308 — Activity seascapes highlight central place foraging strategies in marine predators that never stop swimming

“Central place forager” is the name given to the many types of animals that have a single “home base” to which they return in between venturing out for food or other necessities. One example of a CPF is you, since you live in your house and you only leave to hunt down prey at the local supermarket.

This is in contrast to other foragers whose trajectory may not bring them back to the same place in between trips. If you can say, “wow, we always see [that animal] over there” then that animal is probably a CPF.

One of the core assumptions of the ecological niche of a CPF is that the further a patch of foraging area is from their central place, the greater the incurred cost to travel there; and as a result, it follows that the forager should choose to spend more time there in order to amortize that travel cost.

Many types of shark are considered CPFs. But sharks are an interesting violation of that core assumption: Sharks are constantly in motion, swimming continuously in order to pass enough water through their gills to oxygenate sufficiently.

So for a shark, it’s almost as though everywhere requires travel cost; even when the the shark is just circling or staying in one area, it still incurs energy costs.

The researchers tracked sharks with acoustic telemetry and on-body sensors to detect changes in sharks’ position, depth, and body temperature. They also tagged sharks with video cameras and accelerometers and used a Markov model to begin to understand the logic behind switching from warm, shallower water (with a higher metabolic cost) to cooler, deeper water (a closer analog to central-place ‘resting’ behavior).

Instead of energy expenditure correlating closely with body temperature — which would be expected of the general CPF animal — the recorded sharks’ energy expenditure varied instead with changes in swim speed (and thus metabolism). And so while the basic CPF model is a good approximation for many foragers both above and below the water surface, sharks (and other animals that never stop swimming) are a bite more complicated.