Ancient African elephants and their relatives lived in forests, originally munching leaves on trees. But as climate and habitat changed, so did their food preference. Reporting in the journal Nature, a Natural History Museum scientist explains how the shift to grass reveals itself in the evolution of their teeth, but not until millions of years later.
These new findings, by Museum fossil mammal expert Prof Adrian Lister, provide strong evidence that behavioural change, such as a switch in feeding habit, can shape how species evolve.
‘The theory that behaviour may play an important role in the origin of new adaptations has been discussed for over 100 years, but it has been very difficult to find concrete examples,' says Lister.
'The idea is that, by exploring a new habitat or trying a new food resource, species put themselves in a situation where natural selection will modify their anatomy to more efficiently accomplish the new behaviour.’
Lister analysed fossil remains of East African proboscideans (mammals with tusks and trunks such as elephants and mammoths) from the last 20 million years.
Carbon isotopes in the fossil teeth can reveal details of diet, and Lister compared this information with the tooth measurements.
Ancient proboscidean molar tooth (top) showing a low crown with rounded cusps suitable for eating tree leaves, and molar of a fossil species of elephant (bottom) that shows elongated ridges of enamel, more efficient for grass eating. © Bill Sanders
He found that about 8 million years ago various species changed their diet and feeding behaviour from browsing leaves from trees to grazing on grass. However, the teeth structure and shape only showed an adaptation to eating tougher grass from 5 million years ago, that's 3 million years after the shift in behaviour.
The teeth became three times higher and the number of enamel ridges increased.
'This long delay between the behavioural change and the evolutionary response suggests that behaviour led the way and that it takes time for complex changes in teeth and skulls to adapt to new lifestyles,’ says Lister.
The new research also gives clues about the evolution of some of today's elephants. There are two forms of African elephant; forest and savannah, which some people consider separate species. They have very similar teeth, implying they both evolved in an open, grassy habitat.
'This suggests that the forest elephant did not enter its current habitat until quite late – around 2 million years ago – rather than being the original place where African elephants evolved, as had been previously suggested,' explains Lister.
'It also helps to refine the date of divergence of the forest and savannah types: DNA evidence can only give a broad range of 5 to 2 million years ago; according to the fossil evidence it is more like 2 million'.
Lister concludes, 'I hope my example of the elephants will encourage others to look for this evolutionary mechanism in other groups of animals.'