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| A blue whale in the waters off the coast of Colombo, Sri Lanka, April 2015. |
Many other marine mammals, such as orcas and walruses, are not lightweights either. Scientists over the years have speculated that pressure on body size is more relaxed in water, possibly because the ability to float removes the necessity of supporting body weight on limbs.
New research counters that theory, though, concluding instead that mammal growth is actually more constrained in water than on land. The findings, reported in the Proceedings of the National Academy of Sciences, show that aquatic mammal size is bounded at the small end by the need to retain heat and at the large end by challenges associated with food acquisition.
"We went in just looking to statistically show that aquatic mammals are larger than we would expect by chance," lead author William Gearty, a graduate student at Stanford School of Earth, Energy, and Environmental Sciences (Stanford Earth), told Seeker.
"While we did find that," he added, "we also came away with surprising new knowledge that these mammals are actually under really strong evolutionary pressures, much stronger than those on land, which goes against the expectations of many hypotheses regarding why these animals have gotten so big."
He explained that there are fewer orders of magnitude between the smallest and largest aquatic mammals than there are between the smallest and largest terrestrial mammals.
The smallest known marine mammal is the marine otter (Lontra felina), which weighs up to 13 lbs. The smallest terrestrial mammal by mass is the Etruscan shrew (Suncus etruscus), which weighs only around 0.063 ounces. The largest terrestrial mammal by mass is the African bush elephant (Loxodonta africana), with males weighing around 13,000 lbs. and females weighing about half that much.
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| Marine otter |
All aquatic mammals appear to have had terrestrial ancestors. Seals and sea lions are closely related to dogs, for example, while manatees share ancestry with elephants. Whales and dolphins are related to hippos and to other hoofed mammals.
The researchers found that when the land-based ancestors of marine mammals gradually adapted to aquatic life, they evolved very quickly toward larger sizes and converged at around 1,000 lbs.
"Since mammals are warm-blooded, we need to maintain relatively high internal body temperatures," Gearty said. "Once mammals enter the water, this becomes an even harder process, and the only way to counteract this pressure is to get larger to reduce the relative amount of energy you lose to the water around you."
Marine otter ancestors evolved for an aquatic lifestyle more recently, so they have not yet followed that trend. This could also be because otters still live much of their lives on land.
The largest marine mammals, blue whales, are also clearly an exception to the 1,000-pound trend.
"The tricky thing about an optimum is that, if every species tried to be that size they would all be competing with one another, and that's not good for anyone," Gearty said. "Also, the differences in energy conservation between 100 kilograms (221 lbs.), 500 kg (1,102 lbs.), and 1,000 kg (2,205 lbs.) aren't make-or-break."
"As long as you aren't losing energy, you are doing just fine," he continued. "So, if you can find some niche that lets you survive at a slightly different size, it's better to take advantage of that than try to compete with the other species that are at the optimum."
Blue whales possess a filter-feeder system known as baleen. During peak consumption periods, these marine giants may consume thousands of pounds of krill and copepods, which are a type of small aquatic crustacean.
As large as the blue whale is, the models created by the scientists show that this marine mammal could have an even larger optimal body size. Human-related pressures could be preventing the blue whale from evolving into this bigger size.
McClain told Seeker, "We know that humans have a tremendous impact on the body-size distributions on both land and in the oceans."
He explained that an earlier study came to the same conclusion. Still other research has shown that fishing has resulted in a reduction in overall fish sizes. A prior study led by McClain found that decreases in whale sizes through time have occurred due to whaling.
Other factors affect animal body sizes. During the Dinosaur Era, millions of years before our species emerged, many terrestrial mammals often grew to enormous sizes. Tyrannosaurus rex, for example, is estimated to have grown up to 20 feet in height and to have weighed up to 20,000 lbs.
"Some large sizes were obtained because atmospheric oxygen was greater," McClain said, adding that still other large sizes were obtained because certain animals didn't have many, if any, competitors.
When non-avian dinosaurs and numerous other animals went extinct around 66 million years ago, surviving mammals evolved to fill the ecosystem voids. Mammals that eventually evolved into aquatic species must have been living initially along the edges of bodies of water, consuming prey like fish or marine invertebrates. Research on such a transition from land to water life was published a few years ago in the journal Science.
"Once animals get hooked on that diet, the only way to make that diet more efficient is to move into the water," Gearty said. "It's a gradual process for sure, which we see occur over millions of years in the fossil record."
He added, "While it's likely a very difficult process, mammals have made the full transition at least six times in the fossil record. All of the transitions seem to be at different times in the past, so it doesn't look like it was climate-driven."
One of the most controversial past theories for human evolution is the aquatic ape hypothesis. First proposed by marine biologist Alister Hardy (1896–1985) in 1960, AAH holds that the ancestors of modern humans were more aquatic. Even if some were, they obviously did not evolve into marine mammals.
As Gearty said, "You certainly won't see any fully aquatic humans any time soon, I can say that for certain! At least, not without a lot of technology involved. If we did want to invade the aquatic realm, it would take millions of years, a lot more hair — or blubber — and a lot more mass!"
Our species' average body size, as well as that for all other animals, could be explained by basic principles of physics and chemistry. Coming up with a mathematical equation to explain this has, however, vexed scientists for decades.
McClain said, "While the principles may be basic and the model is elegant in its simplicity, often times the simplest solutions take years to come by. Not that I want to say our work is on par with Albert Einstein's, but E=MC2 is a rather simple formula founded on basic principles, but it took years for physics to get to this point."
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