(CN) – Despite hosting some of the largest creatures on Earth, the ocean limits mammal growth more so than land, according to a new study.
The report, published Monday in the journal Proceedings of the National Academy of Sciences, contrasts previous theories that limits on body size should be more relaxed in water due to the vastness of the ocean and the ability of animals to float, rather than bearing their weight on legs.
Instead, a team of researchers found that marine mammal size is limited, at the small end by the need to preserve internal heat and at the large end by the challenge of consuming enough food to survive.
“Many people have viewed going into the water as more freeing for mammals, but what we're seeing is that it's actually more constraining,” said co-author Jonathan Payne, a professor of geological sciences at Stanford University. “It's not that water allows you to be a big mammal, it's that you have to be a big mammal in water – you don't have any other options.”
While aquatic mammals share a similarly oblong body shape, they are not closely related. Sea lions and seals are closely related to dogs, manatees share ancestry with elephants, and dolphins and whales are kin to hippos and other hoofed creatures.
To investigate how these groups of land mammals grew when they entered the water, the team compiled the body masses of 2,999 fossil and 3,859 living mammal species from existing data sets. The analysis considers roughly 25 percent of extinct species and 70 percent of living species.
They examined the data with a set of models refined in collaboration with Craig McClain, an evolutionary ecologist and executive director of the Louisiana Universities Marine Consortium.
The analysis revealed that once land animals descend to the water, they evolve rapidly toward their new size, converging at about 1,000 pounds. Aquatic mammals with smaller ancestors, such as seals, ballooned in size more so than species with larger ancestors, such as whales, to reach that optimal weight.
This suggests bigger is better for aquatic life – but only up to a certain threshold, according to the researchers. They note that otters, which entered the water more recently, don’t follow that trend, potentially because many species still spend much of their time on land.
“The key is having a phylogenetic tree to understand how these species are related to one another and the amount of time that has taken place between different evolutionary branching events,” said lead author Will Gearty, a graduate student at Stanford.
“The tree of ancestral relationships allows us to build models based on data from modern species to predict what the ancestors' body sizes would have been and see what evolutionary trajectories best fit with what we see in the modern day.”
The team believes that being larger helps aquatic mammals retain heat in water that is colder than their body temperatures. “When you're very small, you lose heat back into the water so fast, there's no way to eat enough food to keep up,” Payne said.
They also suggest the creatures’ metabolisms heighten with size more than their ability to gather food, which limits how large they can grow.
“Basically, animals are machines that require energy to operate. This need for energy places hard limits on what animals can do and how big they can be,” said McClain, who was a co-author of the report. “The range of viable sizes for mammals in the ocean is actually smaller than the range of viable sizes on land.
“To demonstrate that statistically and provide a theory behind it is something new.”
If otters are the aberration at the compact end, baleen whales are the exception at the large end. The whales expend much less energy on eating than their toothed counterparts because they filter their food, which makes them more efficient and enables them to grow larger.
“The sperm whale seems to be the largest you can get without a new adaptation,” said Gearty. “The only way to get as big as a baleen whale is to completely change how you're eating.”
The team is currently analyzing how well similar approaches can be used to clarify body-size distributions in other animal groups, particularly those that have both aquatic and terrestrial species.
“The hope is there's simpler explanations that can apply to other species, including terrestrial animals,” Payne said. “It opens up some possibility that body size can be explained by basic principles of physics and chemistry.”
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