Given the vast expanse of ocean on our planet, compared to the somewhat limited land space, you may be forgiven for thinking animals in water have the potential to grow bigger and bigger.
Yet new research has revealed that mammal growth is more constrained in water than on land, and we may have already reached the maximum.
By analysing data sets of almost 6,000 species, researchers at Stanford have found that animals size is bound at the small end by the need to retain heat, and at the large end by difficulties getting 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’s School of Earth, Energy & Environmental Sciences. “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.”
To study the size of animals on both land and sea, the Payne’s team compiled a so-called phylogenetic tree made up of body mass data from 3,859 living and 2,999 fossil mammal species. These species range from dogs and elephants, which are closely related to seals, sea lions and manatees respectively, and hippos, which are closely related to whales and dolphins
From its analysis, the team discovered that once land animals take to the water, they evolve quickly toward their larger size, converging at around 1,000 pounds. Generally speaking, the smaller the animal on land, the faster they grow to reach their optimal weight when in water – the exception being the otter, which only took to the water more recently, due to the amount of time they spend on land.
This rapid growth is due to the fact that being larger in water helps aquatic mammals retain heat in water that’s lower than their body temperature. “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 explained.
Generally speaking, the smaller the animal on land, the faster they grow to reach their optimal weight when in water – the exception being the otter due to the amount of time they spend on land
An animal’s metabolism similarly with size more than an animal’s ability to gather food, which puts an upper limit on how big aquatic mammals can grow.
“The range of viable sizes for mammals in the ocean is actually smaller than the range of viable sizes on land,” Payne said. “To demonstrate that statistically and provide a theory behind it is something new.”
If otters are the exception at the small end, baleen whales, such as the blue whale, prove the exception at the larger end of the scale. In particular, they expend less energy on feeding than their toothed counterparts because they filter their food, this makes them more efficient and allows them to grow larger than toothed whales.
“The sperm whale seems to be the largest you can get without a new adaptation,” lead author Will Gearty, a graduate student at Stanford Earth said. “The only way to get as big as a baleen whale is to completely change how you’re eating.”
The research is published in Proceedings of the National Academy of Sciences.
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