Whales' large bodies help them consume their prey at high efficiencies, but their gigantism is limited by prey availability and foraging efficiency, according to a more than decade-long study of around 300 tagged whales of various species.
Published Dec. 12 in Science, these results have been difficult to confirm with quantitative data because of challenges studying these gargantuan mammals in the field. Using new technology to tag whales in the open ocean, the researchers of this study were able to obtain "daily diaries" of the whales' feeding habits and behavior in fine detail.
"I am excited that we are just now getting to the point where we can study the largest animals of all time in their natural environment at unprecedented levels of resolution," said Jeremy Goldbogen, a professor of biology at Stanford University and lead author of this study. "These data can help inform conservation and management of these populations, particularly for whales that consume prey also targeted by fisheries."
Whales' food supply can also be affected by a gradually warming climate, warranting increased efforts to preserve these endangered giants by first deciphering their basic biological needs.
Whales are major contributors to the marine food chain, as consumers of copious amounts of marine organisms, contributors of fertilizing waste, and, in death, feeding members of ecosystems at every level. They are also the largest animals on Earth. Many of them, including the largest animal perhaps to ever exist in the world, the blue whale, sustain their size by feeding on tiny prey — a yet unresolved phenomenon.
Growing large depends on food — specifically, the delicate balance between energy gained from food and energy expended. When it comes to eating, whales break off into two groups: a toothed kind that target single prey and toothless filter feeders that engulf masses of tiny crustaceans like krill.
Toothed whales may have evolved larger body sizes to enhance their biosonar so that they could dive deeper for larger and more nutritious prey like giant squid. Their toothless cousins, which branched off from smaller, toothed ancestors, are thought to have grown larger to more efficiently process large amounts of tiny food, as well as cover more of the sea to find such patchy distributed prey.
"The sheer magnitude of a scientific undertaking to study an oceanic, 25-meter-long, 95,000-kilogram [roughly 82-feet-long, 209,400-pound] wild blue whale has long left researchers with little more than brief glimpses of their presence," said Terrie Williams, director of the center for marine mammal research and conservation at University of California Santa Cruz, who wrote a related Perspective on this study.
Using microprocessor technology, Goldbogen and his colleagues designed submersible wildlife tags with sensors that could attach to whales like suction cups. The sensor-containing tags provided direct measures of feeding rates, body size, confirmation of feeding events, and prey quality — the types and sizes of prey found in the stomachs of toothed whales and the acoustically measured biomass, density and distribution of krill and other small creatures at toothless whales' foraging hotspots.
"These biologging tags provided a form of digital natural history that appeals to anyone interested in how animals work and how they interact with their environment," said Goldbogen.
Tagging around 300 toothed and filter-feeding whales— from the smallest porpoise to the colossal blue whale — Goldbogen and his team calculated the whales' energetic efficiency, or energy from captured prey divided by energy spent. They confirmed previous hypotheses that larger body size for both toothed and toothless whales increased their energy efficiency by allowing greater consumption of prey and effective capturing of prey.
However, all toothed whales, whether large or small, sometimes fed on small prey because of a lack of large prey like giant squid in their hunting grounds. This indicated that the energy gained from deep-sea hunting was constrained by limited abundance of prey attainable during one dive, a factor that might ultimately limit their size.
By contrast, filter feeders, consuming prey-laden water via giant gulps or by continuously ramming water into their mouths, consistently exhibited rapid increases in energy from food. The total biomass and energetic content of their tiny prey exceeded on average those of the largest toothed whale prey.
Unlike toothed whales, the size of filter feeders might be limited by their biology, such as their ability to gulp as much krill-enriched water as quickly as possible, rather than prey availability, Goldbogen postulated. Filter feeding might have fueled an evolutionary pathway to gigantism not available to toothed whales, by exploiting vast quantities of small prey at high efficiencies.
"I think we have made a major step forward in highlighting the physiological drivers and ecological limits to size," said Goldbogen.
He and his team are seeking other clues to illuminate whales' relationship with their food, such as how filter feeders know where to find their krill in a vast and seemingly featureless ocean.
[Credit for associated image: Cyrille Humbert/ Flickr]