Jessica F. Stephenson, Cormac Kinsella, Jo Cable and Cock van Oosterhout. 2016. A further cost for the sicker sex? Evidence for male-biased parasite-induced vulnerability to predation. Ecology and Evolution. http://onlinelibrary.wiley.com /doi/10.1002/ece3.2049/full
Ubiquitous parasites are a major challenge to wild organisms. The pathology they cause can be severe – but the direct damage isn’t the only concern for infested individuals. In recent years the indirect consequences of species interactions such as parasitism have increasingly been recognised and studied. The phenomenon of parasite-induced vulnerability to predation is one such indirect effect, whereby parasite infested individuals are victims of predators more often than healthy conspecifics. For some parasites, this is a desirable outcome that allows them to complete their life cycle. Echinococcus sp. requires its moose intermediate host to be eaten by the wolf definitive host, and Toxoplasma gondii is known to remove rodent’s aversion to cat scent, and has recently been shown to do the same for chimpanzee’s aversion to leopard scent. Other parasites, such as Gyrodactylus spp. increase the odds of their host being caught by predators as an unintended consequence of the symptoms and response to infection. Everyone is familiar with the lethargy brought on by viral infections, during which a lower productivity is expected. In nature however, there are no sick days, and the effect on hosts such as guppies can be lethal.
Males in nature tend to contract more and heavier parasite infections, and are less capable of mitigating the consequences than females. In this study we hypothesised that the logical consequence was also true – that males were more likely to fall victim to predators as a result of infection than females. To test this we examined the escape velocities of guppies in response to a scare stimulus, with or without Gyrodactylus infection. As guppies are highly sexually dimorphic (males are smaller), we size matched our guppies in order to distinguish between the effects of size and sex. Our results showed that infection slowed down small guppies by twenty percent – a significant effect for wild fish facing fast-moving predators – but didn’t slow down large guppies. Inherent sex differences were not identified in this experiment, that is to say a male performed the same as an equally sized female. However, the size effect identified implies that males in the wild fall victim to parasite-induced vulnerability to predation more often than females, due to their lower average size. The findings have implications for the life history evolution of guppies, and probably many other species, highlighting the complexity of dynamic natural ecosystems.