) show no tendency to remain in or return to a localized area of the transect, and (e) investigate flying conspecifics within their visual range but do not respond aggressively to the presence of conspecific males. 3. Feeding—Males feed from flowers or perch on moist soil with proboscis extended (mudpuddling). Some flight may be associated with this behavior (to switch from flower to flower or to move along the ground), but males do not respond to moving objects within their visual range and do not act aggressively toward conspecifics [which appears to be the norm in territorial butterflies (see Lederhouse, 1982; Rosenberg, 1989)]. 4. Indeterminate—This category was reserved for individuals seen too briefly to make an accurate behavioral assessment, and those exhibiting behaviors not in complete compliance with the above classifications.

Quantification of Male Behavior Starting November 1997, the census route was censused fortnightly, but more frequently at the beginning (August–September) and end (April–June) of the reproductive season. In these “seasonal-scale” censuses, the transect was censused only during clear sunny conditions, with light wind (less than approximately 5 ms−1 ), between 1000 and 1300 h. In the diel investigation, censuses were conducted hourly, from 0700 to 1800 h local time, on 12 days between 21 January and 16 February 1998. This time period was chosen because male density at the site reached extremely high levels (refer to Fig. 1). Also, weather conditions at this time were favorable for testing predictions of the thermal constraint hypothesis (refer to introduction). In all censuses, the route was walked at a roughly constant speed (1– 2 ms−1 ), and the behavior of all individuals seen along the track classified (with respect to the defined behavioral categories). Individuals were watched for as long as possible, sometimes using binoculars, to make an accurate behavioral assessment. However, rarely was it necessary to observe an individual for longer than 30–40 s to classify his behavior. The behavior of males seen for less than 5–6 s was always classified as indeterminate. In most cases, the census route took 25–35 min to traverse. Two additional aspects of male behavior were recorded during diel censuses. First, to investigate male thermal status, the wing position of all


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Fig. 1. The abundance of males along the census route from November 1997 to October 1999. Diel censuses were conducted during the shaded period (January–February 1998).

individuals seen perching continuously for more than 20–30 s was classified as either open or closed. Males of this species are dorsal baskers and will perch with their wings horizontally fully open when their thoracic temperature falls below approximately 34.5◦ C (D. J. Kemp, unpublished data). In contrast, actively mate locating males perching with wings closed have thoracic temperatures ranging from 34.5 to 39.0◦ C, across varying ambient temperatures (D. J. Kemp, unpublished data). Hence, although the actual thoracic temperature of all males could not be measured, some indication of this parameter was afforded through the posture of perched males. Second, all pairwise male interactions seen in their entirety were timed to the nearest second using a digital stopwatch. Interaction duration was defined as the period of time between when the two males came to within 0.1 m of each other and to when they either spontaneously separated or when the winner broke off pursuit and returned to the perching site. Interactions between males and conspecific females were also noted but not timed. Environmental Data The method of collection of environmental data is described in detail by Kemp and Rutowski (2001) and, hence, is treated only briefly here. Ambient temperature (Ta ) was measured immediately prior to and following


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all censuses, using a shaded mercury thermometer situated in underneath vegetation adjacent to the transect. In addition, during diel censuses only, blackbody (Tb ) temperature was measured in an open clearing adjacent to the transect. Ambient temperature, Ta , was subsequently subtracted from Tb to yield blackbody excess (Te ) values. Blackbody excess temperatures represent the potential for elevation of body temperatures above ambient and provide an indication of the basking potential of a butterfly (refer to Pivnick and McNeil, 1987). Statistical Analyses Kolmogorov–Smirnov goodness-of-fit tests (Sokal and Rohlf, 1995) were used to screen all data sets for normality, with nonnormal data either transformed or analyzed using nonparametric tests. Transformations are stated where used. When comparing the proportional incidence of specific behaviors with environmental variables, significance levels were adjusted using the Bonferroni method and an experimentwise error rate of α = 0.05 (Sokal and Rohlf, 1995). Two tailed probabilities are employed in all tests, and sample means throughtout this paper are accompanied by their standard errors. RESULTS Seasonal Male Density Male density at the study site varied considerably throughout the 2year sampling period (Fig. 1). Although generally fewer than 15 males were counted in most months, counts almost trebled in late January/early February 1998 (the period when diel censuses were conducted; see Fig. 1). There was some evidence of seasonality in male counts, with numbers generally lowest in the months of June–September. The reproductive seasonality of H. bolina at this site, and other sites in North Queensland, is dealt with in detail by Kemp (2001). Male Behavior As suggested by the preliminary observations, most males observed at this site (88%) exhibited consistent behavior that fully satisfied the criteria of either or perching or patrolling (as defined). Of the total 2301 males observed, 1933 were classified as perching, 90 as patrolling, 34 as feeding, and 244 as indeterminate. Perching was the primary behavior of H. bolina across


