Sperm whales are tagged by the research team in the Caribbean Sea near the island of Dominica. The research team's findings, reported online Dec. 12 in the journal Science, reveal that body size in all whales is limited by the availability of their prey, but only filter-feeding whales have evolved a feeding strategy that rewards and drives them to achieve the largest body sizes to have ever evolved on Earth. In contrast with filter-feeding whales that strain dense patches of krill and other small prey from ocean water, toothed whales, like sperm whales, use echolocation to forage and are limited to feeding on one prey target at a time. The team's data showed that hunting takes a lot of energy for toothed whales--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 prey to maintain their body size if they were any larger--there simply are not enough large prey in the ocean to sustain bigger sperm whales.