Many spectacular cases of biological diversity are associated with sexual selection, and structures under sexual selection often show positive static allometry: they are disproportionately large for the size of the animal's body in larger individuals. Other sexually selected structures, however, show negative allometry or isometry. Theory fails to account for this variation and recent summaries do not agree regarding the frequency of positive allometry in sexually selected structures. We propose explanations for why sexually selected structures with different functions (courtship, threat signals, and weapons) should differ in allometry. Positive allometry is predicted for threat structures (including most weapons) because larger individuals tend to win fights and threat signals are used to avoid unwinnable fights with larger opponents, the reproductive payoffs for contests tend to be higher for larger males, and discriminating the sizes of relatively larger traits requires greater absolute differences due to Weber's Law of sensory physiology. Male courtship signals, in contrast, convey many types of information, much of which is not consistently related to male size, so positive allometry is expected less often. We tested these predictions empirically by comparing the allometries of male structures with relatively "pure" functions. Our predictions were confirmed, thus helping to explain differences in previous empirical surveys. 208Volume 93 THE QUARTERLY REVIEW OF BIOLOGY This content downloaded from 192.236.

No abstract

No abstract

In arthropods, most cases of morphological dimorphism within males are the result of a conditional evolutionarily stable strategy (ESS) with status-dependent tactics. In conditionally male-dimorphic species, the status' distributions of male morphs often overlap, and the environmentally cued threshold model (ET) states that the degree of overlap depends on the genetic variation in the distribution of the switchpoints that determine which morph is expressed in each value of status. Here we describe male dimorphism and alternative mating behaviors in the harvestman Serracutisoma proximum. Majors express elongated second legs and use them in territorial fights; minors possess short second legs and do not fight, but rather sneak into majors' territories and copulate with eggguarding females. The static allometry of second legs reveals that major phenotype Electronic supplementary material The online version of this article (expression depends on body size (status), and that the switchpoint underlying the dimorphism presents a large amount of genetic variation in the population, which probably results from weak selective pressure on this trait. With a mark-recapture study, we show that major phenotype expression does not result in survival costs, which is consistent with our hypothesis that there is weak selection on the switchpoint. Finally, we demonstrate that switchpoint is independent of status distribution. In conclusion, our data support the ET model prediction that the genetic correlation between status and switchpoint is low, allowing the status distribution to evolve or to fluctuate seasonally, without any effect on the position of the mean switchpoint.

No abstract