INFLUENCE OF COMPETITION ON GROWTH RATES AND MAXIMUM SIZE OF PREDATORY GASTROPODS: ECOLOGICAL VERSUS EVOLUTIONARY PERSPECTIVES

While seeking food, predators often risk becoming food themselves. In response to this risk, predators may alter their foraging behaviors, sometimes accepting a lower rate of food intake and decreased growth potential in exchange for greater safety.

Plio-Pleistocene predatory gastropods of Florida offer an apparent contradiction to this general pattern. Despite independent evidence for increased competition and predation risk in the Pliocene than afterwards, most Pliocene predators were substantially larger than their congeners in the Pleistocene and today. Stable isotope sclerochronology indicates that large size in one of these predators, the muricid Phyllonotus, was primarily a result of more rapid growth rather than a longer lifespan.

Behavioral experiments with living Phyllonotus in the lab were conducted to examine in greater depth the seemingly counterintuitive fossil evidence for increased growth potential in a hazardous environment. Six snails between 38.7 and 71.7 mm in shell length were monitored for prey consumption rates in isolation from other conspecific predators (i.e., a non-competitive environment) followed by consumption rates with three other conspecifics and potential cannibals present (i.e., a competitive environment). Prey were kept more numerous than predators at all times during the experiment.

Results of these experiments showed that all individuals consumed less in competition than in isolation re-confirming the consensus view that biological hazards alter feeding behaviors and reduce consumption rates in predators. However, larger snails consumed absolutely higher quantities of prey in the competition experiments than did smaller ones, suggesting that rapid growth and large size may be selected for even when, overall, growth potential for individuals is severely diminished. These results provide a likely explanation for large bodied predators in the biologically hazardous Pliocene of Florida.