JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. Sex-based differences in diets are widespread among animals, but may often reflect sex differences in body size or in the times and places that feeding occurs, rather than sex-specific prey-selection behaviour. We examined these hypotheses with detailed information on the types and sizes of fishes consumed by filesnakes, Acrochordus arafurae McDowell, 1979, based on a 4-year field study in the Wet-Dry tropics of northern Australia. 2. These aquatic snakes show extreme sexual dimorphism in body length, body shape, relative head size, head shape, foraging habitat and diet. The composition of the diet varied seasonally and with the size as well as the gender of the snakes. Both sexes ate more frequently during the Wet season, apparently because of higher prey availability at this time. 3. Both sexes showed significant selectivity of prey species, but differed profoundly in food habits. Female filesnakes generally consumed only a single large prey item at a time, whereas the stomachs of males typically contained many small prey items. 4. Males were relatively unselective foragers, with little evidence of ontogenetic shifts in prey size or prey type. In contrast, large females ate larger prey items, and showed an ontogenetic shift from a generalized (male-like) diet to 'sleepy cod' (eleotrids) and finally to plotosid catfish. 5. When comparison was restricted to snakes of the same head length, males and females differed significantly in dietary composition and in prey sizes, but not in feeding frequencies or the relative numbers of snakes containing single versus multiple prey items.

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ABsrkd.-A m.rk-reeprure 8tu.ty of fil6nak6 (A.ro.nordus aranaae) in freshrard bill.bonF of rorth.m Austr.li. previded drta on >3400 duk6 over. folFy.d period, popuhdon denside;rc esti@ted by a noalidcd j.clknife technique, fal were nuch hi8h.r thd ir most p!.vio6ty-srudied 3Pe.i6 of dnak6, with > 40 sn.ke6 per he.t.re surf..e @a (> lOO ka ha 1) in rhe @in stuaty bill.bons. Thb hiSh bioda$ .an be naitrt.ined b.c.6. of the hiAh abundance of prey (ff.h6), oupled wirh rhe low oetabolic reqrir€aent! {and hen.e, low fealing r.t6} of acrc.hordid snak6. Comp.rtsons of sex r.ti@ in h.pp.d smp16 .nd in popularior estimates 3hosed th.t m.l€ 616!at6 w€re 16,.rt(h.btein oE fyke-nele thr wde f.mler Ser raric ditrerd among bill.bonas, with a tEnd fot morc tules in bro.

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