Biophysical Determinants and Constraints on Sperm Swimming Velocity

Abstract

Over the last 50 years, sperm competition has become increasingly recognised as a potent evolutionary force shaping male ejaculate traits. One such trait is sperm swimming speed, with faster sperm associated with increased fertilisation success in some species. Consequently, sperm are often thought to have evolved to be longer in order to facilitate faster movement. However, despite the intrinsic appeal of this argument, sperm operate in a different biophysical environment than we are used to, and instead increasing length may not necessarily be associated with higher velocity. Here, we test four predictive models (ConstantPower Density, Constant Speed, Constant Power Transfer, Constant Force) of the relationship between sperm length and speed. We collated published data on sperm morphology and velocity from 141 animal species, tested for structural clustering of sperm morphology and then compared the model predictions across all morphologically similar sperm clusters. Within four of five morphological clusters of sperm, we did not find a significant positive relationship between total sperm length and velocity. Instead, in four morphological sperm clusters we found evidence for the Constant Speed model, which predicts that power output is determined by the flagellum and so is proportional to flagellum length. Our results show the relationship between sperm morphology (size, width) and swimming speed is complex and that traditional models do not capture the biophysical interactions involved. Future work therefore needs to incorporate not only a better understanding of how sperm operate in the microfluid environment, but also the importance of fertilising environment, i.e., internal and external fertilisers. The microenvironment in which sperm operate is of critical importance in shaping the relationship between sperm length and form and sperm swimming speed.

Short Communication: Characteristics of the endangered Javan banteng (Bos javanicus) spermatozoa

“…Interestingly, spermatozoa morphology serves some unique function, such as in the hydrodynamics of spermatozoa. It means that the design or shape of the head of the spermatozoa, accompanied by force generated by the spermatozoa's tail, will affect the spermatozoa's speed and movement (Nosrati et al 2015;Soulsbury and Humphries 2022). Any abnormalities in spermatozoa morphology will compromise fertility and even cause sterility in males (Cao et al 2017;Feyisa et al 2018;Oehninger and Kruger 2021).…”

Parthenogenesis, sexual conflict, and selection on fertilization rates in switching environments

“…This can be achieved by having a population of pheromone emitters and receivers [29, 30] and by having a large stationary target egg and small motile sperm [31, 32]. Although the assumption of an inverse speed-size relationship in [32] has justification in some gametic systems, it is also worth noting that positive speed-size relationship have been observed in C. reinhardtii , due to larger cells having greater propulsive forces; at scales such as these the precise speed-size relationship is complicated by details of cell morphology [33]. Lastly, the prior evolution of internal fertilization has been proposed as a mechanism that could generate selection for oogamy [34], consistent with empirical evidence from volvox (external fertilization and anisogamous) and its sister lineage platydorina (internal fertilization and oogamous) [35].…”