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Trimorphism in Dung Beetles (Image 2)

Types of horn development in the male trimorphic beetle <em>Oxysternon conspicillatum</em>

An illustration showing horn development in the male trimorphic beetle Oxysternon conspicillatum. Clockwise from top right: A female, a gamma male, a beta male and an alpha male. Both the alpha and beta males produce horns, but the relative size of the horn is larger in the alpha. The gamma male and female lack head horns entirely.

In a National Science Foundation (NSF)-supported study, University of New Mexico biology professor J. Mark Rowland and University of Montana professor Douglas J. Emlen studied the evolution of male dimorphism in insects and were surprised to find that many species of beetles are capable of producing not only two, but three different types of males.

The researchers found male trimorphism in dung beetles and in other families of beetles, as well as in other weapon systems--which include head horns in dung beetles, mandibles in stag beetles and ventral spines in weevils. The dung beetle has an alpha, beta and gamma (completely hornless) male--three qualitatively distinct forms. The researchers also found that in these trimorphic species, one of the three male forms was always female-like in morphology. The researchers believe these gamma males may use this as a tactical deception to evade combat and earn sexual encounters.

To learn more, see the UNM news story Sexual Encounters of the Third Kind: Darwin's Beetles Still Producing Surprises. [Research supported by NSF grant IOS 06-42409.] (Date of Image: 2008) [See related image Here.]

Credit: J. Mark Rowland/Douglas J. Emlen
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