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.