Fossils of whale ancestors reveal the path to becoming Earth's largest mammals

Whales were surprisingly consistent on their evolutionary journey from the land to the sea.

Modern cetaceans (whales and dolphins) have one of the largest size ranges of any living animal group, from as small as 1.5 metres long to as many as 30. However, a new study reveals this diversity came much later in their evolution than expected.

By comparing body lengths from living and fossil species, the paper shows that the evolutionary trends in cetacean size remained much the same for over 20 million years after they entered the ocean.

While whales of all sizes existed over this period, they were all evolving towards an optimum length of around 12.5 metres long, which is about the size of a humpback whale.

Dr Gustavo Burin, a Leverhulme fellow at the Museum and co-lead author of the new study, says, 'While previous studies have looked at the evolution of cetacean body size, few had ever combined measurements from living and fossil species, and none was this comprehensive in terms of included species.'

'The inclusion of fossils fills a lot of gaps in the evolution of these animals, showing that the patterns of evolution 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.'

The findings of the study were published in Current Biology.

Why did whales become so big?

The first cetaceans were goat-sized animals which lived on the edge of lakes and rivers, spending time both in and out of water. Over time, their descendants became increasingly adapted to life in water, before leaving land behind altogether.

Species which 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 getting big is one way to solve many of them.

Dr Travis Park, a co-lead author of the paper, says, 'Many lineages which returned to the water tend to increase in size soon after making this transition, as it is often an advantage. For example, small species lose heat rapidly underwater, so getting large can help to maintain body temperature.'

'Returning to the water might also release evolutionary constraints imposed by forces such as gravity, which would allow much larger body sizes to develop. It's difficult to say which is the most prominent cause, as body size is decided by a range of different factors all acting together.'

After a shift towards evolving larger sizes early in their evolution, this pattern remained constant for millions of years. The optimum size of cetaceans only began to fall around 30 million years ago as the ancestors of modern dolphins adapted to become fast-moving, agile predators.

The ancestors of river dolphins such as the baiji go even further, with their optimum body size shrinking to just over two metres as they adapted for life in shallow coastlines, and eventually freshwater.

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

'These whales may have been able to get so big because of their feeding style,' Travis says. 'Lunge feeding is more efficient for bigger animals, so this could have driven adaptation for greater size.'

Only the minke whale, the smallest member of the group, experienced a general decline in size. A recent paper suggests that this may allow the species to target smaller and more manoeuvrable patches of krill at night, when other whales aren't as active.

What are adaptive landscapes?

The researchers observed the changes by visualising them as adaptive landscapes, where evolution is seen as a series of peaks and troughs. The best characteristics in a given environment are represented as peaks, and the taller they are, the more of an advantage they provide.

As a species evolves, its course of evolution can be seen as it climbs the hill towards becoming well-adapted. Previous research into the evolution of body size in whales has found peaks at very large and very small body sizes, but these tended to be focused on either living species or fossils – not both at the same time.

It's important to look at these landscapes over a long period because the peaks can move as different characteristics become more or less important for survival.

To get a more detailed look at cetacean evolution, the team gathered body length measurements of 345 different species, including 89 living animals and 256 fossil lineages, in the largest dataset of its kind.

'Fossils are usually less than complete, so they aren't often incorporated into these datasets,' Gustavo explains. 'To account for this, we used the relationship between skull size and body length in living animals to estimate how large extinct species would have been.'

'We ensured that these estimates were robust by testing the relationship on modern animals, which provided reliable results that were close to their actual values.'

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.

They compared their findings to the surface of an ocean, which tends to be generally flat but with occasional large waves, such as the shrinking of dolphins, causing major change. Meanwhile, ripples on the ocean only affect small areas, representing cetaceans adopting different life strategies.

Going forward, the team want to apply a similar approach to other secondarily aquatic animals, with seals next to be looked at. Over time, they hope to see how these groups changed, and how competition between them could have affected their mutual evolution.