How Whales Became the Largest Mammals on Earth

March 30 () –

The diversity of sizes in modern cetaceans (whales and dolphins) -from 1.5 to 30 meters- it came much later in its evolution than expected, a new fossil study reveals.

To get a more detailed look at cetacean evolution, a team of scientists, including Dr Travis Park and Dr Gustavo Burin, both Leverhulme Post-Docs at the Natural History Museum (NHM), collected body length measurements of 345 different species, including 89 living species and 256 fossil lineagesin the largest dataset of its kind. His article is published in Current Biology.

By comparing the body lengths of living and fossil species, an approach rarely taken, the research revealed that evolutionary trends in cetacean size remained roughly the same for more than 20 million years after they entered the ocean.

While whales of all sizes existed during this period, they all evolved towards an optimal length of around 12.5 meters longwhich is about the size of a humpback whale.

Lead co-author Dr. Gustavo Burin said it’s a statement: “While previous studies have looked at the evolution of cetacean body size, few had combined measures of living and fossil species, and none were as comprehensive in terms of species included.”

“The inclusion of fossils fills in many gaps in the evolution of these animals, showing that evolutionary patterns in body size are much less obvious than expected. This is an interesting finding that shows the limitations of excluding extinct species from studies of evolutionary trends.“.

After a shift toward evolving larger sizes early in their evolution, this trend remained constant for millions of years. A second change occurred about 30 million years ago, when the ancestors of modern dolphins adapted to become fast-moving, agile predators.

However, the most extreme size adaptations only take place in individual branches of the cetacean family much closer to the present. For example, the average ancestor size of baleen whales increased by up to 175%.

The researchers looked at the changes by visualizing them as adaptive landscapes, where evolution is seen as a series of peaks and troughs. The best features in a given environment are represented as spikes, and the higher they are, the more benefits they provide.

The team found that when fossil cetaceans were included with living species, the peaks in body size disappeared. Instead, they found that the adaptive landscape was mostly flat, with few peaks.

THE SIZE OF A GOAT

The first cetaceans were goat-sized animals that lived on the shores of lakes and rivers, spending time both in and out of the water. Over time, their descendants became more and more adapted to life in the water, before leaving the earth completely.

Species that return to the water after living on land are known as secondarily aquatic and, in addition to cetaceans, include animals such as penguins and crocodiles.

These species face a unique set of challenges after moving into the water, and evolving to become larger is one way to solve many of them.

Dr Travis Park, co-lead author of the paper, said: “Many lineages that return to the water tend to increase in size soon after making this transition, as this is often an advantage. For example, small species lose heat rapidly under Water, so getting bigger can help maintain body temperature.”

“Returning to the water could also release evolutionary constraints imposed by forces like gravity, allowing much larger bodies to develop. It’s hard to say which is the most important cause, since body size is decided by a variety of different factors, all acting together“.

For example, the minke whale, the smallest baleen whale, experienced a general decline in size. It has recently been suggested that this may allow the species to target smaller, more maneuverable krill patches at night, when other whales are not as active.

More than a third of all skull measurements used in the article come from Natural History Museum specimens, and total length data for 1,659 individuals were retrieved from the NHM cetacean stranding database, demonstrating the importance of the museum’s collections and research.

In the future, the team of scientists responsible for this latest research wants to apply a similar approach to other secondary aquatic animals, with seals next to be observed. Over time, they hope to see how these groups have changed and how competition between them might have affected their mutual evolution.