Robust calling performance in frogs infected by a deadly fungal pathogen.

Sasha E Greenspan,Elizabeth A Roznik,Lin Schwarzkopf,David A Pike,Ross A Alford

doi:10.1002/ece3.2256

Abstract

Reproduction is an energetically costly behavior for many organisms, including species with mating systems in which males call to attract females. In these species, calling males can often attract more females by displaying more often, with higher intensity, or at certain frequencies. Male frogs attract females almost exclusively by calling, and we know little about how pathogens, including the globally devastating fungus, Batrachochytrium dendrobatidis, influence calling effort and call traits. A previous study demonstrated that the nightly probability of calling by male treefrogs, Litoria rheocola, is elevated when they are in good body condition and are infected by B. dendrobatidis. This suggests that infections may cause males to increase their present investment in mate attraction to compensate for potential decreases in future reproduction. However, if infection by B. dendrobatidis decreases the attractiveness of their calls, infected males might experience decreased reproductive success despite increases in calling effort. We examined whether calls emitted by L. rheocola infected by B. dendrobatidis differed from those of uninfected individuals in duration, pulse rate, dominant frequency, call rate, or intercall interval, the attributes commonly linked to mate choice. We found no effects of fungal infection status or infection intensity on any call attribute. Our results indicate that infected males produce calls similar in all the qualities we measured to those of uninfected males. It is therefore likely that the calls of infected and uninfected males should be equally attractive to females. The increased nightly probability of calling previously demonstrated for infected males in good condition may therefore lead to greater reproductive success than that of uninfected males. This could reduce the effectiveness of natural selection for resistance to infection, but could increase the effectiveness of selection for infection tolerance, the ability to limit the harm caused by infection, such as reductions in body condition.

Highlights

Animals maximize their fitness by adjusting their allocation of resources throughout their lifetimes (Stearns 1992; Roff 2002)
By the mid-1990s, chytridiomycosis outbreaks had extirpated this species from high-elevation portions of its range (>400 m a.s.l.; Richards et al 1993; McDonald and Alford 1999)
Model selection indicated that both body temperature and body length influenced call attributes (Table 2, Fig. 1) and that call duration, pulse rate, call rate, and intercall interval were largely mediated by temperature (Fig. 1A–D)

Summary

Introduction

Animals maximize their fitness by adjusting their allocation of resources throughout their lifetimes (Stearns 1992; Roff 2002). Pathogen infections can alter reproductive investment in complex ways. Individuals carrying infections may reduce their present investment in reproduction so that they can divert energy to manage the costs of infection, for example, by producing and increasing immune responses (Cade 1984; Bonneaud et al 2003; Martin et al 2003; Madelaire et al 2013). Male field crickets, Gryllus integer, call less frequently when parasitized by Euphasiopteryx ochracea flies (Cade 1984), and male Burmeister’s treefrogs, Hypsiboas prasinus, decrease their calling rate with increasing helminth parasite intensity (Madelaire et al 2013). Infected individuals may increase their investment in reproduction to compensate for a potentially shortened life span (Agnew et al 2000). Male Drosophila nigrospracula court more when parasitized by mites (Macrocheles subbadius; Polak and Starmer 1998)

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Results