Many ectothermic vertebrates raise their preferred body temperature after feeding, likely expediting digestion. However, most studies documenting this phenomenon have relied upon laboratory thermal gradients, which grossly oversimplify an animal's environment. We explored the validity of thermal gradient methodology by investigating post-prandial thermophily in an Australian pygopodid lizard (Lialis burtonis Gray). Mean body temperatures did not differ between fed and unfed lizards in field enclosures. Feeding influenced body temperature in a thermal gradient, but in opposite directions depending on details of the methodology. When we introduced L. burtonis into the gradient at the warmer end, fed lizards had higher body temperatures than unfed conspecifics. However, the opposite was true when lizards were introduced at the cooler end. These contrasting results indicate that lizards with food in their stomachs did not seek out higher temperatures, but instead were more sedentary than unfed lizards. Our study highlights the need for caution in interpreting thermal gradient results unaccompanied by field data, and it demonstrates how minor changes in equipment design or procedures can significantly alter conclusions from laboratory studies.

Although garter snakes at communal overwintering dens on the Canadian prairies have attracted considerable behavioural ecology research, previous studies have relied upon sampling of active animals to describe broad patterns of distribution and abundance of snakes within the den population. We conducted a mark–recapture study to directly quantify temporal and spatial variation in the phenotypic traits (sex, size, body condition) of snakes at the den itself, and those dispersing through woodland 50 m away. Captures of 909 snakes on the days they emerged, and 6653 snakes as they dispersed, revealed massive spatiotemporal heterogeneity in phenotypic traits among samples. Day-to-day variation in weather conditions affected numbers and sex ratios of emerging and dispersing snakes; for example, small females dispersed in greater numbers after unusually cold nights, when harassment by courting males was reduced. Most snakes stayed at the den only briefly (<5 days) prior to dispersal, so that sampling at the den itself (the only evidence available from most previous studies) underestimates the number of animals in the population, as well as the proportions of females, of small adult males and of juvenile animals. Overall, the heterogeneous and temporally dynamic distributions of phenotypic traits (such as sex and size) among our samples are predictable on the basis of the central roles of male–male competition and sexual conflict in the mating system of these snakes. Surprisingly, however, many of the snakes that overwinter at this den play no part in den-based breeding aggregations

Occasional use of arboreal habitats is common in many otherwise-terrestrial species, but the functions of facultative arboreality remain unclear. Red-sided garter snakes (Thamnophis sirtalis parietalis) are small nonvenomous terrestrial snakes, recorded to climb only rarely. During springtime fieldwork near a large communal overwintering den in Manitoba, we observed occasional episodes when hundreds of snakes climbed into low trees and remained in the branches for periods ranging from minutes to hours. Our data suggest a thermoregulatory basis for this behaviour. During daylight hours when the snakes were active, ambient temperatures generally were higher on the ground than in trees; but this thermal cline reversed occasionally, especially after the ground was cooled by light rain. Arboreality was most common during those thermal reversals. Experimentally simulating rain in the field (sprinkling with water) induced climbing behaviour; and in laboratory enclosures, snakes climbed to avoid a cold substrate. Thus, red-sided garter snakes ascend trees to access higher temperatures than are available on the ground.

Females of many species behave in ways that make it difficult for males to locate, court, and inseminate them. Two hypotheses have been advanced to explain such behavior: either a female thereby minimizes costs of harassment (sexual conflict model) or by playing "hard to get" she discourages inferior suitors (indirect mate choice model). Our studies of garter snakes (Thamnophis sirtalis parietalis) at a communal den in Manitoba support an interpretation of sexual conflict rather than indirect mate choice. Female snakes dispersed rapidly from the den through areas with relatively few males rather than waiting for additional courtship. Many females dispersed without mating. Experimental (pheromonal) manipulation of the intensity of courtship accelerated rates of female dispersal rather than delaying dispersal, as would be predicted if females wait to obtain matings. The behaviors of females escaping from courting groups were maximally effective in losing their suitors regardless of the number of courting males or whether or not the female was capable of mating (recently mated females cannot mate again because of a mating plug). In total, our data are most consistent with the hypothesis that female garter snakes at communal dens evade males to escape harassment rather than to enhance mate quality.

