Natural Selection

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.


What’s the deal with fiddler crabs?


Image: CC via Wikimedia Commons

We have a new case study lesson that was published last week over at the National Center for Case Study Teaching in Science. The center is run through the University of Buffalo and is supported by the National Science Foundation. This collection is very beneficial to any science teacher, and I encourage you to look through the topics for use in your classroom.

Our case (which you can find here) deals with animal weapons, specifically extreme weapons. Our minds typically go to moose or elk antlers, but there are lots of examples from the animal world. The case looks at this from a bioenergetic perspective. For example, male moose have large antlers even though allocating resources towards growing those antlers deplete their bones in other areas by 60%. That’s incredible! It seems like their has to be a better answer than just for fighting. This case looks at fiddler crabs. Why do males have one large claw that sometimes is as big as their whole body? This large claw would seem to come with some disadvantages.

Screenshot 2016-03-10 09.06.20

This case is designed to be used in a flipped classroom. However, there are many different ways you can use it. Take it, and make it your own. Here’s the abstract:

In most animals, the drive to breed and produce offspring is strong. However, most males live their whole lives without having the chance to breed. The events leading up to mating can be very dangerous and also very costly to an individual. Some males have evolved elaborate structures, or weapons, as a result. The structures do help males in both combative situations and with attracting females, but ironically, the structures themselves come with certain costs. This flipped case study provides students with the opportunity to not only see how animal structures and functions are linked, but also to see how certain animal structures are needed and costly. There are videos that students are expected to view before the case. The case was initially designed for a second semester college general biology class for majors. However, it can also be used in non-major biology classes. Students should have some background knowledge of natural selection, specifically sexual selection as well as energetic demands of certain structures.