Meeting Abstract
Forage fish have been involved in evolutionary tug-of-wars with predators for more than 100 million years yielding finely balanced predator-prey interactions with thin margins for error. Engulfment predation by gigantic filter feeding whales, in contrast, is a relatively recent (< 5 Ma) phenomenon that typically occurs at extreme predator-prey size ratios that mitigate the effect of prey escape responses (e.g. microphagy on krill). Rorqual whales, however, also commonly hunt forage fish whose performance capabilities suggest that they should easily evade whale-sized predators. To address this paradox we determined, in a laboratory setting, when individual anchovies initiated escape from virtually approaching whales, then used these results along with in vivo humpback whale attack data to model how predator speed and engulfment timing affected capture rates. Anchovies were found to respond to approaching visual looming stimuli at expansion rates that give ample chance to escape from a sea lion-sized predator, but humpback whales could capture as much as 40-50% of a school at once because the increase in their apparent size does not cross their prey’s response threshold until their jaws are already rapidly expanding. Humpback whales are thus incentivized to delay engulfment until they are very close to a prey school, even if this results in higher hydrodynamic drag. This potential exaptation of a microphagous filter feeding strategy for fish foraging enables humpback whales to achieve nearly 7x the energetic efficiency (per lunge) of krill foraging, allowing for flexible foraging strategies that may underlie their ecological success in fluctuating oceanic conditions.
