Whale size may be held in check by the availability of prey, a new study reports. While baleen whales have evolved to leverage size as an advantage while feeding — which put them on an “energetic knife’s edge” — toothed whales instead stand to benefit from being less massive.
Growing to more than 100 tons, blue whales are considered to be the largest creatures to have ever roamed the Earth. Seeking to understand why baleen (filter-feeding) whales and toothed whales are so different in body size — and what factors limit their growth — a new study is looking into how much energy the two groups spend when feeding.
“Blue whales and sperm whales are not just kind of big,” said Nicholas Pyenson, curator of fossil marine mammals at the Smithsonian’s National Museum of Natural History and the corresponding author of this study. “They are among the biggest animals ever to have evolved.”
“They rival and, in some cases, exceed the heaviest dinosaurs. That’s pretty spectacular. But why aren’t they bigger?”
Since whales spend most of their time deep underwater, it’s very hard for researchers to actually monitor what they’re doing. The current study, written by an international team of researchers led by Pyenson and Stanford University biologist Jeremy Goldbogen stuck multi-sensor arrays onto the backs of whales, porpoises, and dolphins of various sizes using suction cups.
The sensor arrays were used to track the animals’ underwater activities using accelerometers, pressure sensors, cameras, and hydrophones. Sonar sweeps in the surrounding waters, alongside older records of prey in whale stomachs, were used to estimate the density of prey in the vicinity of each tagged animal. Over 10,000 distinct feeding events were analyzed as part of the study, with the team calculating the energy cost and payoff for each.
“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,” Goldbogen said.
“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.”
The net energy return on feeding, they found, largely depended on how each whale fed — i.e. by hunting individual prey (toothed whales) or by filter-feeding (baleen whales). Body size in all whales is limited by this net return of energy.
Less bang for your kill, more bang for your krill
Filter-feeding whales are the only ones to have evolved a feeding strategy that favors larger body sizes — which turned them into the largest animals to have ever evolved on Earth. Size, they explain, plays right into the feeding style of baleen whales. They feed by straining patches of krill from ocean water — which aren’t particularly smart or able to move out of the way, so catching them is like shooting fish in a barrel. Having a larger body (and mouth) therefore means the whales can generate more calories for the same effort. In other words, their net energy return from feeding increases with the size of the whale.
Toothed whales, in contrast, need to hunt down individual prey, like a cheetah would on land. The whales use echolocation to spot a target and then hunt it down — so they are, in essence, limited to feeding on one target at a time. This hunting process requires a lot of energy, as the prey obviously would rather not be eaten, and does its best to escape. The larger a toothed whale becomes, the more energy it needs to give chase and catch prey. In other words, their feeding strategy favors smaller body sizes.
In some cases, the team reports, larger toothed specimens like sperm whales actually lost energy on some food dives; it simply takes more calories to dive and eat than they get out of what they ate. In effect, energy expenditure related to hunting acts as the hard cap for the size of toothed-whales. There aren’t enough large animals in the ocean for them to grow larger.
“They literally can’t eat enough to achieve a higher energetic payoff before they have to return to the surface and breathe,” Pyenson said.
“Being a sperm whale today is really pushing a serious biological limit.”
Filter-feeding whales, on the other hand, only seek out the densest patches of krill and almost always, the data showed, consume significantly more calories than they expend. The largest whales in this category saw the best energy return rates in the whole study. But they’re also limited in size by prey availability.
Krill population numbers explode but only for short periods of time every year in specific areas of the globe. This seasonal variation is what keeps baleen whale size in check.
Are whales the limit?
“The largest baleen whale species must reap the energy gains of krill patches in only a few of the most productive summer months at high latitudes,” Goldbogen said.
“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.”
While it may seem that filter-feeding whales have it nailed down, Pyenson explains that they’re actually playing a dangerous game here. They’re superbly adapted to take advantage of one resource. It’s an abundant resource, for sure, but if anything were to happen to the supplies of krill, filter-feeding whales have nothing to fall back on. Given their huge bulk, it’s hard to imagine any source of food in the ocean that could serve as a fall-back.
“You have to wonder just how perilous it is for whales living on an energetic knife’s edge,” Pyenson said, noting that climate change, overfishing, and other threats to the oceans are rapidly impacting their food suply.
“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 said. “It’s a good reason for us to try to better understand these predator-prey relationships.”
This study identifies food sources as a limiting factor in whale size, but previous research has suggested that it may be biologically impossible for blue whales to grow any larger anyway due to mechanical constraints on their cardiocirculatory systems.
Beyond the immediate value of the findings, the team says it helps us better understand the dynamics between other huge beasts — particularly dinosaurs — and their food source, and how this limited their growth. “We don’t know how badly a herd of sauropod dinosaurs would chomp down on a forest in the Cretaceous period,” Pyenson adds, “but they probably did a number on it.”
The paper “Why whales are big but not bigger: Physiological drivers and ecological limits in the age of ocean giants” has been published in the journal Science.