If we understand the proximate criteria that animals use in discriminating between ‘accepted’ and ‘rejected’ potential food items, we may then be able to clarify the ecological and evolutionary bases to those criteria. An alternative way to gain insight into the foraging tactics of animals is to work at the mechanistic level. Mathematical modelling of optimal feeding tactics has generated many predictions ( Pyke, Pulliam & Charnov 1977), but also has stimulated considerable scepticism ( Krebs & McCleery 1984 Greene 1986 Perry & Pianka 1997). 1999), but the reasons for this diversity remain unclear for most kinds of organisms. Auffenberg 1981 Kephart & Arnold 1982 Nakano et al. Dietary composition can vary enormously even among individuals within a population, and among populations within a single species (e.g. For example, an animal's foraging decisions may affect not only its individual energy budget, but also the nature and outcomes (competitive exclusion, coexistence, etc.) of interactions between sympatric taxa ( Stephens & Krebs 1986 Vitt 1987 Arnold 1993). These dietary decisions define the trophic niche of the organism, and have significant implications for ecological processes at many levels ( Holling 1966 Charnov 1976). Most animals feed selectively, and hence need to discriminate between objects that are or are not acceptable as food ( Schoener 1971 Caraco & Gillespie 1986 Greene 1986 Arnold 1993 Nakano, Fausch & Kitano 1999).
The absence of small prey items from the diet of larger snakes is due to active refusal to strike at small prey, as well as a behavioural shift to scavenging and to terrestrial rather than arboreal ambush-sites (and thus, higher rates of encounter with large prey items) by larger pit-vipers. The absence of large prey from the diet of small snakes is due to gape-limitation (these snakes strike and attempt to swallow much larger prey). The ontogenetic shift in prey size thus reflects a combination of processes. Larger snakes also scavenge dead birds too large for smaller snakes to ingest, but do not ignore live birds: even the largest snakes use prey movement and prey temperature as cues to elicit feeding strikes. A pit-viper's body size influenced its prey-size selectivity, with larger snakes refusing to strike at smaller prey items. The snakes struck at the prey item in 101 of these trials, and this ‘decision’ was influenced by the size, movement and temperature of the prey item. To clarify the reasons for this ontogenetic shift, 251 snakes in ambush postures were approached and offered potential prey items (dead birds, or models covered in feathers) to clarify the cues that trigger a foraging strike. Both minimum and maximum prey sizes increase with predator size. Why do larger predators generally consume larger prey items? Endemic pit-vipers ( Gloydius shedaoensis) on a small island in north-eastern China ambush passerine birds, usually from the branches of small trees.
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