The metabolic costs of sexual signalling in the chirping katydid Plangia graminea (Serville) (Orthoptera: Tettigoniidae) are context dependent: cumulative costs add up fast

Abstract

Katydids produce acoustic signals via stridulation which they use to attract conspecific females for mating. However, direct estimates of the metabolic costs of calling to date have produced diverse cost estimates and are limited to only a handful of insect species. In this study, we therefore investigated the metabolic cost of calling in a unstudied sub-Saharan katydid, Plangia graminea. Using wild-caught animals, we measured katydid metabolic rate using standard flow-through respirometry while simultaneously recording the number of calls produced. Overall, the metabolic rate during calling in P. graminea males was 59% higher than the resting metabolic rate (0.443±0.056 vs. 0.279±0.028 CO2 ml g−1 h−1) although highly variable among individuals. While individual call costs were relatively inexpensive (ranging from 0.02–5.4% increase in metabolic rate per call) the individuals with cheaper calls called more often and for longer than those with expensive calls resulting in the former group having significantly greater cumulative costs over a standard amount of time (9.5 h). The metabolic costs of calling are however context dependent since the amount of time spent calling greatly influenced these costs in our trials. A power law function described this relationship between cumulative cost and percentage increase per call (y=130.21x−1.068, R2=0.858); where y=cumulative cost, and x=percentage increase per call. The choice of metric employed for estimating energy costs (i.e. how costs are expressed) also affects the outcome and any interpretation of costs of sexual signalling. For example, the absolute, relative and cumulative metabolic costs of calling yield strongly divergent estimates and any fitness implications depend on the organism's energy budget and the potential trade-offs in allocation of resources that are made as a direct consequence of increased calling effort.