Species and size-selective predation by raccoons (Procyon lotor) preying on introduced intertidal clams

Abstract

Raccoons (Procyon lotor (L., 1758)) are known for their dietary plasticity and ability to exploit new resources. We studied raccoons preying on introduced intertidal clams and hypothesized that raccoons maximized energetic profit by foraging selectively. Raccoons discarded Manila clams (Venerupis philippinarum (A. Adams and Reeve, 1850)) but selected large varnish clams (Nuttallia obscurata (Reeve, 1857)), although varnish and Manila clam densities did not differ significantly and small varnish clams were more abundant than large ones. We determined the energy content of different-sized varnish and Manila clams by subtracting the cost of cracking a clam from its soft-tissue energy. Varnish clams with less shell mass than Manila clams required less energy to open, but for their size Manila clams were more profitable. We suggest that raccoons, limited to preying on clams when the tide is low and at risk feeding on an open beach, select varnish clams because they need less handling, but maximize profit by selecting large clams. Our calculations indicate that a raccoon eating large varnish clams could obtain up to 8.4% of its daily basal metabolic needs in 10 min, making varnish clams a potentially valuable new prey resource.