Effects of sex, body size, temperature, and location on the antipredator tactics of free-ranging gartersnakes (Thamnophis sirtalis, Colubridae)

Gartersnakes (Thamnophis sirtalis parietalis) in southern Manitoba are subject to intense predation (primarily by crows) duringtheir spring breeding season. The huge numbers of snakes providea unique opportunity to quantify behavioral traits. We simulatedpredator attacks by "pecking" more than 500 free-ranging snakes,to explore the determinants of snake response. Snakes respondedto a human finger in the same way as they did to a more realisticstimulus (a model crow). A snake's response to attack dependedon several factors, which interacted in complex ways. The primaryinfluences on response were body temperature (warmer snakes tended to flee, whereas colder snakes remained cryptic or flattenedand/or gaped and struck) and sex (males were more likely toflee). Responses also depended on microhabitat (i.e., insidethe winter den versus in adjacent grassland) and on the snake'sprior activity (e.g., courting snakes often ignored our closeapproach). These factors interacted in significant ways; for example, snakes outside the den were smaller and warmer thanthose inside, male snakes were smaller and warmer than females,and mean body temperatures were higher in larger snakes withineach sex. Thus, a snake's body size and its location affectedits defensive response indirectly (via their influence on bodytemperature). Our results differ from those of previous studiesand suggest that antipredator responses in these animals dependin a flexible and complex way upon biotic and abiotic variables.Interactions among these variables also must be consideredbefore we can identify underlying causal processes.     

Size-dependent predation by snakes: selective foraging or differential prey vulnerability?

I staged replicate encounters between unrestrained lizards andsnakes in outdoor enclosures to examine size-dependent predationwithin the common garden skink (Lampropholis guichenoti). Yellow-facedwhip snakes (Demansia psammophis) forage widely for activeprey and most often consumed large skinks, whereas death adders(Acanthophis antarcticus) ambush active prey and most oftenconsumed small skinks. Small-eyed snakes (Rhinoplocephalusnigrescens) forage widely for inactive prey and consumed bothsmall and large skinks equally often. Differential predationmay reflect active choice by the predator, differential preyvulnerability, or both. To test for active choice, I presentedforaging snakes with an inert small lizard versus an inertlarge lizard. They did not actively select lizards of a particularbody size. To test for differential prey vulnerability, I quantifiedvariation between small and large lizards in behavior thatis important for determining the outcome of predator—prey interactions. Snakes did not differentiate between integumentarychemicals from small and large lizards. Large lizards tendto flee from approaching predators, thereby eliciting attackby the visually oriented whip snakes. Small lizards were moremobile than large lizards and therefore more likely to passby sedentary death adders. Additionally, small skinks were more effectively lured by this sit-and-wait species and less likelyto avoid its first capture attempt. In contrast, overnightretreat site selection (not body size) determined a lizard'schances of being detected by small-eyed snakes. Patterns ofsize-dependent predation by elapid snakes may arise not becauseof active choice but as a function of species-specific predatortactics and prey behavior.     

Why do kangaroo rats (Dipodomys spectabilis) footdrum at snakes?

We examined alternative hypotheses for the benefits of footdrummingin the presence of snakes by the banner-tailed kangaroo rat,Dipodomys spectabitis, by testing whether the target of thesignal includes conspecifics, the predator or both. Footdrummingrecorded in the field revealed that rats altered their footdrummingsignatures when drumming at snakes. In playback tests, however,neighbors failed to show any measurable change in behavior tobroadcasts of the snake drumming pattern, but mothers footdrummedsignificantly more than nonmothen in the presence of a tetheredsnake. Gopher snakes, Pituophis melanolsucus affinis, respondedto footdrumming vibrations created by a mechanical thumper.Nonhungry snakes avoided footdrumming, while hungry snakes approachedthe seismic footdrumming. Snakes decreased stalking rates asfootdrumming increased, but they spent more time stalking drummingthan nondnimming rats. We conclude that D. spectabilis footdrumsin individual defense and in parental care, rather than to warnadult conspecfics. Footdrumming deters pursuit by informingthe snake that the rat is alert and the chances of predationare low. We find little evidence that footdrumming startles,confuses, or harasses the snake. Hungry gopher snakes, however,may locate prey by eavesdropping on territorial footdrumming     

