Mating opportunities and energetic constraints drive variation in age-dependent sexual signalling

Abstract

When males repeatedly produce energetically expensive sexual signals, trade-offs between current and future investment can cause plasticity in age-dependent signalling. Such variation is often interpreted as alternate adaptive strategies: live fast and die young vs. slow and steady.  An alternative (yet rarely tested) explanation is that condition-dependent constraints on allocation cause variation in signalling with age (‘late bloomers’ do not have early investment options). Testing this hypothesis is challenging because resource acquisition and allocation are difficult to measure, and energetic reserves both affect and are affected by reproductive effort.  We simultaneously manipulated acquisition (through dietary nutrition) and access to potential mates (as a proxy for manipulating sexual trait allocation) in male decorated crickets (Gryllodes sigillatus), while measuring age- and signalling effort-mediated changes in energy storage components.  Increased diet quality caused increased signalling effort and energy storage, while access to females increased both the likelihood of and time spent signalling. Males with lower resource budgets signalled less, but still suffered energetic storage loss and viability costs.  Our results suggest that energetic constraints, rather than strategic resource accumulation, reduced signalling levels in males with lower resource acquisition ability. Our findings imply a non-adaptive explanation for age-dependent variation in sexual signalling, and an important role for energetic constraints in maintaining the honesty of costly behavioural displays.