Secrets of the sengi: the world's fastest small mammal

A Rufous elephant shrew investigates a butterfly Super swift sengis rarely pause for long

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What's small, furry, related to an elephant and really fast?

Not a bizarre punchline as you might expect but the sengi: the world's fastest small mammal.

This seemingly contrary combination of features has enthralled scientists since they first discovered these animals at the turn of the 19th Century.

At first glance they might look like just another shrew - a small, furry mammal that loves to eat insects - and this is how they were initially described.

But decades of scientific attention has shown the tiny tearaways could hold substantial secrets about how mammals evolved.

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Their long noses earned the animals comparisons to elephants and the family of 18 species native to Africa are also known as the elephant shrews.

Strangely, scientists in the 1990s discovered that despite their diminutive size, they are genetically more closely related to their elephant namesakes than to shrews.

Their motley branch on the tree of life was named Afrotheria and includes aardvarks, sea cows such as dugongs and manatees, elephants, hyraxes and Madagascar's tenrec.

The habits of sengis have continued to surprise biologists: they eat a termite and ant diet which is more common to larger mammals, do not nest like other small mammals, hibernate overnight and have remarkably long bones in their feet.

According to experts the latter two of these features are a result of their adaptation to running, which makes them unique among small mammals.

Need for speed

Start Quote

Elephant shrews are the fastest mammals on Earth that are smaller than half a kilogram”

End Quote Prof Barry Lovegrove University of KwaZulu-Natal, South Africa

"I first became fascinated by elephant shrews when I was walking in the veld in the Namib Desert in 1990 and a [short-eared elephant shrew] ran past me. I was stunned at the speed that this creature could attain." says Prof Barry Lovegrove from the University of KwaZulu-Natal, South Africa.

Alongside graduate student Metobor Mowoe, Prof Lovegrove investigated two particular species and the results have just been published in the Journal of Experimental Biology.

Sengis live in a variety of different habitats, from dense forest to sparse areas with only boulders for cover, but they all have one thing in common: they are always on the move, running fast to avoid predators and stay safe.

According to Prof Lovegrove a simple sum to calculate how fast the animals can cover their own body length would show that "elephant shrews leave cheetahs in their dust."

"Evolutionary physiologists get a bit cranky when you 'correct' for an animal's size for comparative purposes by dividing a measure such as maximum running speed by the body length of the animal," he concedes.

But he adds that alternative calculations considering residual running speeds still fail to accurately compare the performance of both animals because the relationship between speed and body mass differs so greatly for large and small mammals.

"Nevertheless, what you can claim using this method, is that elephant shrews are the fastest mammals on Earth that are smaller than half a kilogram," he says.

In their analysis of western rock elephant shrews (Elephantulus rupestris) and Cape elephant shrews (E. edwardii), Prof Lovegrove and Ms Mowoe found that the animals could reach speeds of 28.8 km/h.

"I am convinced that species that occupy more open habitats, such as [short-eared elephant shrews], can run faster than 30 km/h," says Prof Lovegrove.

All in their stride

In addition to speed tests, the research looked at the animals' physical attributes.

"To run fast, an animal needs long legs to increase stride length. The only bones that were available for natural selection to 'play around with' for elongation of the limb, without compromising its strength, were the small bones in the foot, the metatarsals," he explains.

Looking at existing records, researchers found that giraffes have an exceptional leg to foot bone ratio due to their uniquely long proportions but the next highest ratios were found in a handful of antelopes which have adapted to outpace athletic predators.

Immediately after these big mammals came the small sengis.

The tunnel constructed to test elephant shrew speeds Researchers constructed a tunnel to test the sengis' running speeds

"All mammals smaller than half a kilogram have [leg to foot bone] values much less than one. Elephant shrews have a value of 1.3!" says Prof Lovegrove.

"For such small mammals, they have the only hindlimbs which are adapted specifically for running fast, very fast."

All this dashing around results in a very high metabolic rate and costs the animals a lot of energy.

Previous research revealed how the small speedsters enter a state of torpor at night, effectively a short-term hibernation to lower their body temperature and conserve energy for their fast lifestyles.

For Prof Lovegrove, every new revelation about sengis is another piece in the jigsaw of mammalian evolution.

"Why this is all so fascinating to me is that the elephant shrews show physiological characteristics, such as the highest body temperatures of all Afrotherian, more typical of mammals that evolved in the Americas," he says, describing how he is currently researching "the evolutionary driving force for elevated metabolism".

"I am trying to piece together the conditions under which the ancestral placental mammals evolved so incredibly rapidly once the asteroid impact killed off the non-avian dinosaurs 65 million years ago."

The further study of these strange small mammals could provide answers to some very big questions.

Hidden Kingdoms features the secretive lives of sengis and begins on Thursday 16 January at 2000 on BBC One.

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