Modern cetaceans—whales and dolphins—have one of the largest size ranges of any living animal group, ranging from as small as 1.5 meters long to as many as 30. However, a new study reveals that this diversity came much later in their evolution than expected. .
To get a more detailed look at the evolution of cetaceans, a team of scientists, including Dr. Travis Park and Dr. Gustavo Buren, both Leverhulme postdoctoral researchers at the Natural History Museum, collected body length measurements for 345 different species, including 89 living and 256 fossil genealogical species, in the largest data set of its kind. Their article, “The Dynamic Adaptive Landscape of Cetacean Body Size” is published in Current Biology.
By comparing the body lengths of both living and fossil species—an approach rarely taken—the research revealed that evolutionary trends in cetacean size remained the same for more than 20 million years after they entered the ocean.
Although there were whales of all sizes during this period, they were all evolving towards an optimal length of about 12.5 metres, which is about the size of a humpback whale.
“While previous studies have looked at the evolution of whale body size, few have collected measurements from living and fossil species, and none has been comprehensive in terms of the species included,” said co-lead author Dr. Gustavo Buren.
“The inclusion of fossils fills in many gaps in the evolution of these animals, demonstrating that evolutionary patterns in body size are much less clear than expected. This is an interesting finding that illustrates the limits of excluding extinct species from studies of evolutionary trends.”
After shifting toward developing larger sizes early in their evolution, this trend has remained constant for millions of years. The second shift occurred about 30 million years ago, as the ancestors of modern dolphins adapted to become fast-moving and agile predators.
However, the most extreme size modifications occur only in the individual branches of the family Cetaceans closest to the present. For example, the average size of the ancestors of baleen whales increased by up to 175%.
The researchers observed the changes by visualizing them as adaptive landscapes, where evolution is seen as a series of peaks and troughs. The best properties in a given environment are represented as peaks, and the higher they are, the more advantage they provide.
The team found that when fossil cetaceans were included with living species, the body size peaks disappeared. Instead, they found that the adaptive landscape was mostly flat, with few peaks.
The first cetaceans were goat-sized animals that lived at the edge of lakes and rivers, spending time in and out of the water. Over time, their descendants increasingly adapted to life in the water, before leaving land completely behind.
Species that return to the water after living on land are known as secondary aquatic, and in addition to cetaceans include animals such as penguins and crocodiles.
These species face a unique set of challenges after they move into the water, and evolving to be larger in size is one way to solve many of them.
Dr Travis Park, co-lead author of the paper, said, “Many species that return to water tend to increase in size soon after they make this transition, as it is often an advantage. For example, small species quickly lose heat underwater so An increase in body size can help maintain body temperature.”
“Returning to the water might also trigger evolutionary constraints imposed by forces such as gravity, which would allow the evolution of much larger body sizes. It is hard to pinpoint the most prominent cause, as body size is determined by a combination of different factors all working together.” .
For example, the minke whale, the smallest baleen whale, has experienced an overall decrease in size. It has recently been suggested that this may allow the species to target smaller and more maneuverable spots of krill at night, when other whales are not active.
More than a third of all cranial measurements used in the paper came from Natural History Museum specimens, and total length data for 1,659 individuals retrieved from the NHM cetacean strandings database demonstrates the importance of museum collections and research.
Going forward, the team of scientists responsible for this latest research wants to apply a similar approach to other secondary aquatic animals, with seals next door to consider. Over time, they hope to see how these groups have changed, and how competition between them might affect their mutual development.
Gustavo Buren et al., The Adaptive Dynamic Landscape of Cetacean Body Size, Current Biology (2023). DOI: 10.1016/j.cub.2023.03.014
This story is republished with permission from the Natural History Museum. Read the original story here
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