DO INCOMPLETE DRILLHOLES INDICATE PREY EFFECTIVENESS?

Drill holes made by predatory gastropods are often used to estimate predation intensity in the fossil record. Incomplete drill holes are possible evidence of failed or interrupted predation attempts, and their frequency has been used as a metric of the prey’s defensive capability, or prey effectiveness. In this study, we test whether other processes besides adaptive improvement of prey defenses could elicit incomplete drillholes and possibly bias the prey effectiveness metric, which is often used in tests of escalation theory.

We conducted an experiment with the naticid gastropod Neverita duplicata and its bivalve prey Chione elevata in which naticids were assigned to one of two treatments. In the first treatment, the isolation group, naticids were assigned singly to individual aquaria, and each was offered three Chione prey, which varied in size and thickness but were selected randomly. In the second treatment, the competition group, three naticids were grouped each aquarium to simulate threat of cannibalism and kleptoparasitism. The number of prey available to each predator (3) was kept constant in this competition treatment, and prey were replaced immediately after being eaten.

Results show that incomplete drillhole frequency was higher in the competition (35.7%, N=28) than the isolation treatments (7.4%, N=68). Because there were no differences in the distribution of shell sizes or thicknesses of the single prey species offered in the two treatments, this difference in incomplete drilling cannot be attributed to the prey. Rather, increased density of predators appears to be the primary disruptive factor in the drilling process. However, in the competition treatment, unsuccessfully drilled prey were still killed, perhaps by suffocation, and consumed. This suggests that competition may disrupt drilling but may not necessarily reduce the effectiveness of the predator. These factors should be considered in interpreting the fossil record of incomplete drilling.