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.

Reproductive isolation between sympatric taxa can be maintained by specific mate-recognition behaviours or by ecological divergence that reduces interspecific contact during reproduction. Common garter snakes, Thamnophis sirtalis (L., 1758), and plains garter snakes, Thamnophis radix (Baird and Girard, 1853), are sympatric over large areas, but morphological data suggest that the prezygotic isolation between these two species partially breaks down in a severely cold part of their joint range in Manitoba. Courtship trials show that male T radix court intensely over a narrower range of (higher) ambient temperatures than do male T sirtalis. Males selectively court females of their own species, but male T radix are less choosy than male T sirtalis. Hexane extracts of female skin lipids also elicited species-specific courtship. Although this male preference for species-specific pheromones contributes to species isolation, it is not strong enough to completely separate the two taxa. The absence of hybridization over most of the sympatric range may depend on the timing of mating (early spring, near the hibernation den). Differences between the species in hibernation-site selection and the timing of spring emergence break down in central Manitoba because severely cold winter temperatures force both species together into the few available hibernation (and thus, mating) sites, and the short warm season reduces temporal separation in emergence (and thus, mating) seasons.

Cues that females use to select potential mates have attracted substantial research effort, but the criteria for male mate choice remain very poorly known. Red-sided garter snakes (Thamnophis sirtalis parietalis) court and mate in large aggregations around overwintering dens in southern Manitoba, Canada. Both courtship and mating are size-assortative: small male snakes court small as well as large females, whereas larger males court only large females. This system provides a unique opportunity to assess the cues that males use in selecting mates, and in particular the mechanisms that generate a size-related shift in mate preference. Experiments in which we manipulated body sizes and scents showed that both vision and scent (sex pheromones) were important. Large males directed intense courtship only when the stimulus provided both visual and chemical (skin lipid) evidence of large body size. Small males were much less discriminating in both respects. Thus, size-assortative mating in this system is generated not by larger males excluding their smaller rivals from the largest females (as has been reported in other reptile species), but by a size-related shift in the visual and pheromonal cues that elicit courtship. Males of some species may thus show complex patterns of mate choice, with the cues that stimulate courtship differing even among males within a single population based on traits such as age or body size.

When choosing between two potential mates, a male may benefit by picking a larger (longer and/or more heavy-bodied) female because she is likely to produce more or larger offspring. Males of many species use visual cues to evaluate the sizes of their mates, but in some situations (at night or in a crowded mating swarm), vision may be useless. Potentially, males may be able to use chemical cues that convey information about female body size. We manipulated cues available to free-ranging male garter snakes (Thamnophis sirtalis parietalis) in large courting aggregations near communal dens in Manitoba, Canada. Males not only directed disproportionate courtship to longer and heavier-bodied females, but also courted most vigorously in response to lipids extracted from the skins of such females. Our data show that with a flick of his tongue, a male garter snake can identify not only a female's body length, but also her body condition.

Abstract Mechanisms that maintain species isolation within sympatric congeners have attracted analysis in many kinds of organisms, but not in snakes. We studied two sibling species of amphibious sea snakes (Laticauda colubrina and L. frontalis) on the island of Efate, in the Pacific Ocean republic of Vanuatu. The two taxa are almost identical morphologically, except that L. colubrina grows much larger than L. frontalis. No natural hybrids have been reported, and geographic distributions of the two taxa suggest the possibility of sympatric speciation. Our fieldwork shows that the two taxa are often syntopic and overlap in breeding seasons. Behavioral studies in outdoor arenas show that the separation between these two taxa is maintained by species-specific cues that control male courtship. Males of both species courted conspecific females but not heterospecific females. The proximate mechanism driving this separation involves chemical cues. Adult females of both taxa possess distinctive lipids in the skin. Males directed courtship behavior (chin-pressing) to hexane-extracted samples of lipids from conspecific but not heterospecific females. Males of the dwarf species (L. frontalis) were more selective courters than were those of the larger taxon (L. colubrina), perhaps because a preference for courting larger females means that L. colubrina males would be unlikely to court L. frontalis–sized (i.e., small) females even in the absence of pheromonal barriers.

She-male garter snakes exploit the amorous attentions of other males to warm up. Males of several animal species mimic females either in appearance or in the chemical cues they release1,2, and this mimicry has generally been interpreted in terms of alternative mating strategies –– for example, a male that mimics a female may obtain stolen inseminations or avoid aggression from larger rivals3. Our studies of snakes suggest a different explanation, which relies on natural selection rather than sexual selection. Male garter snakes that produce female-like pheromones (she-males) may benefit simply because large 'mating balls' of amorous males form around them, transferring heat to the she-male after it emerges from hibernation and reducing its exposure to predators.