Rodent Horns: A Case of Natural or Sexual Selection?

Mylagaulidae is an extremely unique family of rodents that lived during the Miocene period. Thought to have developed from Aplodontinae, a family that includes the mountain beaver (Shotwell 1958), Mylagaulid fossils have been found in almost all of the North American faunas during this time period (Korth 1999). Mylagaulids, which are now extinct, are the only rodents known to have had horns. To better understand these extinct rodents, researchers must determine why no horned rodents exist today. Evidence containing fossil remains must be analyzed to determine the functions of the horns, the possible motives for their evolution, and the reasons for the animals’ extinction.

By R. Bruce Horsfall [Public domain], via Wikimedia Commons

By R. Bruce Horsfall [Public domain], via Wikimedia Commons

One important question yet to be answered among researchers is the exact origin of these rodent horns. Natural selection, the direct result from outside environmental pressures to survive, could have caused the evolution of horns. They could have also initially increased the success of mating for the males, making the horns the result of sexual selection (Arnold 1994).

If horns were exclusively the result of natural selection, the rodents should have exhibited several specific behaviors. First, these rodents would have needed the horns to survive everyday environmental pressures. Mylagaulids were large rodents that show some definite adaptations to their habitat (Hopkins 2005). For example, these rodents could have lived primarily above ground for a time until they were pressured from predators to find another home. While most rodents today use their claws and teeth to help with digging (Hopkins 2005), horns may have made mylagaulids more efficient burrowers. Therefore, horns could have helped select mylagaulids survive by digging underground tunnels to escape the threat of predators. Mylagaulids had characteristics such as large neck muscles and thickened nasal bones to indicate that they did use their heads for digging (Hopkins 2005), and some research has shown that the horns were thick and flat (Hopkins 2005). Other research shows that the skulls of these rodents were low and broad, characteristics not ideal for head-lift digging (Korth 1994). This does not necessarily mean that the rodents did not use their horns for digging, but it does indicate that the horns may have evolved for some other purpose.

A second behavior that would prove natural selection would be the mylagaulids’ use of the horns in defending against predators. The mylagaulids probably became extinct because of competition from species of the gopher family (Baskin 1980). It was this competition along with the changing landscape of the Miocene period that eventually drove the mylagaulids to live underground (Baskin 1980).  The horns, which were broad enough at the base to cover the neck and eyes and seemed to be adapted for defense (Hopkins 2005), may have evolved as mechanisms to help the rodents fend off predators. An example this type of evolution can be seen in the Bovidae family, which includes sheep, cattle, antelope, and bison. Interspecific competition led the horns of these animals to be shaped differently because of various fighting techniques (Lundrigan 1996). While arguments can be made for horn development due to environmental pressure, it is also possible that the horns evolved earlier because of sexual selective pressure.