Socio-sexual environments (e.g., sex ratio and population density) can influence individual’s growth and development, mating behavior, physiology and reproduction [1]. In this seminar, I reviewed papers related to plastic males mating strategies in response to the presence of rival males in their social environment.
Males are predicted to switch their mating strategies, such as strategic allocation of ejaculate and mating behaviors when their socio-sexual environments are variable [2, 3]. For example, in polyandrous butterfly, Pieris napi, males in the high male density treatments transferred larger ejaculates than males in the low male density treatments [2]. Moreover, in Drosophila melanogaster, although increasing rival male number or density did not alter significantly male mating duration, the longer the males were exposed to the rival males prior to mating, the longer the mating duration, which lead to increased transfer of ejaculate components, occurred subsequently [3]. Taken together, males of P. napi and D. melanogaster seemed to assess the potential level of sperm competition, regardless of their different ways of response to social environment (i.e. the presence of competitor males) [2, 3]. Nevertheless, males of Australian field cricket, Telegryllus oceanicus, did not adjust ejaculate (measured as sperm viability) in response to the relatedness of their rival males (i.e. full- sibling males) as predicted by sperm competition theory [4]. Interestingly, in the two species of Drosophila subobscura and D. acanthoptera in which females rarely remate, copulation duration was significantly longer when males were with a rival males prior to copulation. The plasticity in copulation duration affected by social environment does not seem to predict post-copulatory sperm competition advantage in these two species, like other polyandrous species. Thus, it will be valuable to find out how plasticity relates to fitness in monandrous mating system [5].
In some species, social experiences have profound effect on certain aspects of male physiology and reproductive behaviors while other processes are not. For example, in male German cockroaches, Blattella germanica, pair-housed males produced significantly more juvenile hormones (JHs) than isolated males, although JH production increased with age in both treatments. However, there were no significant differences in the quantities of accumulation of cuticular hydrocarbons (CHCs) and courtship behavior between isolated and paired males. In this species, paired males are likely to mature faster than isolated males because social interactions may facilitate an endocrine-regulated process by stimulating higher JH production at younger age, which in turn stimulate faster production of accessory gland secretions and more rapid maturation [6]. Similarly, in Drosophila melanogaster, plastic responses by males to rivals did not match between behavior and fitness outcomes. In exposures pattern of (0àR, no rivals, then exposure to rivals), male significantly increased mating duration and offspring output as predicted when competition increased. By contrast, in switched exposures pattern of (Rà0, exposure to rivals, then no rivals), male significantly decreased mating duration but did not fully decrease offspring production when competition levels dropped. Behavioral plasticity is generally predicted to be an inexpensive, rapid expression and reversible response to the environment. These findings show that while behavior (mating duration) remains flexible, investment patterns (offspring number) do not remain flexible to the some extent [7].
In conclusion, sociality can be beneficial for males because it may create opportunities for matings.
References
1. Pulliam HR, Caraco T. 1984. Living in groups: is there an optimal group size?Behav. Ecol. 122-147.
2. Mellström HL, Wiklund C. 2009. Males use sex pheromone assessment to tailor ejaculates to risk of sperm competition in a butterfly.Behav. Ecol. 20:1147–51.
3. Bretman A, Fricke C, Hetherington P, Stone R, Chapman T. 2010.Exposure to rivals and plastic responses to sperm competition in Drosophila melanogaster.Behav. Ecol. 21:317–321.
4. Thomas ML, Simmons LW. 2008. Rival male relatedness does not affect ejaculate allocation as predicted by sperm competition theory. PLoS ONE, 3, e 2151.
5. Lize A, Doff RJ, Smaller EA, Lewis Z, Hurst GDD. 2011. Perception of male-male competition influences Drosophila copulation behavior even in species where females rarely remate. Biol. lett. 8:35-38.
6. Uzsak A, Schal C 2013. Social interaction facilitates reproduction in male German cockroaches, Blattella germanica. Anim. Behav, 1-9.
7. Bretman A, Westmancoat JD, Gage MJG, Chapman T. 2012. Individual plastic responses by males to rivals reveal mismatches between behavior and fitness outcomes. Proc. R. Soc. B 279: 2868-2876.