MOBILE PREDATORS AND MOBILE PREY: COMPOUND TRACES INDICATE ACTIVE HUNTING BEHAVIOR

Observations of the intertidal gastropods, Agaronia propatula and Olivella semistriata on the sand flats of Playa Grande, Costa Rica in March 2009, revealed a recurring trace morphology that may indicate active hunting and predator-avoidance behaviors. Signatures of sensing, stalking, chasing, evasion, capture, and killing or escape, are incorporated into complex sequences of traces indicative of active hunting behavior, with or without interaction between trace makers. Changes in behavior produce traces indicating sudden shifts from low-energy sensing, foraging, or burrowing activities, to high-energy chase or evasion tactics, ending in disturbed areas of capture, killing, and burrowing for ingestion. Other areas show apparent escapes and returns to lower-energy movements, such as foraging or burrowing. Successful captures often resulted in a single trail from the predator intersecting with that of its prey, making for a composite trace. For example, Agaronia follows the scent and slime trail of its prey closely causing overlapping trails or burrows. Not all of such shifts are present in every trail, but some components, especially those indicating sudden transitions, such as traces left by high-energy expenditures associated with explosive movements or struggles to subdue the prey, may themselves indicate active predation. A preliminary examination of trace fossils in the Smithsonian Institution collection has revealed possible evidence of active, high-energy hunting in the form of irregular trails from the La Meseta Formation (Eocene) of Seymour Island, Antarctica. Traces similar to modern examples described from Costa Rica thus may be useful in determining early dates for the onset of chase-and-capture hunting behaviors, and better define evolutionary transitions from foraging to hunting in marine benthos.