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Fig. 2. The behavior of males throughout the day as indicated by the proportional abundance of each behavioral class (columns) and the proportion of basking males (points and solid line). Columns do not reach 100% at any hour due to the presence of males along the transect of indeterminate behavior.

the year, with over 88% of males counted in all months classified as perchers (except for June 1998, when only one perching and two indeterminate males were counted). Perching was also favored throughout the course of a day, with an average of over 75% of all males engaged in this behavior until late afternoon (Fig. 2). In 476 cases when a perching butterfly was seen to detect a conspecific male, an aggressive interaction ensued on all occasions. These interactions led to one male being pursued some distance away from the territory area (see Rutowski 1992; Kemp, 2000). The perfect association between perching and aggression (Yates-corrected χ12 = 474, P < 0.0001), shows that all males observed perching were most likely engaged in site defense. As with those individuals seen during preliminary observations, patrolling males flew almost constantly, often low to the ground (0.1–0.5 m), along the shaded edges of foliage or through the undergrowth of the cane plantation. In diel censuses, males were more likely to patrol and feed later in the day (Fig. 2); however, the proportional incidence of these two behaviors was not closely related (Spearman rank correlation: rs = 0.11, n = 75, P = 0.37). Also, butterflies seen previously patrolling during these censuses were observed to pause and feed on only three occasions. In contrast to perchers, males classified as patrollers were nonaggressive. In the 19 observed interactions involving patrolling and perching males (where each could be


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identified), patrollers always assumed the submissive role, immediately displaying the typical submissive flap-glide flight described by Rutowski (1992). Interactions involving one patrolling male lasted an average of 6.32 ± 1.18 s (N = 19), which is significantly less than the average duration of 19.67 ± 2.29 s for all other 457 observed male–male interactions (independent t test on log-transformed data: t = 2.66, df = 474, P < 0.01). No patrolling male was observed to engage in an escalated contest (lasting >25 s), and the only observed meeting between two patrolling males lasted a fleeting 1.12 s. Of 59 observed male–female interactions, 41 involved males classified as perching (2.1% of perching counts), whereas only one involved a male seen previously patrolling (1.1% of patrolling counts). All interactions consisted of failed courtship attempts, with females curtailing male approaches using stereotyped rejection behaviors (Kemp, 1998). Most butterflies classified as having indeterminate behavior were seen too briefly to make an accurate assessment. These included individuals that were being chased at high speed through the site by a territorial male and those involved in ascending aerial “harassment” of nonreceptive females (see Kemp, 1998). In addition, 20–30% of males classed as having indeterminate behavior were seen flying with fast, direct flight 15–25 m above the ground, possibly migrating to and from the site. The remaining 10–15% of males in this behavioral category were seen apparently resting on foliage adjacent to the transect. These individuals did not respond to pebbles thrown within their visual range and were seen mainly in late afternoon censuses (Fig. 2). When startled, these “resting” males left the site with fast flight and were not seen to return.

