Sex and crime in a beetle: Female choice and deadly male - male aggression in Malachius bipustulatus

T. Meiners 1 & E. Strohm 2

1 Institut für Biologie, Freie Universität Berlin, Haderslebener Str. 9, D-12169 Berlin, Germany; 2 Zoologie III, Biozentrum, Am Hubland, D-97074 Würzburg, Germany

Sexual selection has given rise to the evolution of unique and bizarre structures and behavioural traits. Male beetles of the coleopteran family Malachiidae are endowed with remarkable chitinous structures that are associated with complex glands (= exciter organ). The elaboration and location of these organs vary considerably among the Malachiidae. We hypothesised that these exciter organs play a role during courtship and might provide females either with valuable substances and/or with a signal of male quality. In M. bipustulatus the exciter organ is located on the forehead. Its structure allows females to insert their mouthparts in a fashion that looks like the female would bite into the male's head. Field observation of a total of 123 courtship interactions of this species revealed that interactions lasted 31 ± 36min (range: 1-160min, n = 92). During courtship the female repeatedly bit into the exciter organ. The mean duration of single bites was 25 ± 17s (range: 2-26s, n = 25). The total duration of female contact with the male exciter organ was 28 ± 28min (range: 1-100 min, n = 26). In some cases the occurrence of droplets at the exciter organ or female's mouthparts suggested that actually some substance was transferred from the male to the female. After each biting the female turned around and the male touched the female's abdomen tip with its mouthparts. In 85 out of 123 cases the female left without copulation; only 10 interactions resulted in a final copulation (mean duration: 26 ± 9s, range: 17-39s, n = 9). In 21 cases, other males aggressively interfered with courting pairs and seemingly tried to take-over the females. Six M. bipustulatus males were found in the field feeding on other males (preferably on the head). Even direct attacks and killing of paired and unpaired males was observed in four cases. This was surprising since malachiids are actually pollen feeders. Our observations suggest that males offer valuable secretions during courtship to the female. Males try to attain these secretions even by killing and feeding on congenders. Whether female choice is based on qualitative or quantitative aspects of the secretion is not yet known.

Index terms: Malachiidae, courtship feeding, female choice, male competition, cannibalism

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