The Mothematics of Female Pheromone Signaling: Strategies for Aging Virgins

Abstract

Although females rarely experience strong mate limitation, delays or lifelong problems of mate acquisition are detrimental to female fitness. In systems where males search for females via pheromone plumes, it is often difficult to assess whether female signaling is costly. Direct costs include the energetics of pheromone production and attention from unwanted eavesdroppers, such as parasites, parasitoids, and predators. Suboptimal outcomes are also possible from too many or too few mating events or near-simultaneous arrival of males who make unwanted mating attempts (even if successfully thwarted). We show that, in theory, even small costs can lead to a scenario where young females signal less intensely (lower pheromone concentration and/or shorter time spent signaling) and increase signaling effort only as they age and gather evidence (while still virgin) on whether sperm limitation threatens their reproductive success. Our synthesis of the empirical data available on Lepidoptera supports this prediction for one frequently reported component of signalingtime spent calling (often reported as the time of onset of calling at night)-but not for another, pheromone titer. This difference is explicable under the plausible but currently untested assumption that signaling earlier than other females each night is a more reliable way of increasing the probability of acquiring at least one mate than producing a more concentrated pheromone plume. Submitted February 24, 2014; Accepted October 3, 2014; Electronically published February 2, 2015 Online enhancement: appendix.abstract: Although females rarely experience strong mate limitation, delays or lifelong problems of mate acquisition are detrimental to female fitness. In systems where males search for females via pheromone plumes, it is often difficult to assess whether female signaling is costly. Direct costs include the energetics of pheromone production and attention from unwanted eavesdroppers, such as parasites, parasitoids, and predators. Suboptimal outcomes are also possible from too many or too few mating events or near-simultaneous arrival of males who make unwanted mating attempts (even if successfully thwarted). We show that, in theory, even small costs can lead to a scenario where young females signal less intensely (lower pheromone concentration and/or shorter time spent signaling) and increase signaling effort only as they age and gather evidence (while still virgin) on whether sperm limitation threatens their reproductive success. Our synthesis of the empirical data available on Lepidoptera supports this prediction for one frequently reported component of signaling-time spent calling (often reported as the time of onset of calling at night)-but not for another, pheromone titer. This difference is explicable under the plausible but currently untested assumption that signaling earlier than other females each night is a more reliable way of increasing the probability of acquiring at least one mate than producing a more concentrated pheromone plume.

Sex pheromones in mate assessment: analysis of nutrient cost of sex pheromone production by females of the mothHeliothis virescens

“…If there is an insignificant direct metabolic cost of sex pheromone production for female H. virescens, then females must incur an indirect fitness cost for producing sex pheromone if it indeed acts as a mate assessment signal (Umbers et al, 2015). The most likely indirect fitness cost to female moths is interception of the chemicals by various natural enemies.…”

“…In spite of numerous reports of pheromone-based mate assessment, few, if any, show direct evidence of any such costs (Kotiaho, 2001). In particular, there is a paucity of work on the direct metabolic costs of chemical mating signals (Johansson and Jones, 2007;Umbers et al, 2015).…”

Diet-Mediated Pheromones and Signature Mixtures Can Enforce Signal Reliability

“…The specific costs of pheromone production have rarely been explicitly identified (Johansson et al, 2005;Foster and Anderson, 2015;Umbers et al, 2015), perhaps reflecting a widely held assumption that pheromone production is cheap (Alberts, 1992) and the difficulties of measuring the amount of pheromone released in model species such as moths (Umbers et al, 2015). Nonetheless, pheromone production could have significant physiological costs if its profile depends on nutrients that have other important biological functions.…”

Sexual signalling by females: do unmated females increase their signalling effort?

“…In general, mate searching by males is the most commonly observed pattern; however, there are circumstances under which females are expected to search [1]. Indeed, female mate searching is found in a variety of taxa; female moths emit pheromones [2], female fireflies emit flashes of light [3] and female bushcrickets produce acoustic signals [4]. However, while mate searching by males has been the subject of considerable research effort, relatively little attention has been paid to elucidating the costs and benefits of mate searching by females.…”

“…In a recent theoretical analysis, Umbers et al [2] showed that even with very small costs associated with signalling, females should signal less intensely when young and increase their signalling effort the longer they remain unmated. In support of their prediction, Umbers et al [2] presented evidence from the literature that a common pattern among moths is for unmated females to increase their signalling effort as they age. The cost of pheromone signalling for females has been difficult to document, but adaptive adjustments in signalling effort in response to elevating risk of reproductive failure provide good evidence of signalling costs [2].…”