VARIATION IN THE EFFECT OF PROFITABILITY ON PREY SIZE SELECTION BY THE LACERTID LIZARD PSAMMODROMUS-ALGIRUS

Maximizing the average rate of energy intake (profitability) may not a lways be the optimal foraging strategy for ectotherms with relatively low energy requirements. To test this hypothesis, we studied the feedi ng behaviour of captive insectivorous lizards Psammodromus algirus, an d we obtained experimental estimates of prey mass, handling time, prof itability, and attack distance for several types of prey. Handling tim e increased linearly with prey mass and differed significantly among p rey types when prey size differences were controlled for, and mean pro fitabilities differed among prey taxa, but profitability was independe nt of prey size. The attack distance increased with prey length and wi th the mobility of prey, but it was unrelated to profitability. Thus, lizards did not seem to take account of the rate of energy intake per second as a proximate cue eliciting predatory behavior. This informati on was combined with pitfall-trap censuses of prey (in late April, mid -June and late July) that allowed us to compare the mass of the prey c aptured in the environment with that of the arthropods found in the st omachs of sacrificed free-living lizards. In April, when food abundanc e was low and lizards were reproducing, profitability had a pronounced effect on size selection and lizards selected prey larger than averag e from all taxa except the least profitable ones. As the active season progressed, and with a higher availability of food, the number of pre y per stomach decreased and their mean size increased. The effect of p rofitability on size selection decreased (June) and eventually vanishe d (July-August). This variation is probably related to seasonal change s in the ecology of lizards, e.g. time minimization in the breeding se ason as a means of saving time for nonforaging activities versus movem ent minimization by selecting fewer (but larger) prey in the postbreed ing season. Thus, the hypothesis that maximizing profitability could b e just an optional strategy for a terrestrial ectothermic vertebrate w as supported by our data.