When do acoustic cues matter? Perceived competition and reproductive plasticity over lifespan in a bushcricket

Individuals often modify their behaviour in response to environmental cues and their own condition. Here we asked whether males modify ejaculates based on information from the sociosexual environment and their physiological age, and how those two factors may interact. We used two populations of the chorusing bushcricket, Ephippiger diurnus, to test whether males strategically adjust large, costly spermatophores they transfer to females during mating based on experience of rivals' calls, and whether males change their investment strategies with age. Males broadcast highly consistent, rhythmically repeated calls of syllables in daily choruses. Populations differ in average syllable number, an important trait under selection via female preference, and females from our two populations prefer calls with more syllables than the population mean. We reared males in one of five acoustic environments that varied in call syllable number. We then mated males twice, as young and old adults, measuring spermatophore size each time. We found that acoustic experience, age and their interaction all significantly influenced male investment, resulting in reaction norms with different slopes. Young males differentially invested in spermatophores in response to acoustic experience, whereas old males generally invested in larger spermatophores across environments. We then tested for a broad pattern of age-related investment with eight different field-collected populations, finding the majority of old males significantly increased spermatophore investment. Our findings demonstrate that both environmental context and an individual's life history state influence plasticity in reproductive investment, and such adjustments may optimize their reproductive success.