Nonrandom mating in the two-spot ladybird (Adalia bipunctata): the influence of weight on mating success.

In some populations of the two-spot ladybird, Adalia bipunctata, there is evidence that melanics of both sexes have a general mating advantage over the typical morph. There is also evidence that some female ladybirds possess a simple mating preference for melanic males. We have determined the influence of body weight on mating success in the two-spot ladybird and assessed whether weight differences might influence the mating success of the melanic morph. We found that the "formal mating tests" used in previous studies of the two-spot ladybird did not detect any influence of weight on mating success. Using more sensitive "singleton" tests, however, a significant mating advantage to heavier males was detected, irrespective of morph. There was also evidence in favor of a similar advantage to heavy females, but this was not present in all ladybird groups that were tested. Heavy individuals neither produced more eggs in matings nor showed higher activity rates. We suggest that some form of mate competition favors large individuals: for example, the larger an individual, the greater is the chance of an encounter with a potential mate. There is no evidence in this study that either sex chooses heavier mates. Effects of weight might contribute to the general melanic mating advantage found in some populations but cannot account for all the data in favor of a female mating preference for melanic males.