Effects of food supplementation on the physiological ecology of female Western diamond-backed rattlesnakes (<Emphasis Type="Italic">Crotalus atrox</Emphasis>)

Food availability is an important factor in the life histories of organisms because it is often limiting and thus can affect growth, mass change, reproduction, and behaviors such as thermoregulation, locomotion, and mating. Experimental studies in natural settings allow researchers to examine the effects of food on these parameters while animals are free to behave naturally. The wide variation among organisms in energy demands and among environmental food resources suggest that responses to changes in food availability may vary among organisms. Since most supplemental feeding field experiments have been conducted on species with high energy demands, we conducted a supplemental feeding study on free-ranging, female Western diamond-backed rattlesnakes (Crotalus atrox), a species with low energy demands and infrequent reproductive investment. Snakes were offered thawed rodents 1–4 times per week. Over two active seasons, we collected data on surface activity, home range size, growth, mass change, and reproduction of supplementally fed and control snakes. Fed and control snakes did not differ in surface activity levels (proportion of time encountered above versus below ground) or home range size. Fed snakes grew and gained mass faster, and had a dramatically higher occurrence of reproduction than control snakes. Also, fed snakes were in better body condition following reproduction than snakes that were not fed. However, litter characteristics such as offspring number and size were not increased by feeding, suggesting that these characteristics may be fixed. These data experimentally demonstrate that food availability can directly impact some life history traits (i.e., growth and reproduction for C. atrox), but not others (i.e., surface activity and home range size for C. atrox). The relationship between food availability and life history traits is affected in a complex way by ecological traits and physiological constraints, and thus interspecific variation in this relationship is likely to be high.     

Snake predation on North American bird nests: culprits,patterns and future directions

Predation is the leading cause of nest failure for most birds. Thus, for ornithologists interested in the causes and consequences of variation in nest success, knowing the identity and understanding the behavior of dominant nest predators is likely to be important. Video documentation of nests has shown that snakes are frequent predators. Here we reviewed 53 North American studies that used nest cameras and used these data to identify broad patterns in snake predation. Snakes accounted for 26% (range: 0–90%) of recorded predation events, with values exceeding 35% in a third of studies. Snakes were more frequent nest predators at lower latitudes and less frequent in forested habitat relative to other nest predators. Although 12 species of snakes have been identified as nest predators, ratsnakes Elaphe obsoleta, corn snakes E. guttata and fox snakes E. vulpina were the most frequent, accounting for > 70% of all recorded nest predation events by snakes and have been documented preying on nests in 30–65% of studies conducted within their geographic ranges. Endotherm‐specialist snakes (Elaphe and Pituophis genera) were more likely to depredate nests in forests and the canopy relative to other snakes, due to their affinity for edge habitat. Predation by only ratsnakes and corn snakes was predominantly nocturnal and only ratsnakes were more likely to prey on nests during the nestling stage. Snakes were not identified to species in over 30% of predation events, underlining the need for more complete reporting of results. A review of research to date suggests the best approach to investigating factors that bring snakes and nests into contact involves combining nesting studies with radio tracking of locally important snake nest predators.     

Food resources influence spatial ecology, habitat selection, and foraging behavior in an ambush-hunting snake (Viperidae: Bothrops asper): an experimental study

Prey availability affects many aspects of predators' life history and is considered a primary factor influencing individuals' decisions regarding spatial ecology and behavior, but few experimental data are currently available. Snakes may represent ideal model organisms relative to other animal groups for addressing such resource dependency, due to a presumably more direct link between food resources and many aspects of behavior and natural history. We experimentally investigated the relationship between food intake and spatial behavior in a population of the snake Bothrops asper in a Costa Rican lowland rainforest. Six adult snakes were allowed to forage naturally while six were offered supplemental food in the field, with both groups monitored using radiotelemetry. Mean home range size did not differ between groups presumably due to small sample size, but supplementally fed snakes demonstrated altered patterns of macro- and microhabitat selection, shorter and less frequent movements, and increased mass acquisition. Fed snakes also devoted less time to foraging efforts, instead more frequently remaining inactive and utilizing shelter. Because snakes were always fed in situ and not at designated feeding stations, observed shifts in habitat selection are not explained by animals simply moving to areas of higher food availability. Rather, B. asper may have moved to swamps in order to feed on amphibians when necessary, but remained in preferred forest habitat when food was otherwise abundant. The strong behavioral and spatiotemporal responses of snakes in this population may have been influenced by an overall scarcity of mammalian prey during the study period.     

