Chemical signals are the most widely used form of sexual communication throughout the living world. However, there is in general little knowledge about what these signals actually communicate. The role of chemical signals, i.e. pheromones, in sexual behaviour has traditionally been seen as restricted to mate attraction and species recognition. This thesis reviews the evidence for pheromones as indicators of mate quality, and then investigates three important factors for mate quality signals – individual variation, heritability and cost – by using the male pheromone of the lekking fruit fly Drosophila grimshawi as a model.
The experiments presented indicate that the pheromone of D. grimshawi has multiple functions, and that these functions vary with social context. Thus, with regard to females, the pheromone seems to act mainly as a species/mate recognition signal, since females show little preference for the amount of pheromone deposited by a male. Moreover, males invest less in pheromone production when subjected to females as compared to when subjected to rival males. However, the pheromone seems to be costly in production since males that invest much in pheromone deposition has a shorter lifespan. This suggests a function for the pheromone in male-male interactions. Males can distinguish their own pheromone depositions from those of a strange male, and also discriminate between pheromone depositions from one and two strange males. This might give them the ability to assess the size of a lek and the competitive capacities of rivals, information that should be useful when optimizing sexual behaviour.
In conclusion, the pheromone seems to act as an honest mate/competitor quality signal in some social contexts, and as a non-costly species/mate recognition signal in other. In addition, I show for the first time that a chemical signal has differential fitness costs, and that an insect is able to distinguish between individual odour signatures.