Battling Beetles

Antlers and horns are usually associated with the male of the species, but sometimes the females bear battle armor.

Female dung beetles have horns that are used in challenging other female dung beetles for dung. Dung beetles lay their eggs in dung, and researchers have found that females with more robust horns produced more offspring than females of similar size but with smaller horns.

Experiments also showed that, unlike males, who may sport extravagant ornament to advertise their superior genetic makeup and suitability as a mate, female ornament does not play a role in sexual selection, at least among dung beetles. So it appears that development of showy weaponry in female dung beetles is related to survival in procuring resources, while in males it functions for survival of a different sort.

Sources: Millus, S., Female beetles ready for dung wars. Science News March 27, 2010

Watson, Nicola and Simmons, Leigh, Proceedings of the royal Society B, March 3, 2010

Watson and Simmons, Behavioral Ecology, March-April, 2010

Photo credit: Sean Stankowski

Charles Darwin & Sexual Selection

Here, in his own words, Darwin’s observations of differences between the sexes that led to his hypothesis of sexual selection:

In the several great classes of the animal kingdom - in mammals, birds, reptiles, fishes, insects, and even crustaceans - the differences between the sexes follow nearly the same rules. The males are almost always the wooers; and they alone are armed with special weapons for fighting with their rivals. They are generally stronger and larger than the females, and are endowed with the requisite qualities of courage and pugnacity. They are ornamented with infinitely diversified appendages, and with the most brilliant or conspicuous colours, often arranged in elegant patterns, whilst the females are unadorned. These various structures for charming or securing the female are often developed in the male during only part of the year, namely the breeding-season.

Photo credit: http://www.crbs.umd.edu/crossingborders/ai2008/lessonplans08.html