Snake bioacoustics: toward a richer understanding of the behavioral ecology of snakes

Snakes are frequently described in both popular and technical literature as either deaf or able to perceive only groundborne vibrations. Physiological studies have shown that snakes are actually most sensitive to airborne vibrations. Snakes are able to detect both airborne and groundborne vibrations using their body surface (termed somatic hearing) as well as from their inner ears. The central auditory pathways for these two modes of "hearing" remain unknown. Recent experimental evidence has shown that snakes can respond behaviorally to both airborne and groundborne vibrations. The ability of snakes to contextualize the sounds and respond with consistent predatory or defensive behaviors suggests that auditory stimuli may play a larger role in the behavioral ecology of snakes than was previously realized. Snakes produce sounds in a variety of ways, and there appear to be multiple acoustic Batesian mimicry complexes among snakes. Analyses of the proclivity for sound production and the acoustics of the sounds produced within a habitat or phylogeny specific context may provide insights into the behavioral ecology of snakes. The relatively low information content in the sounds produced by snakes suggests that these sounds are not suitable for intraspecific communication. Nevertheless, given the diversity of habitats in which snakes are found, and their dual auditory pathways, some form of intraspecific acoustic communication may exist in some species.     

Why do Juvenile Chinese Pit‐Vipers (Gloydius shedaoensis) Select Arboreal Ambush Sites?

Ontogenetic shifts in habitat use are widespread, especially in ectothermic taxa in which juveniles may be an order of magnitude smaller than large adult conspecifics. The factors that generate such habitat shifts are generally obscure, but we studied an unusual system that allowed us to compare consequences of habitat selection between adults and juveniles. Pit‐vipers (Gloydius shedaoensis) on a small island in north‐eastern China feed almost entirely on seasonally migrating birds. During the spring bird‐migration period, individual snakes consistently re‐used either arboreal or terrestrial ambush sites. Snakes in trees were smaller (and more philopatric) than snakes on the ground. This ontogenetic shift in habitat use may reflect the difficulty of capturing birds on the ground, especially by small snakes. In laboratory trials, large (adult) pit‐vipers struck faster, further and more accurately than did small (juvenile) snakes. In experiments with free‐ranging snakes, the proportion of strikes hitting the bird was lower for juveniles than for adults, and lower for terrestrial snakes than for arboreal snakes. Additionally, adult snakes generally seized the bird by the head whereas juveniles frequently struck the body or wings (and thus, obtained a less secure grip). Arboreal ambush sites may facilitate prey capture not only because they give access to smaller birds but also because they render the bird's location more predictable and, hence, enable the snake to position itself optimally prior to the prey's arrival. Because juvenile pit‐vipers are less capable strikers, and are small relative to available prey items, they may benefit from the greater ease of prey capture from branches. Thus, the ontogenetic shift in habitat selection within this species may be because of ontogenetic shifts in the vipers’ ability to capture and ingest large, mobile prey.     

Husbandry of the little file snake,Acrochordus granulatus

Aquatic snakes of the family Acrochordidae are unusual in terms of appearance, biology, and natural history. In spite of many attractive and fascinating features, there are few zoological exhibits of acrochordid snakes, and as a result many aspects of their husbandry are poorly understood. The present paper summarizes aspects of acrochordid biology related to health and welfare of captive snakes, with emphasis on the little file snake, Acrochordus granulatus. Several key points emerge having crucial relevance to successful husbandry. (1) File snakes are sensitive to low temperatures and to rapid thermal change. Captive snakes do well when maintained at water temperatures of 27–30°C and will not thrive if water temperatures are below 25°C. (2) File snakes can be kept in either fresh water or seawater. Snakes in sea or brackish water dehydrate, however, and must be allowed to drink fresh water periodically. If snakes from marine populations are maintained in saline water, 60–70% seawater is recommended. In all cases, water should be filtered or changed periodically to maintain quality. (3) File snakes feed almost exclusively on fishes which are usually captured in body coils. Snakes are more inclined to feed well if live prey are offered in shallow water where they are more easily captured. (4) File snakes are nocturnal and prefer quiescent seclusion within darkened refugia during daylight hours. Providing snakes with refugia such as sections of PVC pipe (which simulate burrows) helps reduce stress and improves the chances of snakes feeding regularly. (5) Snakes tend to burrow, and they locomote by crawling as well as by swimming. Use of sharp or rough materials in aquaria should be avoided because of possible skin abrasion which increases permeability and provides sites for bacterial infection. © 1996 Wiley-Liss, Inc.     

