Scientists may have solved an evolutionary riddle of how the ancestors of elephants changed their diet from soft leaves to relatively tough grasses and in the process became one of the dominant herbivores of the African savannah.

A study of the fossils of animals with tusks and trunks, such as elephants and mammoths, has revealed that the crucial change in diet occurred about 3 million years before elephants had actually developed the big teeth needed for a grass diet.

Professor Adrian Lister of the Natural History Museum in London said that the fossils, which cover a period of 20 million years, indicate that the ancestors of today’s elephants changed their feeding behaviour long before they acquired the high-crowned teeth needed for chewing tough grasses.

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A study, published in the journal Nature, found that about 8 million years ago various species changed their diet and feeding behaviour by switching from a diet based on browsing for leaves from trees to one based on grazing the ground for grass.

Professor Lister said that the change – identified by analysing carbon isotopes in the fossils – was relatively rapid and occurred at a time when there were still plenty of trees and forests, indicating that it was a behavioural “choice” rather than a necessity. Yet it took another 3 million years for the elephants to evolve the high-crowned teeth that were better suited to dealing with the grittier, grassy food.

“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. After that, the dental changes show a clear example of progressive evolution for another four million years,” Professor Lister said.

“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. The idea is that, by exploring a new habitat or trying a new food resources, species put themselves in a situation where natural selection will modify their anatomy to more efficiently accomplish the new behaviour,” he said.