Fossil Tusks Indicate Earliest Sexual Differentiation

Hillary Mayell
for National Geographic News
January 28, 2003
In the beginning, males and females of species were created equal—or at least, other than reproductive equipment, the same. But look around today; especially among mammals, there are lots of differences. Males are often bigger, more muscular, and have built-in weaponry—tusks, antlers, horns, spines, bigger teeth, or skulls built for butting.

Sexual dimorphism, different physical traits in opposite sexes of the same species, is an extremely prevalent characteristic among mammals today. When and why did these differences evolve?

At least 260 million years ago, says a team of researchers writing in the January issue of the Proceedings of the Royal Society of London, Biological Series.

"Among tetrapods—four-legged land vertebrates—you went from a situation where there was very little dimorphism between the sexes to a situation in this one animal, Diictodon, where you had quite evident differences. The males are slightly larger and had formidable tusks," said Corwin Sullivan, a graduate student at Harvard University and lead author of the study.

The reason for their weaponry? Competition among males for the opportunity to reproduce.

"Sexual dimorphism with armament implies these animals have a relatively complex mating system characterized by antagonistic competition among males," said Robert Reisz, a biology professor at the University of Toronto at Mississauga and co-author.

"It's really a fascinating story," he said. "Here we are 260 million years ago and we have complex behaviors strikingly similar to mammalian populations today taking place well before the advent of dinosaurs."

The Era Before Dinosaurs

Dicynodonts, a group of four-legged plant eaters, thrived from the Late Permian, around 260 million years ago to the Late Triassic, around 208 million years ago, a time when the Earth's land formed one giant super-continent, Pangaea.

These mammal-like reptiles were a biologically diverse group, very abundant, and globally distributed. They were also remarkably hardy, having survived one of the Earth's greatest mass extinction events that occurred about 250 million years ago.

One dicynodont, Diictodon was somewhat unique even within this very diverse group. On the small side at roughly 3 feet (1 meter) long, Diictodon had an extremely large head, a beak and no teeth, a barrel-shaped body, and a short tail. It made its home in burrows carved in ancient floodplains and river banks. But what truly interested the research team was the fact that some had tusks, and some didn't.

Several possible explanations for the presence of two forms, tusked and tuskless, exist: the fossils could be of different species; different ages within a species; or the same species, opposite sexes.

The researchers were able to eliminate the first two possibilities, and concluded that the tusked animals were the males of the species. [See sidebar]

"If you look at vertebrate history, there's a basic split that occurs, and one line leads to reptiles, dinosaurs, and birds, and the other side to mammals," said Sullivan. "Dicynodonts are on the mammalian side of vertebrate history, like a second or third cousin to mammals."

"What these findings tell us is that sexual dimorphism goes a long way back in the lineage of mammals, and that this very early group had relatively complex social behavior," he said.

Living in a Sexually Dimorphic World

By looking at sexually dimorphic animals living today, scientists can infer certain kinds of social behaviors.

"In species characterized by the male having some form of armament, whether horns or spurs, there is almost always some form of male aggression," said J. Michael Plavcan, a paleontologist at the University of Arkansas.

The males could be competing directly for females or for territory.

"In a situation in which males can exclude other males to gain a reproductive advantage, there's a tremendous selective pressure for the males to develop weaponry and large size because those attributes will give them more opportunities for mating," said Plavcan.

The competition need not be a fight to the death.

"It's very common for antagonistic displays to be the first stage of a confrontation, and it can frequently stop at that point, with one male deciding the other guy is bigger, or has bigger horns and 'I'd better back down,'" said Sullivan.

Why would a group go from monomorphic (no physical differentiation) to sexually dimorphic?

"There's no real way of knowing what might have been going on in this population," said Sullivan. "I can imagine a scenario where they went from having lots of food or territory to a situation where those resources became more limited. Or there may have been a more internal reason for their having evolved a more elaborate mating system. Those are things we just don't know."

© 1996-2008 National Geographic Society. All rights reserved.