Male moths that shed their genital spines inside the female: A survey of male mating costs and material investment during copulation

Abstract

In some insects, males produce costly ejaculates that reduce the typical sexual imbalance in potential reproductive rates thus influencing the mode and intensity of sexual selection. In Lepidoptera, in particular, males tend to invest heavily in mate searching, courtship and ejaculate production; furthermore, in some species males shed genital spines (deciduous cornuti) within the female genital tract during copulation. We studied experimentally the potential mating costs incurred by males of a moth with deciduous cornuti, Amorbia cuneana (Tortricidae). In the first experiment, we tested the prediction that mating costs result in a reduction in male longevity and survival rate. Our results did not support this prediction because virgin and mated males had similar longevity and survival rate. This result is somewhat surprising considering the substantial resource investment in mating found in the second experiment. In their first mating males invested in the ejaculate 9% of their body weight and transferred virtually all their deciduous cornuti; furthermore, few males were able to mate a second time (although we do not know if this was due to female rejection), these males produced smaller ejaculates and there was no evidence of cornuti regrowth. We hypothesize that reduced mating opportunities have selected for males that are ready to transfer large ejaculates at any moment and thus against the diversion of resources from reproductive tissues to somatic maintenance in unmated males; the transfer of the full set of deciduous cornuti in first copulations is consistent with this hypothesis.