Female choice, secondary effect of ``mate check''? A hypothesis

A new hypothesis is formulated to explain the diversity and the range of complexity of secondary sexual characters (SSC). It is based on the observation that in many animal groups an important somatic radiation took place but the SSC remained fairly uniform and their complexity low, while in some other well-studied groups it can be shown that, apparently at a later stage, complexity increased dramatically while somatic morphology remained stable. SSC are therefore hypothesised to be linked to hidden (behavioural), but crucial traits that have been acquired in the last steps of the evolution of the taxon. The mating process is postulated to guarantee the presence of these characters. During this process the << mate is checked >>. The reason far this mechanism is hypothesised to be the avoidance of the loss of crucial behavioural adaptations through deleterious mutations. The hypothesis might explain why taxa with a flexible checking system (e.g. stridulation, nuptial dance) are more speciose than those using only morphological clues which may be more limited in complexity and variation. Systems that allow larger variation without compromising the survival of the adult male will allow a wider radiation. Since complexity of SSC is hypothesised to be correlated with specialisation, animal groups with smaller species can be expected to have more complex SSC. Female choice is presumed to be a secondary effect of the << mate check >> mechanism. The former only operates in optimal habitats where a wide range of the signal strength of the male is to be expected. In marginal habitats (sinks) it is likely to be insignificant because both female coyness and range of male signal strength are assumed to drop. It is precisely in sinks where speciation will occur when behavioural adaptations, consolidated by SSC, allow more efficient use of underexploited resources. Therefore, in contrast to female choice, mate check is viewed as a stabilising mechanism.