Spatial ecology of arboreal snakes (Hoplocephalus stephensii,Elapidae) in an eastern Australian forest

Abstract Stephens' Banded Snakes (Hoplocephalus stephensii Krefft 1869) are large (to 1 m), highly arboreal elapid snakes, restricted to mesic forested areas along the eastern coast of Australia. Radiotelemetric monitoring of 16 individuals at Whian Whian State Forest in north‐eastern New South Wales over 25 months provided the first data on spatial ecology of this threatened taxon. Two major influences on movements by Stephens' Banded Snakes were identified: the distribution of large hollow‐bearing trees, and the avoidance of conspecifics. Radiotracked snakes were sedentary inside tree hollows for extended periods (mean = 8 days) during their active season, interrupted by occasional long (mean = 124 m) nocturnal movements to another shelter tree. Snakes travelled on the ground rather than within the canopy, and thus were potentially exposed to terrestrial predators. Although the home ranges of the radiotracked snakes overlapped substantially (mean = 27%), simultaneous occupancy of ‘shared’ shelter trees was less common than expected by chance. Hence, we conclude that adult Stephens' Banded Snakes generally avoid the presence of conspecifics. Snakes used from five to 30 shelter trees and home ranges of male snakes were larger than those of females (mean = 20.2 vs 5.4 ha). The large spatial scale of these movements, and limited overlap among individuals, means that a viable population of this taxon requires a large area of contiguous forest. This requirement may explain why the species has not persisted in small forest fragments.     

Development of the renal sexual segment in immature snakes: effect of sex steroid hormones

The renal sexual segment (RSS) of immature Northern and Diamondback Water Snakes and Red-Sided Garter Snakes exhibited varying responses to testosterone or 17beta-estradiol. In both male and female water snakes, kidney mass was not a reliable indicator of hormone treatment, whereas tubule diameter, epithelial height and number of sexual granules responded to hormone treatment. In male water snakes, either hormone initiated granule development by day 16; by day 23, only testosterone increased granule density. Female water snakes receiving either hormone exhibited a small number of granules by day 16; by day 23, granules increased only in Diamondback Water Snakes receiving testosterone. Hormones did not initiate RSS hypertrophy in female Red-Sided Garter Snakes. Tubule diameter and epithelial height of testosterone-treated males exhibited significant hypertrophy, while 17beta-estradiol initiated significant increases in tubule diameter. Garter snakes initiated sexual granule development in response to hormone treatment with males exhibiting a greater response than females and testosterone stimulating a greater response than 17beta-estradiol. Sex steroids appear to mimic sexual maturity in immature snakes initiating RSS development. Whereas the RSS of adult males respond to testosterone, our data suggest specific changes in the RSS of females during maturation effectively negates the effect of 17beta-estradiol evident in immature female RSS.     

Variation in size of cover used by watersnakes (Nerodia): Do smaller snakes use smaller rocks?

Snakes often seek shelter under rocks or other physical cover objects, which provide protection from both predators and unfavourable weather and also may be used actively in thermoregulatory behaviour. Presumably, snakes are limited, by their own body size, to some minimum rock size, so that we should expect a positive relationship between a snake's size and the size of the rock it uses. I tested this hypothesis for the watersnake, Nerodia sipedon, along a creek in a peri-urban setting in southern Ontario, Canada. Although larger snakes tended to be found under larger rocks, the relationship was weak. A composite measure of rock size from PCA was not markedly better than simple rock length. Sizes of rocks used by snakes did not change over the active season. Snakes tended to be found under the largest rock in their immediate vicinity, but distances from used rocks to equally big or bigger rocks rarely exceeded a few metres. Similarly, measurements of rocks on transects along the creek showed that, although small rocks predominated, all transects contained rocks large enough for snakes. Thus, it seems unlikely that cover is limited for watersnakes at this site. However, unmeasured factors such as temperature or other physical characters of rocks could change that conclusion. These factors would best be determined by manipulative experiments.     

