Lack of Female Preference for Nuptial Gifts May Have Led to Loss of the Male Sexual Trait

Evolutionary loss of sexual traits may occur if the forces that maintain those traits weaken or disappear. Females may evolve resistance or a change in preference if the male sexual trait decreases their fitness (e.g., coercive or deceptive traits). In nuptial gift-giving spiders, males offer a food gift wrapped in silk during courtship, taking advantage of female foraging motivation. Males may also produce worthless gifts, which could select for female emancipation from deception and subsequent loss of gift function. This might be the case in the two known species of the spider genus Trechaleoides (Trechaleidae). Here, we examined the females' preference for nuptial gifts, and gift function as male mating effort and/or male protection in both species. Trechaleoides keyserlingi males offering gifts acquired significantly fewer matings than males without gifts and thus, we verified no female preference for the gift. In T. biocellata males never produced a gift, although they experienced a high risk of pre-copulatory cannibalism. To assess whether T. biocellata females possess a pre-existing sensory bias for nuptial gifts, they were presented with heterospecific T. keyserlingi males with and without gifts. No female preference was detected, and the gift did not protect males from sexual cannibalism. If silk-wrapped nuptial gifts are ancestral in the spider family Trechaleidae, a basal loss of female preference for the gift in the genus Trechaleoides could be hypothesized. This may subsequently have changed the gift's sexual function in T. keyserlingi and led to the complete loss of the gift-giving behaviour in T. biocellata.