Scientists at the University of Bristol have found that body size is more important than body shape in determining the energy economy of swimming for aquatic animals.
This study, published today in Communications biology, shows that large bodies help overcome the excess drag produced by extreme morphology, debunking a long-standing idea that there is an optimal body shape for low drag.
An important finding from this research is that the large necks of extinct elasmosaurs added further resistance, but this was offset by the evolution of the large bodies.
Tetrapods, or “four-limbed vertebrates,” have returned repeatedly to the oceans over the past 250 million years and come in many shapes and sizes, ranging from streamlined modern whales over 25 meters in length, to extinct, four-finned plesiosaurs. and extraordinarily long necks and even extinct fish-shaped ichthyosaurs.
Dolphins and ichthyosaurs have similar body shapes, adapted to move quickly through water while producing low drag or drag. On the other hand, plesiosaurs, who lived side by side with ichthyosaurs in the Mesozoic era, had completely different bodies. Their huge four fins that they used to fly underwater and varying neck lengths are unmatched among living animals. Some elasmosaurs had truly extreme proportions, with necks up to 6 meters long. These necks likely helped them bite into fast-moving fish, but were also believed to make them slower.
Until now, it was unclear how shape and size affected swimming energy needs in these different marine animals. Paleobiologist Susana Gutarra Díaz of the School of Earth Sciences in Bristol and the National History Museum in London who led the research, explained: “To test our hypotheses, we created various 3D models and performed computer flow simulations of plesiosaurs. ichthyosaurs and cetaceans. These experiments are performed on the computer, but they are like experiments on water tanks. “
Dr Colin Palmer, an engineer involved in the project, said: “We showed that although plesiosaurs experienced more resistance than ichthyosaurs or whales of equal mass due to their unique body shape, these differences were relatively minor. We found that when size was taken into account, the differences between the groups became much smaller than the differences in shape. We also show that the ratio of body length to diameter, which is widely used to classify these aquatic animals as more or less efficient, is not a good indicator of low drag ”.
Dr Gutarra Díaz further explains: ‘We were also particularly interested in the necks of elamosaurs and so we created hypothetical 3D models of plesiosaurs with various neck lengths. Simulations of these models reveal that beyond a certain point, the neck adds extra strength. , which would potentially make swimming expensive. This “optimal” neck limit lies about twice the length of the animal’s trunk. “
Dr Benjamin Moon, another contributor and marine reptile expert, continued: “When we examined a large sample of plesiosaurs modeled on really well-preserved fossils at their actual size, it was found that most plesiosaurs had necks at the back. below this high resistance threshold, within which neck can lengthen or shorten without increasing resistance. But more interestingly, we showed that the extremely long necked plesiosaurs had also developed very large torsos, and this made up for the extra stamina!
Dr Tom Stubbs, another co-author, summarized: “This study shows that, in contrast to prevailing popular knowledge, very long-necked plesiosaurs were not necessarily slower swimmers than ichthyosaurs and whales, and this is partly due to to their great bodies. We found that neck proportions changed very quickly in elasmosaurs. This confirms that long necks were beneficial to the elasmosaurs when hunting, but they could not take advantage of this adaptation until they became large enough to offset the cost of the high drag on their bodies. “
Professor Mike Benton, also part of the research, commented: ‘Our research suggests that large aquatic animals can afford to have crazy shapes, such as in elasmosaurs. But there are limitations: the body size cannot become indefinitely large, as there are some constraints even for very large sizes. The maximum neck lengths we observe seem to balance the benefits of hunting against the costs of growing and maintaining such a long neck. In other words, the necks of these extraordinary creatures have evolved in balance with the overall size of the body to minimize friction. “
“The large size in aquatic tetrapods compensates for the high aerodynamic drag caused by extreme body proportions” by S. Gutarra, T. Stubbs, B. Moon, C. Palmer and M. Benton in Communications biology.
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