University of Tübingen: Unexpected differences between males and females of early deer piglets

Tübingen scientists are examining skulls found around eleven million years old from the Hammerschmiede clay pit in the Allgäu
Deer piglets are among the smallest ruminants in the world. Today they live in the tropics of Africa and Asia and are hardly larger than rabbits. Externally, males and females differ only slightly. That was different around eleven million years ago: Josephina Hartung and Professor Madelaine Böhme from the Senckenberg Center for Human Evolution and Palaeoenvironment at the University of Tübingen discovered a previously unknown gender difference when examining two fossil deer piglet skulls from the Hammerschmiede clay pit in the Allgäu. They discovered conspicuous bony ridges above the eyes on the skull of a male deer piglet, which the females lacked. The study was recently published in the journal PLOS ONE .

“In today’s deer piglets, the males differ from the females only in their enlarged and externally visible upper canine teeth,” says doctoral student Josephina Hartung. These saber-like tusks were used as a weapon in a fight between two males or as a show of strength. The deer piglets of the extinct species Dorcatherium naui , which lived eleven million years ago in what is now southern Germany around Pforzen, were somewhat larger than modern species. They were contemporaries of the first walking great ape, Danuvius guggenmosi , whose remains from the hammer mill were examined some years ago.

Air-filled bone structures
“The unusual thing about the male skulls of the deer piglets from the hammer mill is the well-pronounced bony ridges that enclose the skull cap almost like a wreath,” explains Hartung. This feature was previously unknown in both modern and fossil deer piglets. Female deer pig skulls from the same locality did not show this feature. The researchers concluded that there must be a previously undiscovered gender difference, also known as sexual dimorphism. They obtained confirmation of this finding by comparing them with other fossil skulls of this deer piglet species.

It is interesting that the male’s skull ridges were filled with air, says Hartung. That would have been the result of measurements using micro-computed tomography, an X-ray method used to obtain three-dimensional images of the inner structure of small samples. “These data showed us that the ridges were not filled with dense bone material, but rather had many small cavities, similar to those in modern-day giraffes.” Whether these cavities served to reduce the weight of the skull or had another function is still unclear not clear. “It is possible that the bulge section over the eye area protected the eye from combat injuries from the dagger-like canine teeth of another male, as is known from muntjac deer,” speculates Madelaine Böhme.

Indicative of an ecologically diverse family
The researchers also think it likely that the bony ridges served as some sort of display feature, adornment or display to impress females or scare off other males. The same is known of the closely related hoofed animals such as giraffes, deer and antelopes, which carry horn-like protuberances, horns or antlers as forehead weapons. “The fact that the deer piglets, which are more primitive in terms of evolutionary history, had bony ridges on their skulls is a novelty for the biology of these small ruminants,” says Hartung. This suggests that the deer piglets once formed an ecologically more diverse family.

“Once again, the excavations in the hammer mill have shown the unique potential of the fossils. They help us to learn more about the evolution and biology of extinct species,” says Böhme.

University of Tübingen