Reproductive strategies in snakes

Snakes of both sexes display remarkable flexibility and diversity in their reproductive tactics. Many features of reproduction in female snakes (such as reproductive mode and frequency, seasonality and multiple mating) allow flexible maternal control. For example, females can manipulate not only the genotypes of their offspring (through mate choice or enhanced sperm competition) but also the phenotypes of their offspring (through allocation 'decisions', behavioural and physiological thermoregulation, and nest-site selection). Reliance on stored energy ('capital') to fuel breeding results in low frequencies of female reproduction and, in extreme cases, semelparity. A sophisticated vomeronasal system not only allows male snakes to locate reproductive females by following scent trails, but also facilitates pheromonally mediated mate choice by males. Male-male rivalry takes diverse forms, including female mimicry and mate guarding; combat bouts impose strong selection for large body size in males of some species. Intraspecific (geographical) variation and phenotypic plasticity in a wide array of reproductive traits (offspring size and number; reproductive frequency; incidence of multiple mating; male tactics such as mate guarding and combat; mate choice criteria) provide exceptional opportunities for future studies.     

The effects of substrate and vertebral number on locomotion in the garter snake Thamnophis elegans

1. Locomotor performance of limbless vertebrates depends on the substrate through which individuals move and may result in selection on vertebral number in different habitats. To evaluate the effect of push-point density on snake locomotion, the density of vegetation and other potential push-points was quantified at two sites in California (coastal and inland), where conspecific snakes differed greatly in vertebral number (230 and 256 average total vertebrae, respectively; Arnold 1988). The coastal site had significantly higher push-point densities than the inland site.
2. Five experimental push-point densities that fell within the natural range of push-point densities were employed in laboratory trials of juvenile snake locomotion. Density of push-points significantly affected both crawling speed and head-to-tail distance (HTD), an indirect measure of lateral bending. The fastest speed was achieved at an intermediate push-point density. The shortest HTD occurred when snakes moved through the lowest push-point density.
3. Sex, total number of vertebrae and total length significantly affected HTD, regardless of push-point density. Snakes with relatively more vertebrae had a shorter HTD, suggesting they were able to achieve greater lateral bending than snakes with fewer vertebrae. Coastal and inland populations did not differ in HTD during locomotion.
4. Numbers of body and tail vertebrae significantly influenced speed at different push-point densities. In general, snakes with more body vertebrae were slower than those with fewer, while snakes with more tail vertebrae were faster than those with fewer. Snakes of greater total length were faster at all densities. Coastal snakes crawled faster than inland snakes at all push-point densities.     

Population size and survivorship of the common garter snake, Thamnophis sirtalis, near the northern limit of its distribution

The common garter snake, Thamnophis sirtalis . has frequently been studied, but never in the far north where climatic influences on distribution and abundance might be expected to be severe. We examined the structure and dynamics of a communal den population at the northern limit of the species' range. Snakes were sampled at the hibernaculum from spring 1983 to spring 1987. Males were more easily caught than females, and few inferences could be made about the latter. Population estimates, though possessing wide standard errors, seemed to indicate that the denning population of 500–600 snakes, mostly adults, was stable. Annual survivorship of large males (≥400 mm SVL) was high (0.68–0.98) and was not correlated with body size. Thirty-four snakes that were first marked in 1983 were recaptured in the spring of 1987, indicating that they had survived at least six years, assuming they were a minimum of two years old when first marked. Recruitment to the denning population consisted of an influx of 'new' individuals (450–550 mm SVL) each autumn. It is not known where these snakes spend the first few years of their existence. The relative Stability of numbers and high overwinter survivorship observed over the course of this study contrast sharply with the only previous study of T. sirtalis in cold climates.     

Function of snake mobbing in spectral tarsiers

Numerous species are known for their tendency to approach and confront their predators as a group. This behavior is known as mobbing. Snakes seem to be one of the more consistent recipients of this type of predator-directed behavior. This paper explores individual differences (sex and age) in the mobbing behavior of the spectral tarsier toward live and model snakes. This study was conducted at Tangkoko Nature Reserve (Sulawesi, Indonesia) during 2003-2004. During this research, 11 natural mobbing events and 31 artificially induced mobbing events were observed. The mean number of individuals at a mobbing was 5.7. The duration of mobbing events was strongly correlated with the number of assembled mobbers. Adults were more likely than other age classes to participate in mobbings. Males were more likely than females to participate in mobbings. Mobbing groups often contained more than one adult male, despite the fact that no spectral tarsier group contains more than one adult male. No difference in body size between extragroup males and resident males was observed, refuting the "attract the mightier" hypothesis. The number of mobbers did not affect whether the tarsier or the snake retreated first, countering the "move-on" hypothesis. The "perception advertisement" hypothesis was tentatively supported, in that live snakes were rarely seen in the area following mobbing calls, in comparison to when tarsiers either ignored the snake or alarm call.     

