Team swimming in ant spermatozoa

Abstract

In species where females mate promiscuously, competition between ejaculates from different males to fertilize the ova is an important selective force shaping many aspects of male reproductive traits, such as sperm number, sperm length and sperm-sperm interactions. In eusocial Hymenoptera (bees, wasps and ants), males die shortly after mating and their reproductive success is ultimately limited by the amount of sperm stored in the queen's spermatheca. Multiple mating by queens is expected to impose intense selective pressure on males to optimize the transfer of sperm to the storage organ. Here, we report a remarkable case of cooperation between spermatozoa in the desert ant Cataglyphis savignyi. Males ejaculate bundles of 50-100 spermatozoa. Sperm bundles swim on average 51% faster than solitary sperm cells. Team swimming is expected to increase the amount of sperm stored in the queen spermatheca and, ultimately, enhance male posthumous fitness.

Keywords: ants; sexual selection; sperm cooperation.

Figure 1.

Sperm bundles ejaculated by mature males of C. savignyi. (a) PSA-FITC staining of sperm bundles. Sperm nuclei (N) are stained in blue with DAPI, while sperm acrosome (A) and bundle cap (C) stain with PSA-FITC (green) (Picture: L. Twyffels). Scale bar, 10 µm. (b) Scanning electron micrograph of a sperm bundle showing the cap and the sperm flagella. The white arrow (W) designates the base of the cap (Picture: D. Monteyne and D. Perez-Morga). Scale bar, 10 µm.

Figure 2.

Motility of sperm bundles and solitary spermatozoa in C. savignyi. (a) Average motility of sperm bundles (solid line) and solitary spermatozoa (dotted line) with increasing viscosity of medium (PVP addition, in per cent). Sperm bundles display significantly greater average velocity than solitary spermatozoa (Kruskal–Wallis χ² = 114.13, d.f. = 1, p < 0.001). Treatments with different lower case letters differed significantly (after Bonferroni correction, Kruskal–Wallis χ²). See table 1 for numeric values and sample sizes. (b) Mean sperm bundle velocity versus solitary sperm velocity for each male. Dotted line indicates a similar velocity for bundles and solitary spermatozoa. Sperm bundles display higher velocity for 27 males (above the line) out of 34.