Ontogenetic changes in foraging behaviour and habitat use by the Oregon garter snake, Thamnophis atratus hydrophilus

Foraging behaviour, stream habitat use and food habits of a population of the aquatic, Oregon garter snake were studied in the field during the spring and summer of 1987 and 1988. Continuous records of behaviour and habitat use were obtained for each snake. Adults foraged more actively and in a wider variety of stream habitats than juveniles or neonates. Adults also fed on a wider variety of prey types and sizes, especially concentrating on large, Pacific giant salamander, Dicamptodon tenebrosis, larvae and neotenes, in the mid-stream substrates. Juvenile and neonate snakes fed on relatively smaller prey that inhabited shallow stream margins. Consumption of relatively large prey by adults and smaller prey by juveniles indicated a shift in foraging ‘strategy’ from frequent feeding on small prey to infrequent feeding on large prey. These ontogenetic changes in foraging behaviour, habitat use and food habits of Oregon garter snakes are probably the result of a combination of proximate ecological, morphological and physiological constraints. Ultimately, these behavioural shifts may have a phylogenetic origin that reflects the strong advantage of single large meals versus many small meals for many modern snake lineages. Foraging and feeding behaviour comprise the primary defining attributes of predatory organisms in ecological (Morse 1980) and evolutionary contexts (Greene 1986). Ontogenetic changes in foraging behaviour and foraging modes (e.g. ‘sitand-wait’ and active foraging; Schoener 1971) have rarely been studied in reptiles (Burghardt 1978; Mushinsky 1987), although understanding such shifts has important implications for animal learning, development, optimal foraging, phylogenetic systematics and evolution. For example, behaviour patterns are increasingly used as character states in phylogenetic analyses and lack of knowledge of the behaviour of different ages of snakes could result in faulty conclusions about phylogenetic relationships. Similar problems plague optimal foraging theory which typically has not reflected development of behaviour over time (Stephens & Krebs 1986; Stephens 1990). Finally, snakes were probably derived from a fossorial lizard or lizard-like ancestor (Rage 1987) that fed frequently on small prey (Greene 1982; Regal 1983). Changes in feeding behaviour have been proposed as a key development in the early evolution of snakes, with a trend towards the capture of larger prey leading to the derivation of many modern forms (Greene 1983). Studies of ontogenetic changes in feeding biology across a variety of taxa might clarify that scenario. Some information is available on the foraging behaviour of snakes of the largest family, Colubridae (Arnold 1980; Drummond 1983; Drummond & Macias Garcia 1989; Jones 1990), and also the family Viperidae (Reinert et al. 1984; Duvall et al. 1985). Laboratory studies of Thamnophis and Nerodia spp. have provided categorizations of foraging behaviour along with important information on foraging in controlled settings (Drummond 1983, 1985; Halloy & Burghardt 1990). Studies of the food habits of several species of water snakes (Colubridae: Nerodia and Regina spp.) and sea snakes (Hydrophiididae) indicate that both size and type of prey change as snakes mature (Godley 1980; Voris & Moffett 1981; Mushinsky et al. 1982; Plummer & Goy 1984). In water snakes, prey size increases with snake size, whereas sea snakes take a wider range of prey sizes as they mature, but do not forage exclusively on larger prey. These studies 0003-3472/94/l 2 126 1 + 13 $08.0010 o 1994 The Association for the Study of Animal Behaviour