Scentlessness is a derived character state within male hairstreaks, a condition that only appears in some species as opposed to being the ancestral state of all species in the genus. Although the scentless state exists in multiple clades of hairstreaks, it evolved through independent lineage-specific events rather than being inherited from a shared ancestor.
Several factors are thought to contribute to this evolutionary shift from emitting pheromones to relying on alternative non-chemical cues:
Resource allocation: Investing in scent production can be costly for males in terms of energy and nutrients. Scentlessness could be a result of reallocating these resources into other aspects of their life history, such as flight performance or longevity, which may enhance their mating success.
Avoidance of predators and parasites: Producing scents may attract not only potential mates but also natural enemies, such as predators or parasitoids. Going scentless could provide an advantage by reducing the risk of being detected and attacked.
Habitat and climate characteristics: The effectiveness of chemical signals is influenced by environmental factors, including habitat structure, temperature, and humidity. In certain environments, visual cues may be more reliable and efficient for long-distance communication and mate attraction.
Population density and intraspecific interactions: Scentlessness could be an adaptive strategy in species or local populations where males are at low densities and encounter mates infrequently. In such situations, relying on scent for long-distance communication may be ineffective, and males may benefit from investing more in visual signals or male-male interactions.
It's important to note that while some male hairstreak butterflies lack androconia and don't produce a fragrance, they still emit other non-sexual scents, for instance, those involved in deterring predators or as components of defensive secretions.
The evolution of scentlessness in hairstreak butterflies is a testament to the diverse adaptations that can occur in mate attraction strategies. Different environmental and ecological conditions can shape the evolution of sensory communication, influencing the prevalence and significance of scent in mate choice.
