How whale body sizes are kept in check decoded
The energy that whales spend during feeding may exceed what they get from their prey, keeping the body size of the Earth's largest animals in check, according to a study. The study, published in the journal Science, noted that the size to which whales grow is limited by prey availability, and some of the marine mammals have evolved a strategy to achieve the largest body sizes of any animal to have ever lived on the Earth.
The researchers, including those from the Smithsonian National Museum of Natural History in the US, sought and tagged whales, porpoises, and dolphins of various sizes -- from 5-foot-long harbour porpoises to gigantic blue whales.
They used multi-sensor tags temporarily affixed via suction cups, reaching from their boats with long poles to stick them on to the animals' backs.
The accelerometers, pressure sensors, cameras, and hydrophones in these high-tech tag devices reported on the mammals' movements as they submerged to feed, the scientists said.
They also used sonar devices, and past records of prey in whale stomachs, to estimate the density of prey in each tagged predator's vicinity.
Collecting these measurements, the researchers obtained data from more than 10,000 whale feeding events in the waters from Greenland to Antarctica, and calculated the energetic costs, benefits, and total payoff of foraging for each whale.
The results of the study revealed that the relationship between body size, and energetic payoff depended on what feeding strategy a whale had evolved to use. The researchers explained that whales were either filter feeders -- which gulped down schools of prey, and strains them from ocean water -- or instead, toothed hunters that caught prey individually.
"Energy is a key currency for all life, and we wanted to know how energy gain compares to energy use in foraging whales with different body sizes and feeding strategies," said Jeremy Goldbogen, study co-author from Stanford university in the US.
"The ratio of energy gain relative to energy use reveals a whale's foraging efficiency and that provides clues as to why different whales are big and why they aren't bigger," Goldbogen said.
The scientists explained that blue whales, humpbacks, and other filter-feeding whales used baleen -- rows of flexible hair-like plates in their mouths -- to strain krill and other small prey from ocean water.
These whales seek out dense patches of their prey, and almost always consumed more calories than they expended while feeding, they said.According to the study, filter-feeding whales had no impediment to foraging because of their large size. The researchers said blue whales, fin whales, and humpback whales -- the largest whales in the current study -- achieved greater energy payoff during feeding events than the other whales.
Citing another example, they said, toothed whales used echolocation to forage, and were limited to feeding on one prey target at a time. According to the researchers, this usually involved diving deeper than other whales to find the largest and most abundant prey. Nicholas Pyenson, study co-author and curator at the Smithsonian National Museum of Natural History, said below 1,000 feet, "there's nothing else down there except all the squid you can eat."
But he added that the squid must be chased, taking a lot of energy -- especially for the biggest toothed whales. In some cases, the researchers said, the largest toothed whales did not eat enough food during a dive to make up for the energy they spent getting there.
"They literally can't eat enough to achieve a higher energetic payoff before they have to return to the surface and breathe," Pyenson explained. The study also noted that at high latitudes, closer to the poles, the largest baleen whale species must reap the energy gains from pathes of krills -- in only a few of the most productive summer months.
"Highly efficient filter-feeding strategies mean that these whales can build up fat stores that can then power their migrations across ocean basins to breeding grounds at lower latitudes that are leaner and provide much less food," Goldbogen explained.
The study also emphasises the precarious position that all whales hold within their ecosystems, especially during this era of human driven climate change, overfishing, and other threats to the oceans. "You have to wonder just how perilous it is for whales living on an energetic knife's edge," Pyenson said. "If you're a blue whale and your only prey item is krill, and something causes krill populations to go into decline, then you are at an evolutionary dead end because you would not be able to eat enough to sustain yourself," he cautioned.
