A case for cannibalism: Confamilial and conspecific predation by naticid gastropods, Cretaceous through Pleistocene of the United States Coastal Plain

8.1 Abstract

Cannibalism is a common phenomenon in animals, but some previous authors have concluded that cannibalism by shell-drilling naticid gastropods was caused by predator ineptitude, especially early in the evolution of naticids. The suggestion that naticids were less efficient predators earlier in their history may be considered consistent with the hypothesis of escalation. According to the hypothesis of escalation, biological hazards, such as predation, have increased through geologic time. If naticids were less efficient early in their history, and if cannibalism is an indicator of predator ineptitude, then the incidence of cannibalism should be greatest early in the history of the naticid predator-prey system. Based on this hypothesis, we predicted a temporal decrease in the frequency of cannibalism. We tested this hypothesis by determining the incidence of predation by naticid gastropods on naticid gastropods, including intraspecific cannibalism, from the Cretaceous through the Pleistocene. Drilling frequencies (percent of naticid specimens with complete naticid drillholes) were determined for samples of naticids from twenty-three stratigraphic levels in the U.S. Atlantic and Gulf Coastal Plain (∼3,400 naticid specimens).

Contrary to this hypothesis, drilling on naticids (including intraspecific cannibalism) increased through time. Drilling frequency for all Cretaceous samples combined was 0.02; comparable results were 0.11 and 0.26 for the Paleogene and post-Paleogene respectively. Spearman rank correlation coefficients between drilling frequency and stratigraphic position were statistically significant for both confamilial and conspecific naticid predation. Drilling frequencies were not correlated with the abundance of naticids in the fauna, nor were they correlated with drilling frequencies on the gastropod fauna as a whole. Most attempts to drill naticid prey were successful, and no trends in frequency of incomplete drillholes (prey effectiveness) occurred between the Cretaceous and Pleistocene. These results may indicate that naticids were not less efficient early in their history. More plausibly, cannibalism may be an inappropriate measure of predator ineptitude. In that case, the increase in cannibalism may indicate increasing naticid predatory capabilities through time; because naticids are highly mobile prey, cannibalism may require greater predator efficiency than do non-cannibalistic predation events. Predation on naticids is energetically profitable and typically successful once drilling is initiated, suggesting that cannibalism may be an attractive alternative for an efficient predator rather than a hallmark of ineptitude.