Terminal investment in multiple sexual signals: immune‐challenged males produce more attractive pheromones

Abstract

Summary 1.Trade-offs between current and future resource allocation can select for elevated reproductive effort in individuals facing mortality. Males are predicted to benefit from increasing investment in costly sexually selected signals after experiencing an acute life span reduction, although few examples of such facultative terminal investment are known. 2. In the mealworm beetle, Tenebrio molitor, males' odours become more attractive to females following a life-threatening immune challenge. However, the pheromones involved are unknown, hindering further insight into the proximate mechanisms and ultimate consequences of terminal investment. 3. Using chemical and behavioural analyses, we show that the cuticular hydrocarbons (CHCs) of T. molitor are sexually dimorphic and are used by females to locate and select males. Moreover, both male CHCs and glandular pheromones were affected by experimental immune challenge in a fashion that made them more attractive to females. 4. The results suggest that males terminally invest in both short-and medium-range pheromones when they perceive reduced future survival. Moreover, the constitutive and inducible aspects of male and female CHC production are consistent with sex-specific selection on the signalling and defensive functions of CHCs. The implications of terminal investment for 'dishonest' signalling and the efficacy of sexual selection are discussed.

Female choice for sick males over healthy males: Consequences for offspring

“…Similar to some previous studies (Kivleniece et al, ; Krams et al, ; Nielsen & Holman, ; Sadd et al, ), T. molitor females showed a preference for sick males over healthy ones. Notice that this applies to situations when males were infected with fungus but not to males treated with nylons.…”

Stickleback Males Increase Red Coloration and Courtship Behaviours in the Presence of a Competitive Rival

“…In the males that were presented to a non-coloured rival, red coloration honestly indicated condition in the first experiment but not in the second trial performed later the season, probably because when low future opportunities are expected males should invest heavily in the current sexual signals ('terminal investment' ;Candolin 1999b;Nielsen & Holman 2012). However, the opposite effect of condition and trial on the sexual signal was observed when males faced a coloured rival.…”

Leg impairment magnifies reproductive costs in male Mediterranean fruit flies, Ceratitis capitata

“…It may be that injury to external structures does not provide the correct mortality cues needed to influence reproductive effort. This is supported by the loss of reproductive ability caused by injury in these male medflies and in other species (Carey et al, 2007(Carey et al, , 2009Sepulveda et al, 2008), whereas reproductive effort is shown to be augmented in studies where the risk of mortality is increased due to an immune response (Velando et al, 2006;Nielsen & Holman, 2012). Overall, these results do not support the terminal investment hypothesis; instead they provide further insight to the cost of reproduction in male insects, the effect of age on reproductive effort and ability, and the influence of impairment on survival and reproductive behavior.…”

“…It may be that injury to external structures does not provide the correct mortality cues needed to influence reproductive effort. This is supported by the loss of reproductive ability caused by injury in these male medflies and in other species (Carey et al., , ; Sepulveda et al., ), whereas reproductive effort is shown to be augmented in studies where the risk of mortality is increased due to an immune response (Velando et al., ; Nielsen & Holman, ).…”

“…Reproductive effort in male insects is conditional on age and prior mating experience (Papadopoulos et al, 1998;Liedo et al, 2002;Carey et al, 2006). There is also experimental evidence that the risk of death caused by a pathological condition may influence reproductive effort in males (Velando et al, 2006;Nielsen & Holman, 2012), a result consistent with the terminal investment hypothesis (Clutton-Brock, 1984). Because an outcome of reproductive effort in males is a loss in life expectancy (Simmons & Kotiaho, 2007;Papadopoulos et al, 2010), reproductive effort and the risk of mortality are codependent and potentially mutually affecting.…”