Abstract
BackgroundReproductive success is determined by the interplay between mating and fertilization success. In social insect species with male-biased sex ratios and queen monogamy, males face particularly strong pre-copulatory sexual selection since they must compete with thousands of other males for a unique mating opportunity. Ejaculate quality is also expected to be under selection, because queens are long-lived and store sperm for life, so males with higher quality ejaculates are expected to provide queens with larger and longer-lived colonies, which in turn may produce more daughter queens (the only direct fitness gains of haplodiploid males). Considering the action of pre and post-copulatory sexual selection on male traits, three scenarios might thus be expected: positive, negative or no association between male mating ability and fertilization success. Here we explored these scenarios in the stingless bee Scaptotrigona aff. depilis, where males gather in large aggregations and queens mate with a single male. Male mating ability was assessed through the capacity of a male to reach an aggregation and persist on it; while sperm viability, sperm number, and sperm morphology were used as proxies for sperm quality.ResultsSperm viability was associated with persistence time in the aggregation, and males that persisted longer presented shorter spermatozoa and higher variation in sperm length than recently arrived males. However, sperm traits of males that reached aggregations did not differ from those of males collected inside their colonies. In addition, males that persisted longer in aggregations were smaller than other males. Male size and sperm viability were not correlated, suggesting that the observed patterns were not due to trade-offs in male resource allocation.ConclusionsPersistence in male aggregations thus seems to select more competitive males with higher quality sperm. Our work is the first one to reveal an association between male competitive ability and fertilization success in a monogamous social insect. This finding sheds important light on the evolution of male traits in social insects and the general mechanisms of sexual selection.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0765-2) contains supplementary material, which is available to authorized users.
Highlights
Reproductive success is determined by the interplay between mating and fertilization success
While mating success is subject to pre-copulatory sexual selection, via male-male competition and female choice [1, 2], fertilization success is usually related to post-copulatory sexual selection in species with multiple mating, through sperm competition [3] and cryptic female choice [4]
The probability of reaching an aggregation was not influenced by any male morphological trait, while the number of days that a male persisted at the aggregation was best predicted by male size (Table 3)
Summary
Reproductive success is determined by the interplay between mating and fertilization success. While mating success is subject to pre-copulatory sexual selection, via male-male competition and female choice [1, 2], fertilization success is usually related to post-copulatory sexual selection in species with multiple mating, through sperm competition [3] and cryptic female choice [4]. In monogamous species where egg fertilization does not occur immediately after mating, male fertilization ability may be subject to selection In these species, females normally possess sperm-storage organs, where sperm are kept viable until fertilization [16]. Females normally possess sperm-storage organs, where sperm are kept viable until fertilization [16] Both mating and fertilization abilities are expected to be under selection in males of monogamous species with sperm-storage, making them well suited to study post-copulatory sexual selection in the absence of sperm competition
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