干旱区生态环境知识资源中心

1. The desert toad, Scaphiopus couchii, survives 10 months hibernation (during which it does not feed) by utilizing stored lipid reserves predominantly concentrated in coelomic fat bodies. The majority of toads emerging from hibernation have negligible fat bodies, although 4% of uninfected males have enough reserves to survive a further year.

2. Adults of the monogenean parasite, Pseudodiplorchis americanus, infect breeding toads (with a 50% prevalence) and feed upon host blood. In one area of desert in southeastern Arizona, USA, infected male and female toads emerging from hibernation had significantly smaller fat bodies than those uninfected. Infected females also had a lower packed cell volume (PCV) of the blood. Fat body weight of both sexes and PCV of females declined significantly with increasing parasite burden.

3. Fat body weights increased during the period of foraging and there was no measurable effect of adult parasite infection after 2 week’s feeding. Therefore, the difference observed in freshly-emerged toads probably reflects the actual parasite drain on host fat reserves during hibernation: representing around 7% of the toads’ annual lipid requirement.

4. Ovary weight also increased during foraging, but it was not possible to determine if reproductive investment was lower in infected animals. Paradoxically, testes weight was smaller in male toads without adult parasites. However, first-time breeders are invariably uninfected. Parasite effects on host reproductive success cannot be determined without considering age-related infection levels in conjunction with age-related reproductive output.

5. Field samples were biased towards animals in breeding condition and these inevitably have suffered least. Extrapolation of the effects of adult P. americanus upon host condition suggests that some toads would not breed or might not survive hibernation due to parasite infection. This relationship identifies the potential for parasites to regulate natural host populations.