Blue whale may be heaviest animal ever, scientists discover

The Perucetus colossus was a 17-metre-long whale, believed to have weighed as much as 340 metric tonnes – ALBERTO GENNARI/PA

A 40-million-year-old fossilised whale previously thought to be the heaviest animal of all time is now thought to weigh less than a blue whale.

The huge, 17-metre-long whale, which was believed to have very dense bones, was initially thought to weigh as much as 340 metric tonnes – equivalent to 150 cars piled on top of one another.

But American scientists have now downgraded its weight – saying it wouldn’t have been able to survive or even swim if it weighed as much as previously predicted.

In a new study, published in the journal PeerJ, the ancient animal was returned to the scales and slimmed down to a weight less than half that of the largest known blue whales.

The fossil skeleton of the whale, known as Perucetus colossus, was previously thought to be the heaviest animal ever to have roamed the Earth.

The first vertebra of the sea giant was discovered in Peru’s Ica River Valley more than a decade ago.

It’s estimated to have lived around 39 million years ago and belonged to an extinct group of early whales called the Basilosaurids.

Scientists found the Perucetus’ bones to be unusually dense.

Mammal bones normally have a solid exterior and a spongy or hollow centre, but some animals’ bones have a centre more filled-in with solid bone, making the bones extremely dense and heavy.

In aquatic animals, heavy bones can offset buoyancy from body fat and blubber, allowing the animal to maintain a neutral buoyancy in water or – in the case of the hippopotamus – to walk on river beds.

The Perucetus bones were found to have both extensive in-filling and extra growth of bone on the outside as well; a condition called pachyostosis also seen in some modern aquatic mammals such as manatees.

Based on a series of assumptions, scientists originally estimated the whale’s body mass to have weighed around 180 metric tons, ranging up to a maximum of 340 metric tons – more than 23 double-decker buses.

These estimates put the Perucetus as being as heavy as or heavier than the biggest blue whales ever known – even though it’s considerably shorter at just 17 metres long, compared to blue whales which can measure up to an incredible 33 metres (110ft).

However, a new study undertaken by palaeontologists at the University of California (UC) Davis and the Smithsonian Institution, disputes these weights and insists such estimates would have made the Perucetus impossibly dense.

Blue whales which can measure up to an 33 metres (110ft) – DENIS SCOTT

Prof Ryosuke Motani, a paleobiologist at the UC Davis Department of Earth and Planetary Sciences, explained: “It would have been a job for the whale to stay at the surface, or even to leave the sea bottom.

“It would have required continuous swimming against the gravity to do anything in the water.”

Dr Motani, along with Dr Nick Pyenson of the Smithsonian Institute National Museum of Natural History, reexamined the assumptions used to make those estimates to come up with their own revised weight for the colossal beast.

The first problem they identified with previous weigh-ins was the fact they used fossil bones to estimate the weight of the skeleton and extrapolated this to the weight of the entire animal, assuming that the skeletal and non-skeletal mass would scale at the same rate with increasing body size.

But Dr Motani and Dr Pyenson argue that measurements of other animals show this not to be the case.

They therefore believe the original estimates overestimated how much body mass increases as a result of pachyostosis – a condition of the thickening of bones.

Evidence from manatees, for example, shows that their bodies are relatively light relative to their skeletal mass.

Size comparison of a modern blue whale (Balaenoptera musculus) and the extinct Perucetus colossus – CULLEN TOWNSEND/SWNS

The researchers now estimate that the 17-metre-long Perucetus actually weighed between 60 and 70 tonnes.

According to the new estimates, a Perucetus that grew to 20 metres could weigh over 110 tons, still well short of the largest blue whales at 270 tons.

“The new weight allows the whale to come to the surface and stay there while breathing and recovering from a dive like most whales do,” Dr Motani said.

Palaeontologists have not yet uncovered the skull or teeth of a Perucetus, so it is therefore difficult to grasp what they ate.

Sustaining a huge body takes a lot of food, and the previous researchers from Italy suggested the Perucetus might have feasted on coastal fish and shellfish, or scavenged carcasses as some sharks do.

The newly slimmed-down size estimate puts the Perucetus in a similar weight range to sperm whales – at 80 tons and 20 metres long – which hunt large prey such as giant squid.

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