The dynamics of sperm cooperation in a competitive environment

Abstract

Sperm cooperation has evolved in a variety of taxa and is often considered a response to sperm competition, yet the benefit of this form of collective movement remains unclear. Here, we use fine-scale imaging and a minimal mathematical model to study sperm aggregation in the rodent genus Peromyscus. We demonstrate that as the number of sperm cells in an aggregate increase, the group moves with more persistent linearity but without increasing speed. This benefit, however, is offset in larger aggregates as the geometry of the group forces sperm to swim against one another. The result is a non-monotonic relationship between aggregate size and average velocity with both a theoretically predicted and empirically observed optimum of six to seven sperm per aggregate. To understand the role of sexual selection in driving these sperm group dynamics, we compared two sister-species with divergent mating systems. We find that sperm of Peromyscus maniculatus (highly promiscuous), which have evolved under intense competition, form optimal-sized aggregates more often than sperm of Peromyscus polionotus (strictly monogamous), which lack competition. Our combined mathematical and experimental study of coordinated sperm movement reveals the importance of geometry, motion and group size on sperm velocity and suggests how these physical variables interact with evolutionary selective pressures to regulate cooperation in competitive environments.

The genetic basis and fitness consequences of sperm midpiece size in deer mice

“…Indeed, sperm from the promiscuous P. maniculatus males swim with greater velocity (straight-line velocity [VSL]) than sperm of the monogamous P. polionotus ( t -test: P =0.017, df =8, n =76–549 sperm/male), consistent with our previous results9. Two other means of measuring sperm swimming performance, curvilinear velocity (VCL; t -test: P =0.0024, df =8, n =76–549 sperm/male) and average path velocity (AVP; t -test: P =0.0039, df =8, n =76–549 sperm/male), showed a similar difference as in the VSL results.…”

“…Two closely related Peromyscus rodents with highly divergent mating systems show marked variation in male reproductive traits891011. Within the genus, the deer mouse, P. maniculatus, is considered one of the most promiscuous species: both sexes mate with multiple partners, often in overlapping series just minutes apart12, and females frequently carry multiple-paternity litters in the wild13.…”

“…Under intense competition, sperm motility can be a critical determinant of success17. Previous studies have shown that P. maniculatus sperm swim with greater velocity than P. polionotus 9. A primary energy source for motility is acquired by oxidative phosphorylation in the mitochondria, which are located within the sperm midpiece18.…”

Noncontact Cohesive Swimming of Bacteria in Two-Dimensional Liquid Films

“…A similar example is well known in spermatozoa. Sperm of polyandrous species form cohesive groups due to hydrodynamics interaction between sperm cells [61,62]; these cohesive groups swim with higher linearity than individuals, allowing them to travel faster through the female reproductive tract [63]. Taken together, our results reveal non-contact cohesive swimming as a unique form of individual interaction between flagellated bacteria that may promote bacterial collective motion, self-organization, and dispersal in 2D environments.…”

Effect of body deformability on microswimming

“…In nature, this class of microswimmers is predominant. [26][27][28][29][30] For artificial swimmers, chemically driven mechanisms are as popular as mechanical ones. [31][32][33][34][35][36][37] An important consideration in mechanical microswimming is the way elastic forces interact with the fluid and any external forces present in determining the motion.…”