Ejaculate evolution in external fertilizers: Influenced by sperm competition or sperm limitation?

• DOI: 10.1111/evo.13372
• Publication Date: 2017-10-04T10:17:46Z
• Authors: Wen Bo Liao; Yan Huang; Yu Zeng; Mao Jun Zhong; Yi Luo; Stefan Lüpold
• Keywords: Male; 0106 biological sciences; 1100 General Agricultural and Biological Sciences; Models, Biological; 01 natural sciences; sperm number; 10127 Institute of Evolutionary Biology and Environmental Studies; 1311 Genetics; sperm size–number trade; Animals; sperm length; 14. Life underwater; reproductive investment; off; Sperm Count; Fishes; fishes; Biological Evolution; Spermatozoa; Anurans; 1105 Ecology, Evolution, Behavior and Systematics; Fertilization; 570 Life sciences; biology; 590 Animals (Zoology); Female; Anura

The evolution of sperm quality and quantity is shaped by various selective processes, with sperm competition generally considered the primary selective agent. Particularly in external fertilizers, however, sperm limitation through gamete dispersal can also influence gamete investments, but empirical data examining this effect are limited. Here, we studied the relative importance of sperm competition and the spawning conditions in explaining the macroevolutionary patterns of sperm size and number within two taxa with external fertilization but differences in their reproductive biology. In frogs, sperm swim slowly but for up to hours as they penetrate the gelatinous egg coating, whereas fish sperm typically swim fast, are very short-lived (seconds to minutes), and often face a relatively higher risk of being moved away from the ova by currents. Our phylogenetic models and path analyses revealed different trajectories of ejaculate evolution in these two taxa. Sperm size and number responded primarily to variation in sperm competition in the anurans, but more strongly to egg number and water turbulence in the fishes. Whereas the results across anurans align with the general expectation that sexual selection is the main driver of ejaculate evolution, our findings across the fishes suggest that sperm limitation has been underappreciated.