Aquatic foraging in some new world natricine snakes ; generalists and specialists and their behavioral evolution

The second major issue concerns the predator's evolutionary adjustment of its foraging repertoire to a new feeding niche: When a radical change is made, as from aquatic to terrestrial prey, how does the predator move from one suite of behavioral traits to another that is quite different? To answer this question, two populations of T. Elegans with very different feeding ecologies were examined for hereditary differences in foraging behavior.

The foraging behavior of each species was observed in the field, then controlled laboratory observations were made to establish which similarities and differences were intrinsic to the species rather than a function of environment or experience. Litters born in the laboratory were raised on a fish diet for several weeks, during which time their responsiveness to dead and live fish was recorded. They were then observed preying on live fish in an artificial pool which simulated the major features of the most similar habitats in which the species commonly forage.

There are strong qualitative and quantitative similarities in the aquatic foraging of the two generalists and in that of the two specialists, as well as striking differences between the pairs. By evolving relatively good underwater vision, the specialists have been able to take the slow, low-cost, terrestrial foraging strategy of their genus beneath the water-surface. They crawl and sit-and-wait on the underwater substrate, orient and attack prey from several centimeters distance and use a powerful attack motor pattern for aquatic and air-to-water attacks. The generalists appear to have relatively poor underwater vision and rely on either essentially terrestrial foraging or coursing rapidly through the water, snatching at prey encountered "by chance." Thus they are effective only with amphibious prey or overcrowded aquatic prey. It appears that in this case, the generalist has indeed compromised and that its failure to develop good aquatic vision has placed severe constraints on its foraging skills in relation to a particular prey class; confining it to a behavioral repertoire that is qualitatively different from that of the specialists and limiting it to foraging only in a restricted class of aquatic environments.

The water snakes are in some ways intermediate, but their foraging repertoire and visual skills resemble more closely those of the generalist garter snakes. Their lack of visual specialization, compared to the specialist garter snakes, may be associated with foraging in conditions of poor visibility.

The population of T. Elegans which has entered an almost exclusively terrestrial niche has retained all of the species-typical aquatic foraging behavior patterns, but its tendencies to employ them are diminished. There is marked individual variation: some readily engage in aquatic foraging whereas others only perform behaviors from their aquatic repertoire under extreme food deprivation, and others evidently never do. Thus feeding behavior patterns can become latent in a species or population when a change in feeding niche is made, and remain available for subsequent reinvasion of the original niche.