Female sex pheromone emission is affected by body condition, but not immune system function, in the orb-web spider Argiope bruennichi

Sex pheromones function as species and sex recognition signals but can be equally important for mate assessment and choice. Adaptive mate choice requires the covariation of signal characteristics with fitness-relevant traits, which can be achieved by condition-dependent costs of signalling. Given a physiological link between signal production and other vital processes, such as immune system function, the expression of the sexual signal may result in decreased body condition or increased pathogen susceptibility. According to honest signalling theory, only high-quality individuals should be able to produce high-quality sexual signals. We tested this prediction in the sexual signal of female wasp spiders, Argiope bruennichi. Female A. bruennichi produce the volatile pheromone trimethyl methylcitrate, which not only attracts males from a distance but also is the main foundation for short-distance male mate choice. Recent experiments showed that females use pheromones prudently and tailor the amount of pheromone produced to their needs, suggesting costs of pheromone production. Here we manipulated female physiological condition using both immune challenge and feeding treatments in a fully factorial design. Fed or starved females were injured with a needle that was sterile or coated with bacterial lipopolysaccharides. We found no effect of immune system stimulation on pheromone production, but food stress significantly reduced the amount of pheromone produced. Importantly, this response was apparent in terms of both absolute pheromone amounts and body mass-normalized relative pheromone amounts indicating that this effect was due to reduced pheromone production rather than reduced body surface or volume. Given the close association of the female pheromone and the citric acid cycle, the main catabolism pathway for energy production in the cell, we argue that the female pheromone of A. bruennichi may signal a female's general capacity of energy production according to the "shared pathway hypothesis" for signal honesty.