Direct Benefits Due to Terminal Investment
Life history theories predict a trade off between investment in growth, reproduction, and survival (Stearns 1992; Roff 1993). As opportunities for future reproduction are diminished in iteroparous animals (e.g., via aging, injury, or disease), an individual’s best strategy may be to invest more heavily in their current reproductive event (Williams 1966). This is called terminal investment (Clutton-Brock 1984). Parasites often castrate their host or otherwise decrease their longevity or ability to reproduce in the future, thus reducing the residual reproductive potential of the individual and so promoting increased investment in current reproduction (Agnew et al. 2000; Gandon et al.2002; Duffield et al. 2017). In iteroparous species, selecting terminally-investing males as current reproductive partners could provide direct benefits to females. For instance, infected partners might provide better parental care (Velando et al. 2006), increased fertilization success due to increased spermatogenesis and sperm storage (McCurdy et al. 2000; Derting & Virk 2005; Brannelly et al. 2016), or more nutritious or preferred nuptial gifts (Hurd & Ardin 2003; Duffield et al. 2015).
Tenebrio molitar represents a particularly intriguing example of the potential for terminal investment to influence attraction and reproductive success in the mate of a terminal investor. Cuticular hydrocarbons and volatile glandular pheromones in immune-challengedT. molitor males are more attractive to females than are those of healthy males (Nielsen & Holman 2012). In addition, the nutrient content of spermatophores of infected T. molitor males is superior to that of healthy males, resulting in a positive relationship between male parasite intensity and female reproductive output (Hurd & Ardin 2003). In cases such as this, it is plausible that females may be selected to prefer terminal investors, and for mimics to evolve to capitalize on the attractiveness of apparent infection.
The virulence of parasites is likely to affect how drastically individuals increase investment during terminal investment, and hence the plausible evolution of female preference, and mimicry. In general, with increased parasite virulence, terminal investment becomes adaptive because future prospects become increasingly bleak (Gandon et al.2002). If females attempt to capitalize on the terminal investment of their mates, virulence and attractiveness may be similarly linked. Additionally, increased virulence should increase the deterrent effect on same-sex conspecific rivals (Stoehr & Kokko 2006) as, even if infection were to produce a temporary spike in attractiveness, becoming legitimately infected reduces fitness overall. Virulence, thus, provides a potential link between these effects of mimicry.