Size, operational sex ratio, and mate-guarding success of the carrion beetle, Necrophila americana

Abstract

When male insects guard females until oviposition, the benefits from last-male sperm precedence must outweigh the costs of relinquishing additional fertilizations. The profitability of guarding is increased when males guard large, fecund females and when females are scarce because fewer fertilizations are sacrificed. However, the male reproductive success is not only determined by the profitability of guarding but also by his ability to maintain guarding. In this study, we used male carrion beetles (Necrophila americana) to examine the effects of sex ratio, male relative size, and female quality on the ability to guard. First, we present a model of mate guarding that explores factors, such as sperm precedence, sex ratio, male size, and female quality, that influence the profitability of postcopulatory riding. Our model predicts that large N. americana males should preferentially guard the largest female only when the sex ratio is male biased and sperm precedence is above 80%. In contrast, small males gain little from guarding because they are not likely to maintain it and be the last male to mate. Then, we tested these predictions by manipulating sex ratio, relative male size, and female quality. All males in equal sex ratio and large males in male-biased sex ratio guarded females significantly longer than did males in female-biased sex ratio. In male-biased sex ratio, large males guarded significantly longer and achieved more takeovers than small males. Large females were guarded longer. The success of guarding males in this beetle depends on their size relative to other males and the operational sex ratio.