Strategic adjustment of ejaculate quality in response to variation of the socio-sexual environment

Strategic ejaculate adjustments occur when males modify their investment in sperm and nonsperm components of the ejaculate according to the context. This strategy is expected to evolve when ejaculate production is costly, the returns of the investment in the ejaculate depend on the environment and environmental conditions are variable. While adjustments of sperm numbers have been widely documented, only recently we have begun to investigate how males modify ejaculate quality, despite the recognized importance of this factor for sperm competition. Here I discuss and synthetize existing literature on strategic adjustments of ejaculate quality. I describe which ejaculate quality traits are most typically plastic and which environmental factors elicit such responses, focusing especially on the socio-sexual environment. I summarize information on the timeframe within which adjustments can occur and on the proximate mechanisms responsible for plasticity. I show that this phenomenon is widespread across taxa; it involves responses to several environmental factors and modifications of many ejaculate traits, with seminal fluid composition playing a central role, as a trait per se and as proximate mechanism for sperm performance adjustments. I point out the circumstances favoring the evolution of ejaculate quality plasticity, and evaluate the fitness consequences of these responses, highlighting the complexity of patterns of covariation with other traits. Finally, I consider implications for male and female behavior. I highlight two areas of research on ejaculate plasticity that may be particularly worth exploring further: 1) the proximate mechanisms responsible for plasticity; 2) the adaptive value of strategic ejaculate adjustments.