Male Density and Behavior Although male density varied considerably throughout seasonal censuses (Fig. 1), almost all males in all months of the 2-year period were classified as perchers (94.2% of all counts). In addition, there was no evidence of systematic variation in male behavior relating to seasonal fluctuations in density at the site (Spearman rank correlation between male density and proportion of perching males per census: rs = −0.126, n = 112, P = 0.185 ). Diel censuses were conducted during a period in which the maximum male density reached extreme levels (indicated by Fig. 1). At least some males were present along the transect at all times of the day, but numbers tended to peak markedly in the midmorning (refer to Kemp and Rutowski, 2001). A maximum number of 43 males was counted, which represents an average density of 1 male per 17.9 m. However, individuals were not uniformly distributed along the transect, and therefore “local” densities were much higher


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Fig. 3. The relationship between the male density along the transect and the proportional incidence of perching (open circles) and patrolling (open triangles).

(refer to Kemp and Rutowski, 2001). Extremely high male presence at the site was also indicated by the nature of some observed male interactions, with 18 interactions involving three separate individuals, 4 interactions involving four males, and 1 interaction involving five spiraling individuals. As shown in Fig. 3, the proportion of perching males increased significantly with increasing male density throughout the day (Spearman rank correlation: rs = 0.27, n = 75, P < 0.05 ). Also, at the times of day when male density was highest, more aggressive interactions were observed on average per male (Pearson’s product–moment correlation between the number of observed interaction and the number of male counts, averaged for each hour: r = 0.88, n = 10, P < 0.01 ). Hence, the relative abundance of territorial perchers increased with increasing male density, a result that clearly contradicts the prediction of the territory economics hypothesis. The proportion of patrolling males was also independent of the number of occupied territories (Spearman rank correlation: rs = −0.12, n = 75, P = 0.29 ). This would suggest that, if patrolling is a tactic of mate location, it was not a best-of-a-bad-job tactic employed by males unable to gain a territory. Environmental Influences on Male Behavior Because male behavior was highly seasonally consistent (see above), the influence of weather conditions on behavior is investigated here primarily


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Fig. 4. The mean daily thermal conditions as indicated by values of Ta (open squares, solid line) and Te (filled squares, dashed line). The error bars represent standard errors of each mean, and the number of sampling occasions is given above each Ta point. Fitted lines are weighted least-squares approximations calculated by the STATISTICA computer program.

with respect to diel censuses. The weather experienced during these censuses was generally clear and hot, with the daily maximum Ta ranging from 29.5 to 32.5◦ C. Values of both Ta and Te rose to daily maxima at 1200 to 1300 h and declined thereafter (Fig. 4). Te declined more substantially after midday than Ta due to the presence of afternoon cloud cover. A more detailed report on the weather conditions over the time period when diel censuses were undertaken is given by Kemp and Rutowski (2001). Proportionally more perching butterflies basked (that is, perched with open wings) in early morning and late afternoon transects (Fig. 2). No perching butterfly was observed basking between 1200 and 1500 h. The proportion of basking males was thus tightly correlated with both Ta (Spearman rank correlation: rs = −0.48, n = 70, P < 0.0001 ) and Te (rs = −0.41, n = 70, P < 0.001 ). Basking behavior clearly followed a “U”-shaped distribution through the day (Fig. 2), suggesting that males perched with open wings in a direct response to low thermal conditions. Relationships between temperature variables and specific male behaviors are summarized in Table I. Although patrolling was correlated with Ta , relatively few males patrolled at the hottest, brightest time of day (on average from 1200 to 1300 h.; Fig. 4). The mean proportion of patrolling males reached a peak 2 h later, when both Ta and Te had fallen slightly.


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Kemp Table I. Spearman Rank Correlations Between Behavioral Classes and Environmental Variablesa Behavior

Ambient temperature (◦ C)

Blackbody excess (◦ C)

Perching Patrolling Feeding

0.02 0.44 −0.23

0.43 −0.01 −0.42

a Boldface

correlations are significant at the Bonferroni-adjusted level of significance (α = 0.008); n = 75 for all comparisons.

Figure 5 shows the diel distribution of perching and patrolling behaviors in relation to thermal maxima on four individual days during this study. Regardless of when Ta and Te peaked, the proportion of patrollers always increased after 1300 h and always reached a maximum later in the day than the environmental maxima. Also, on none of these days (nor on average; Fig. 2) did the diel proportion of perching males follow the U-shaped distribution shown by the distribution of basking butterflies or a “W” shape, which may also be predicted by the thermal constraint hypothesis (refer to the Introduction).

