A giant rat as big as a human and weighing nearly thirteen stone walked the Amazon rainforest in prehistoric times, scientists have confirmed.
The creature could reach up to five feet in length – making it the biggest rodent ever to roam South America.
Named Neoepiblema acreensis, it had two huge curved incisor teeth for gnawing nuts and prey.
The skulls of two individuals were found at a fossil site at Acre in the western Brazilian Amazon. One was almost complete and the other included a fragment of crania, the part that encloses the brain. It was so well preserved it even had impressions of olfactory bulbs that process odour and the frontal and temporal lobes that control thoughts and actions.
Lead author Dr Jose Ferreira said: ‘Neoepiblema was about five feet long and weighed around 80 kg (12.6st), which surpassed the capybara, the largest living rodent which is about 60 kg (9.5st).
‘This rodent is an extinct relative of the chinchillas and pacaranas and inhabited the western Brazilian Amazonia about 10 million years ago.
‘It lived in swampy environments that existed there before the emergence of one of the largest tropical forests in the world.’
Its size meant it had few predators, perhaps just large crocodiles that would have used ‘sit and wait strategies’, pouncing if one wandered past.
Neoepiblema, described in the journal Biology Letters, was not very bright. A digital reconstruction of its brain using CT (computed tomography) scans showed it was very small, weighing just 4 ounces. A human brain is about 3lbs.
Dr Ferreira, of the Federal University of Santa Maria, explained: ‘The adaptive value of a low energetic cost and other ecological factors are possibly associated with the relative small brain size of giant rodents.
‘Although Neoepiblema was one of the largest rodents ever, the brain of this giant rodent was very small relative to its body mass.
‘The evolution over time of this relationship between brain and body size is known as encephalisation.’
To analyse the differences among animals of different body sizes, researchers calculate the ‘encephalisation quotient’ (EQ). This is a way to measure the difference between the expected size for an animal of a certain weight and the brain’s actual size.
For example, humans have an average EQ of roughly 6. For most living South American rodents it is around 1.05. But for Neoepiblema it was about 0.3, which is very low, said Dr Ferreira.
And the reason was that brains burn up lots of energy – which the creature needed to power its huge body. It did not need to waste it on looking out for active predators.
Dr Ferreira said: ‘When Neoepiblema inhabited South America, carnivorous placental mammals such as felids, canids and ursids had not yet arrived on the continent, since the Isthmus of Panama was not yet formed and there was no terrestrial connection with Antarctica.
‘Thus, South America was isolated, like a giant island. The main predators of these giant rodents were crocodilians, who were also giants and inhabited the marshy regions. They were not active predators, as are mammals.
‘Thus, predation pressures were different from what they would become from the end of the Pliocene and Quaternary when the Isthmus of Panama was formed and the large carnivores entered the continent during an event known as the Great American Biotic Interchange (GABI).
‘There was no need for giant rodents to have large brains since this would imply unnecessary energy consumption. After GABI, there was a significant increase in the degree of encephalization of rodents
‘The same pattern has been documented in animals that live on islands where there are no large predatory mammals.’
There are more than 2,000 living species of rodents – constituting almost half the different mammals on Earth. They include such diverse groups as porcupines, beavers, squirrels, marmots, pocket gopher and chinchillas.
Dr Ferreira said: ‘Some extinct South American members of this clade reached giant body size during the Late Miocene, some 10 million years ago.
‘Neoepiblema acreensis is one of the largest rodents that inhabited South America. N. acreensis belonged to a diverse group of rodents known as caviomorphs that derived from African forms that rafted to the continent around 50 million years ago.’
Dr Ferreira said: ‘Today’s caviomorphs include the capybaras, porcupines, guinea pigs and chinchillas, among several others. But during the Late Miocene and Pliocene periods between about 10 and 2.5 million years ago several lineages of South American rodents reached gigantic proportions – some more than 500kg (79st).’
To put this in perspective, the average brown rat today has a body length of less than a foot and weighs about half a pound.
The phenomenon was unique to South America. Rodents are cosmopolitan, but only here did they develop gigantism in so many lineages.
Dr Ferreira said: ‘In addition to these large rodents, other groups of mammals exclusive to South America and giant reptiles also lived during that time. This is the case with Purussaurus, a prehistoric alligator that could reach 12 metres (40ft) in length.’
The largest living rodent is the capybara. It can reach up to four feet in length and weighs around 7 stone.
Josephoartigasia monesi, a rodent closely related to guinea pigs, which lived in South America three million years ago, is the largest fossil rodent ever found. With an estimated body mass of 2,200lbs (1,000 kilos), it was similar in size to a buffalo.
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