Trackways freeze time to reveal ancient elephant sociality

An artist’s reconstruction of what the ancient herd may have looked like, here showing Stegotetrabelodon

The evolution of behavior is tricky to study for one very simple reason: behaviors usually don’t fossilize.  While anatomy can be reconstructed based on skeletal remains and imprints, how might one glimpse how a living, breathing organism behaved millions of years ago?

Sometimes the animals themselves may be buried while performing certain actions – two dinosaurs locked in combat, or a mother sitting on her clutch of eggs.  Such findings are rare and exquisite. Alternatively, animals may leave behind traces of their handiwork – nests, hives, burrows.  These are objects that testify to the skills of their makers, sometimes termed an ‘extended phenotype‘ – the outward expression of a behavioral program that may be encoded by genes.

Occasionally, one finds evidence of the behavior of an entire group of animals.  Dinosaur trackways discovered around the world reveal many amazing insights, such as the fact that some long-necked sauropod dinosaurs moved in herds, much as modern-day elephants do.  Perhaps they enjoyed a social life that was as interesting.  But clues to the behavior of ancient elephants themselves had not come to light — until now.

An exciting discovery comes from a site called Mleisa in the United Arab Emirates.  The trackway shows 13 individuals travelling mostly parallel to one another, and a 14th individual passing through diagonally. Their distinctive saucer-like shape identifies them as proboscideans, the clade to which all elephants belong.  The Asian and African elephants are thought to have diverged from their common ancestor about six million years ago.  This site dates back to just about that period – approximately 6-8million years.  That means these tracks may belong to an ancestor of the elephants, or one of its contemporaries.

An aerial photo of the trackway (left) and a reconstruction of individual paths (right).

An aerial photo of the trackway (left) and a reconstruction of individual paths (right).

Based on the sizes of individual footprints, and the distances between them, the researchers were able to estimate the height and weight of the animals that made these impressions.  These humble measurements turn out to be quite revealing.

First, the body sizes of these animals are comparable to that of modern elephants, but the largest of them are larger than the species living today.  Second, the parallel tracks seldom intersect, suggesting that tracks resulted from animals were all travelling side-by-side as a group, rather than the result of multiple crossings by single individuals. Third, tracks of the lone individual are larger than the tracks occurring as a group.  This suggests the former is a male.  But the size difference between the male and the others is not as great as the size difference between males and females in the present-day species, leading the authors to suggest either that the difference between sexes was not as large in the ancient species, or that the male was not fully grown.  I think a third possible explanation is that perhaps the herd itself had some young males in it. Finally, the trackways also have a few very small individuals, suggestive of calves.

The size of the elephant group is within the range of group sizes reported for modern African elephants.  If the interpretation that the herd is composed mostly of females while the lone track is a male is correct, it means that ancestral elephants themselves lived in a manner strikingly similar to their living counterparts, who also exhibit such a difference between the sexes.  Most clearly, the site shows that the social system of elephants has a long history.

ResearchBlogging.orgBibi, F., Kraatz, B., Craig, N., Beech, M., Schuster, M., & Hill, A. (2012). Early evidence for complex social structure in Proboscidea from a late Miocene trackway site in the United Arab Emirates Biology Letters DOI: 10.1098/rsbl.2011.1185

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