SummaryIn a field population in Manitoba, about 16% of Red‐Sided Garter Snakes (Thamnophis sirtalis parietalis) have a different number of ribs on either side of the body, as indicated by anomalous ventral scales. A study on congenerics shows that this asymmetry reflects disruption of embryogenesis by environmental factors, including suboptimal maternal thermoregulation during pregnancy.Correlates of this scalation asymmetry in adult snakes can clarify the degree to which (and pathways by which) the thermal environment that an individual experiences during embryogenesis affects its later fitness.Data on >4000 free‐ranging snakes reveal no significant association between asymmetry and fitness‐related traits such as body size, body condition, locomotor speeds or female ability to resist harassment from males.However, adult male snakes with scale asymmetry dispersed from the den‐based mating aggregations sooner than did symmetric males. In outdoor arenas, asymmetric males were less able to obtain matings than were symmetric males; and if asymmetric males did mate, they consistently used the hemipenis on the side of the body opposite to their extra rib.Our data suggest that careful thermoregulation by a female garter snake during pregnancy provides a fitness benefit to her sons after they mature, via sexual selection.

Alternative mating strategies occur in many animal lineages, often because males adopt tactics best suited to their own phenotypes or to spatiotemporal heterogeneity in the distribution of females. Garter snakes near a communal overwintering den in Manitoba show courtship in two contexts: competition from rival males is intense close to the den, but weak or absent when males court solitary dispersing females in the surrounding woodland. Larger size enhances male mating success near the den, but not mate location rates in the woodland. As predicted by the hypothesis that males match their tactics to their competitive abilities, our mark-recapture data show that larger, heavier individuals remained near the den, whereas smaller and more emaciated males moved to the woodland. To locate mates, woodland males relied upon substrate-deposited pheromonal trails and visual cues (rapid movement), whereas males in the crowded den environment ignored such cues and instead tongue-flicked every snake they encountered to check for sex pheromones. In arena trials, den males adjusted their courtship intensity to the presence of rival males, whereas woodland males did not (perhaps reflecting the lower probability of interruption by a rival). Thus, male garter snakes adjust the times, places, form and intensity of their reproductive behaviours (mate-searching tactics, intensity of courtship) relative to both their own competitive abilities and spatial heterogeneity in mating opportunities.

In communal dens of garter snakes, Thamnophis sirtalis, in Manitoba, males obtain copulations forcibly, by inducing hypoxic stress in females. Females emerging from their 8-month winter inactivity are weak and slow, but recover strength and speed within a day or two of emergence. Thus, males may benefit by targeting newly emerged females that are too weak to resist, and by courting females under conditions that maximize the male's physical advantage. Fieldwork at a Manitoba den supported these predictions. Both in the field and in outdoor enclosures, males focused courtship on to newly emerged (weak) rather than dispersing (postrecovery) females. Experimental trials showed that males based this choice on female skin lipids (pheromones) that predict time since emergence, rather than correlates of recent emergence that reflect female physiological state per se (low body temperature, low resistance to hypoxia, high breathing rates during courtship). Male superiority in locomotor speed was maximal at high temperatures, and males targeted courtship to hotter females. The postemergence recovery of females in this system substantially shifts the balance of power in physical encounters between males and females, and provides a fitness benefit to male ability to recognize and exploit more vulnerable females. Male ability to discern and exploit differentially vulnerable females is likely to be widespread, but less overt, in many other populations. (c) 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

If animals avoid road surfaces or are unable to follow conspecific trails across such surfaces, previously continuous populations may be fragmented. We gathered data on the effects of a small (4-m wide) gravel road on the behavior and trail-following abilities of garter snakes (Thamnophis sirtalis parietalis) in Manitoba, central Canada. As expected, the road surface had less vegetation cover, a more open canopy and, thus, higher incident radiation than did the surrounding grassland. Contrary to expectations, however, substrate temperatures were lower on the road than in its surrounds, because of the higher reflectivity of the road's surface. On a nearby asphalt road, substrate temperatures were relatively high on the road surface only in the evening, as surrounding areas cooled. Focal sampling showed that snakes avoided the gravel road, typically changing direction when they encountered it. If they crossed the road, they did so by the shortest possible route (straight across). Mate-searching male snakes were less able to follow substrate-deposited pheromonal trails left by females if those trails crossed a road than if the trails were entirely within the surrounding grassland. Thus, roads may significantly modify snake movement patterns, as well as the ability of males to locate reproductive females. Our study provides the first detailed information on the effects of roads on snake behavior.