Fig. 5. The diel distribution of perching (open circles) and patrolling (filled squares) for 4 days of this study chosen on the basis of different environmental maxima. The timing of environmental maxima are indicated by open (Te ) and filled (Ta ) arrows. Days shown are (a) January 22, Tamax = 30.5◦ C; (b) January 23, Tamax = 31.5◦ C; (c) January 27, Tamax = 32.5◦ C; and (d) February 3, Tamax = 31.0◦ C.


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DISCUSSION Most male H. bolina observed in this study behaved in a manner consistent with the territorial mate-locating behavior described previously for this species (McCubbin, 1971; Rutowski, 1992). However, as first indicated in the pilot study, a small proportion of individuals also patrolled nonaggressively throughout the vegetative undergrowth of the study site. The behavior of males within each category was highly consistent, such that two primary behaviors were evident—aggressive perching and nonaggressive patrolling. This apparent behavioral dichotomy is superficially similar to that shown by butterfly species that exhibit alternative mating tactics, for example, P. aegeria (Wickman and Wiklund, 1983), C. pamphilus (Wickman, 1985), Lasiommata megera (Dennis, 1982; Wickman, 1988), S. melinus (Alcock and O’Neill, 1986), and Chlosyne californica (Alcock, 1994). Furthermore, at least one patrolling male H. bolina was seen to attempt courtship with a conspecific female, which shows that males engaged in patrolling behavior will court and mate if given the opportunity. These limited data are consistent with the hypothesis that patrolling in this species is an alternative means of locating receptive females. However, due to the lack of information regarding the reproductive significance of this behavior, this hypothesis cannot be fully explored here. Rather, I discuss the occurrence of patrolling behavior, in concert with the occurrence of perching behavior, to the extent that it bears potential relevance to the proposed behavioral plasticity hypotheses. The territory economics hypothesis contends that males should abandon territorial behavior under high male densities (Parker, 1978; Alcock and O’Neill, 1986; Rutowski, 1991). Clearly, this hypothesis is not applicable here, since proportionally more males were observed perching when the male density was highest, and perching was closely related to site defense. Furthermore, aggressive interactions between individual males occurred more frequently under high male density, which would be expected if males did not abandon site defense at these times. This finding is notable, since males were present at extraordinarily high densities at the height of perching activity (compare with the numbers recorded by Rutowski, 1992; Kemp, 2000), and behavioral variation is expected under these conditions (Emlen and Oring, 1977). Animals are expected to abandon territorial behavior when the costs of site defense outweigh the potential reproductive benefits of this activity (Thornhill and Alcock, 1983). In male H. bolina, as in other territorial butterflies, the costs of site defense will relate to the expenditure of time and energy in expelling intruding conspecific males and the associated risk of physical injury (Hernandez ´ and Benson, 1998). These costs should increase proportionally with increasing male density at the encouter site, since conspecific


