Abstract
The effects of non-lethal parasites may be felt most strongly when hosts engage in intense, energy-demanding behaviors. One such behavior is fighting with conspecifics, which is common among territorial animals, including many beetle species. We examined the effects of parasites on the fighting ability of a saproxylic beetle, the horned passalus (Odontotaenius disjunctus, Family: Passalidae), which is host to a non-lethal nematode, Chondronema passali. We pitted pairs of randomly-chosen (but equally-weighted) beetles against each other in a small arena and determined the winner and aggression level of fights. Then we examined beetles for the presence, and severity of nematode infections. There was a non-significant tendency (p = 0.065) for the frequency of wins, losses and draws to differ between beetles with and without C. passali; non-parasitized individuals (n = 104) won 47% of their fights while those with the parasite (n = 88) won 34%, a 13% difference in wins. The number of nematodes in a beetle affected the outcome of fights between infected and uninfected individuals in an unexpected fashion: fighting ability was lowest in beetles with the lowest (p = 0.033), not highest (p = 0.266), nematode burdens. Within-fight aggression was highest when both beetles were uninfected and lowest when both were infected (p = 0.034). Collectively, these results suggest the nematode parasite, C. passali, is associated with a modest reduction in fighting ability in horned passalus beetles, consistent with the idea that parasitized beetles have lower energy available for fighting. This study adds to a small but growing body of evidence showing how parasites negatively influence fighting behavior in animals.
Highlights
By definition, parasites subsist off of the resources of their host, in many cases draining these resources and reducing host performance or fitness [1,2,3,4,5]
We examined the effects of a naturally-occurring nematode parasite on the fighting ability of a saproxylic beetle
Our results provide evidence that Chondronema passali nematode infections are associated with a modest reduction in overall fighting ability in the horned passalus; beetles with these nematodes tended to win less often, and fights between two infected beetles tended to be less intense, with fewer high-energy behaviors
Summary
Parasites subsist off of the resources of their host, in many cases draining these resources and reducing host performance or fitness [1,2,3,4,5]. While some work on this subject has shown parasites have minimal effects on fighting ability [9], most studies have found that parasites reduce their host’s fighting ability or competitiveness [10,11,12], compared to non-parasitized individuals. These studies have been conducted on crickets infected with tachinid flies [11], trout infected with trematodes [10] and salamanders with ectoparasitic mites [12]. We examined the effects of a naturally-occurring nematode parasite on the fighting ability of a saproxylic beetle
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