Seminal fluid enhances sperm viability in the leafcutter ant Atta colombica
The seminal fluid that accompanies sperm in ejaculates has been shown or suggested to affect sperm competition and paternity success of insects by preventing female remating, inducing oviposition, and forming mating plugs. In Atta leafcutter ants, queens have multiple mates but never remate later in life, although they may live and produce fertilized eggs for several decades. The mating biology and life history of these ants therefore suggests that the major function of seminal fluid is to maximize sperm viability during copulation, sperm transfer, and initial sperm storage. We tested this hypothesis by comparing the viability of testis sperm and ejaculated sperm (mixed with seminal fluid) and found a significant positive effect of seminal fluid on sperm viability. We further quantified this positive effect by adding accessory gland secretion (a major component of seminal fluid) in a dilution series, to show that minute quantities of accessory gland secretion achieve significant increases in sperm viability. Sperm stored by queens for 1 year benefited in a similar way from being exposed to accessory gland compounds after dissection in control saline solution. Our results provide the first empirical evidence that seminal fluid is important for the production of viable ejaculates and that the accessory glands of Atta males—despite their small size—are functional and produce a very potent secretion.
KeywordsSocial insects Mating Sperm storage Sperm transfer Spermatheca Accessory glands
We thank Aniek Ivens and Jens Broch for collecting ants in the field, David Nash for statistical support, Francisco García-González for assistance with the Live/Dead™ protocol, and Leigh Simmons for valuable comments on the manuscript. We thank the Smithsonian Tropical Research Institute in Panama for facilities and logistic support and the Autoridad Nacional de Ambiente (ANAM) for issuing collecting and export permits. This work was supported by grants from the Danish National Research Foundation to SPAdB and JJB and an Australian Research Council Fellowship to BB.
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