The Influence of Parasite Infection on Mating Success in Spadefoot Toads,Scaphiopus couchii

Abstract

Synopsis. The desert toads, Scaphiopus couchii, have an annual activity season of less than 8 weeks and experience only one significant parasite infection: the monogenean Pseudodiplorchis americanus is transmitted during host spawning and provides a natural system for testing the influence of parasite burden on host mating success. The 10 month hiber? nation involves total starvation during which the blood-feeding parasites reduce fat reserves and haematocrit. The toads emerge and spawn on the first night after rainfall, before they replenish depleted reserves. Male chorusing is energetically very demanding and mate selection, limited to a 7 hr nocturnal assembly, is determined principally by female choice. Parasite transmission triggered by host sexual activity results in 100% prevalence and high intensities of infection amongst males. Around 50% of toads lose their burdens entirely but the rest carry chronic infections throughout host hibernation. Field data show a consistent reduction in intensity each year strongly suggesting resistance. Parasite infec? tion is pathogenic and creates extra stress during hibernation; therefore, to the extent that elimination of infection is heritable, toads entering spawning assemblies with heavy burdens should make poor mates. However, extensive field studies show no correlation between mate success and parasite burden. Although infection can prejudice survival, it is only one of several inter-related factors (including feeding success, tolerance of hiber? nation). The condition of successful and unsuccessful males in spawning assemblies indi? cates that all exceed a threshold at which parasite-induced pathology is significant. Males which are debilitated by infection?or other factors?are selected against before mate choice begins.