A common mechanism explaining the evolution of female-limited and both-sex Batesian mimicry in butterflies

1. Batesian mimicry describes the situation in which a palatable mimic resembles an unpalatable model. In some species of butterflies, both sexes mimic, but in others only females do. Two mechanisms have been proposed to generate female-limited mimicry: sexual selection via female choice, and sexual selection via male-male competition. Each is not satisfactory because of too many exceptions. 2. I hypothesized that reductions in physiological life span because of mimicry constituted the costs, while extensions of ecological life span because of mimicry constituted the benefits. Mimicry would result from the balance of a costs/benefits relationship; when balances are favourable, mimicry occurs. 3. Non-mimetic females of female-limited mimicry butterfly Papilio polytes lived longer than its mimetic females in a butterfly farm greenhouse. Therefore, reduction in physiological life span must be a cost of mimicry. 4. Male-biased sex ratios were found in 11 of the 14 non-mimetic and non-model species netted in the Kakamega tropical rainforest in west Kenya, in other one species almost equivalent, and in remaining two species female-biased. For each species, the sexes netted less frequently had more beak marks than those netted more frequently. That is, the biased sex ratios could be responsible for the higher predation rates of the opposite sex. 5. Predators may have selectively attacked females with wider thoraxes, which flew at higher levels and flew more quickly. However, attack rates on males of some species may be high or low regardless of thorax width. 6. As the benefits accrue largely to females with wider thorax due to female-biased predation, balances of costs/benefits relationships divide participating species into three theoretical groups: those in which the adaptation is favourable for both sexes; favourable only for females; and unfavourable for both sexes. 7. In female-limited mimics of five species, thorax widths varied, whereas thoraxes tended to be wider in both-sex mimics of four species.