Aposematic colouration does not explain fear of snakes in humans

Snakes elicit a higher level of fear than other vertebrate animals, yet specific cues responsible for fear of snakes are equivocal. The bright colouration hypothesis suggests that fear responses to snakes are triggered by aposematic colouration, not by snakes per se. We investigated the role of aposematic colouration in fear of snakes in a sample of 10- to 15-year-old Slovak children. Both aposematically and cryptically coloured snakes presented as both colour and black-and-white pictures received higher perceived fear scores than other vertebrates. This suggests that aposematic colouration does not play a crucial role in eliciting fear of snakes. Our results support the snake detection theory suggesting that the human visual system has been influenced by long coexistence between predatory snakes and mammals. As a result, humans have evolved an attentional bias ultimately focused on the correct and rapid detection of these threats.     

Correlations between Ontogenetic Change in Color Pattern and Antipredator Behavior in the Racer, Coluber constrictor

Differences in snake color pattern have been demonstrated to affect behaviors involved in antipredator defense. Snakes with blotched or banded color patterns are concealed when not moving, and tend to rely on concealment and aggression for defense. In contrast, snakes with uniform or striped color patterns are easily seen when stationary, but their speed and direction are difficult to track when moving. They tend to rely on flight for protection. Some snake taxa exhibit ontogenetic change in color pattern, but the behavioral consequences of this change have not been investigated. I present results of a behavioral study in the racer, Coluber constrictor, which has a blotched juvenile color pattern but is uniformly colored as an adult. Hatchling racers were significantly more likely than adults to show aggressive behavior when confronted with a model predator, whereas adults were more likely to flee. This supports the hypothesis that changes in behavior and color pattern are correlated in this species to provide effective antipredator defense at different stages of life history. I also examined sprint speed, which may be an important factor in antipredator defense. Juvenile and adult racers showed a similar relationship between length and speed, a pattern also seen in other species that lack color change. This result suggests that sprint speed is not a causal factor in the evolution of ontogenetic color change.     

Kin recognition in rattlesnakes

Snakes are often regarded as the least social of all vertebrate groups, but this assumption stems from the fact that they are secretive and difficult to observe in nature, rather than direct evidence. Recent studies have revealed a surprising degree of social complexity in snakes. Here, I examine the ability of captive-raised timber rattlesnakes (Crotalus horridus) to recognize siblings by measuring the mean separation distance and frequency of contact between pairs of individuals housed together. The results show that female siblings associate more closely with each other than nonsibling pairs. Previous studies have shown that timber rattlesnakes occupying the same hibernacula have higher relatedness than snakes using neighbouring hibernacula, and frequently form social aggregations. Rattlesnakes exhibit other characteristics consistent with advanced sociality, including group defence, conspecific alarm signals and maternal defence of young. These findings reinforce the notion that, rather than being solitary and asocial, some snake species may form family groups.     

Temperature‐induced multi‐species cohort effects in sympatric snakes

In reptiles, reproductive maturity is often determined by size rather than age. Consequently, growth early in life may influence population dynamics through effects on generation time and survival to reproduction. Because reproductive phenology and pre‐ and post‐natal growth are temperature dependent, environmental conditions may induce multi‐species cohort effects on body size in sympatric reptiles. I present evidence of this using 10 years of neonatal size data for three sympatric viviparous snakes, Dekay''s Brown snakes (Storeria dekayi), Red‐bellied Snakes (S. occipitomaculata), and Common Garter snakes (Thamnophis sirtalis). End‐of‐season neonatal size varied in parallel across species such that snout–vent length was 36%–61% greater and mass was 65%–223% greater in years when gestating females could achieve higher April–May (vs. June–July or August–September) operative temperatures. Thus, temperature had a larger impact during follicular enlargement and ovulation than during gestation or post‐natal growth. Multi‐species cohort effects like these may affect population dynamics and the magnitude of these effects may increase with climate change.