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males will encroach on each other’s territories more frequently. In a study on the mate-locating behavior of the territorial hilltopping species S. melinus, Alcock and O’Neill (1986) found that more males switched to patrolling when male density on the hilltop reached a critical level. Although a similar type of behavioral switch was not detected here in H. bolina, individuals may have modified their behavior in more subtle ways under high male densities. This study was not sensitive to changes in site fidelity, site turnover, and territory size, which have been shown to alter with increasing male density in butterflies (Lederhouse et al., 1992). The second hypothesis addressed here was that perching tactics are used by male butterflies only when flight activity is constrained by suboptimal thermal conditions (Wickman, 1985). This hypothesis contends that when not thermally constrained, males should patrol around the encounter site in search of receptive females. Although patrolling behavior (as defined) in H. bolina was significantly correlated with Ta (but not Te ), males did not act in full accordance with the predictions of this hypothesis. First, individuals did not patrol during the warmest time of each day, either on average (Fig. 2) or on specific days (Fig. 5). If patrolling is constrained by low morning temperatures, then individuals should switch to this behavior as soon as thermal conditions permit [that is, when the fitnesses of each tactic equalize (Dominey, 1984; Gross, 1996)]. The occurrence and timing of patrolling behavior observed in H. bolina, relative to the occurrence of perching, did not appear to support this prediction. Second, the diel distribution of perching male H. bolina did not follow either the “U” or the “W” shape, which is expected if patrolling behavior is thermally constrained. In a study on mate-locating behavior in the temperate species C. pamphilus, Wickman (1985) discovered that male behavior changed throughout the day in a manner related to temperature and that the diel distribution of perching approximated a U shape. In H. bolina, a similar diel distribution was approximated only by the daily incidence of basking behavior, with males basking in early and late censuses (Fig. 2). Since male H. bolina bask dorsally to increase their thoracic temperature [above approximately 34.5◦ C (D. J. Kemp, unpublished data)], this shows that males were indeed thermally constrained by cooler temperatures in the early morning and late afternoon. Nevertheless, perching remained as the primary tactic outside this period, when individuals were presumably closer to their optimum operating temperature. It is unlikely that midday temperatures were too high for patrolling activity, since the maximum Ta (32.5◦ C) was not exceptionally high for this tropical region [and only 1.2◦ C higher than that observed by Wickman (1985)], but also because convective cooling may reduce the temperature of a flying butterfly rather than increase it (Stutt and Willmer, 1998). Therefore, it seems likely that butterfly flight was not


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thermally constrained at all times when individuals were perching in large numbers. This study therefore shows that perching in H. bolina is not reserved for times when temperatures are suboptimal, as appears to be the case in some temperate species (Wickman, 1985). This result, coupled with previous reports on the mating behavior of H. bolina (McCubbin, 1971; Rutowski, 1992), and its related congener H. missipus (Stride, 1956), suggest that intercepting passing objects from a perch may be the best method of locating receptive female H. bolina. Nevertheless, at the studied encounter site, a very small number of males were seen behaving in a manner at least superficially reminiscent of the patrolling tactics of other mate-locating male butterflies. The significance of this behavior, with respect to its potential role in mate location, is currently being investigated. ACKNOWLEDGMENTS John Alcock, Rhondda Jones, and Ronald Rutowski all helped me to improve the clarity of an early version of the manuscript. Alastair Stutt also provided some useful suggestions. This research was supported by an Australian postgraduate research award. REFERENCES Alcock, J. (1985). Hilltopping in the nymphalid butterfly Chlosyne californica (Lepidoptera). Am. Midl. Nat. 113: 69–75. Alcock, J. (1994). Alternative mate-locating tactics in Chlosyne californica (Lepidoptera, Nymphalidae). Ethology 97: 103–118. Alcock, J. (1997). Competition from large males and the alternative mating tactics of small males of Dawson’s burrowing bee (Amegilla dawsoni) (Apidae, Apinae, Anthophorini). J. Insect Behav. 10: 99–113. Alcock, J., and Houston, T. F. (1987). Resource defense and alternative mating tactics in the banksia bee, Hylaeus alcyoneus (Erichson). Ethology 76: 177–188. Alcock, J., and O’Neill, K. M. (1986). Density-dependent mating tactics in the grey hairstreak, Strymon melinus (Lepidoptera: Lycaenidae). J. Zool. 209: 105–113. Alcock, J., and O’Neil, K. M. (1987). Territory preferences and intensity of competition in the grey hairstreak Strymon melinus (Lepidoptera: Lycaenidae) and the tarantula hawk wasp Hemipepsis ustulata (Hymenoptera: Pompilidae). Am. Midl. Nat. 118: 128– 138. Arak, A. (1984). Sneaky breeders. In Barnard, C. J. (ed.), Producers and Scroungers, Croom Helm, London, pp. 154–194. Austad, S. (1984). Alternative reproductive behaviors: The maintenance of behavioral variability. Am. Zool. 24: 309–319. Austad, S. N., Jones, W. T., and Waser, P. M. (1979). Territorial defence in speckled wood butterflies: Why does the resident always win? Anim. Behav. 27: 960–961. Clarke, C., and Sheppard, P. M. (1975). The genetics of the mimetic butterfly Hypolimnas bolina. Phil. Trans. R. Soc. Lond. B 272: 229–265.


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