Evolution: food supply limits whale size (Evolution)

by David Turell , Thursday, December 12, 2019, 20:08 (1806 days ago) @ David Turell

It seems to depend on being able to eat enough:

https://phys.org/news/2019-12-limits-ocean-heavyweights-prey-curb.html

"'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. "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?"

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"'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 relationship between body size and energetic payoff, they found, depended on what feeding strategy a whale had evolved to use—whether a whale was a filter feeder that gulps down schools of prey and strains them from ocean water in their mouth or, instead, a toothed hunter that catches prey individually.

"Blue whales, humpbacks and other filter-feeding whales use baleen—rows of flexible hair-like plates in their mouths—to strain krill and other small prey from ocean water. They seek out dense patches of their prey and almost always, the data showed, consume more calories than they expend when they feed. For filter-feeding whales, large size is no impediment to foraging: blue whales, fin whales and humpback whales, the largest whales in this study, achieved greater energy payoff during feeding events than any other whale in the study.

"Toothed whales, instead, use echolocation to forage and are limited to feeding on one prey target at a time. They must also dive deeper than other whales to find the largest and most abundant prey, like deep-sea squid and fish. Few other warm-blooded predators can access the parts of the ocean where large toothed whales feed. Below 1,000 feet, Pyenson said, "there's nothing else down there except all the squid you can eat." But squid must be chased, and that, the data showed, takes a lot of energy—especially for the biggest toothed whales. In some cases, 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 said.

"Sperm whales, which can be up to 60 feet long, are not only larger than any other of today's toothed whales, but are also bigger than all of their fossil ancestors. That makes sense, Pyenson said, because based on the relative energy efficiencies that the team calculated for different-sized toothed whales, "being a sperm whale today is really pushing a serious biological limit." The team's calculations suggest that sperm whales would not be able to find enough of the largest squid prey to maintain their body size if they were any larger—there simply are not enough large squid in the ocean to sustain bigger sperm whales.

"In contrast, large filter-feeding whales are not limited in their body size by the availability of their prey in the same way as toothed whales. Filter-feeding whales feed on small but very abundant krill prey that flourish at high population densities for short periods of time in specific parts of the world. As a result, Goldbogen, Pyenson and colleagues speculate that the seasonal availability of their abundant prey is what ultimately limits size in today's filter-feeding ocean giants like fin whales and blue whales.

"'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."

"The new study underscores the precarious position that all whales hold within their ecosystems. "You have to wonder just how perilous it is for whales living on an energetic knife's edge," Pyenson said—particularly in the face of climate change, overfishing and other threats to the oceans.

"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.'" (my bold)

Comment: Very educational study of whale size limits. They can evolve to a dangerous size for their continuing existence. My bold is a major point. Top predators control ecosystems unless or until they eat themselves out of a job, or prey disappear. The bush of life requires active econiches/systems