A competitive environment influences sperm production, but not testes tissue composition, in house mice

Abstract

Due to the physiological cost of sperm production, males are expected to be prudent in their expenditure and adjust their investment according to current social conditions. Strategic adjustments in sperm expenditure during development can be made via changes in testes size, sperm production rates or testes tissue composition. Here, using house mice, we test the hypothesis that elevated sperm production is driven by a plastic response in the spatial organization of the testes. We reared males under different social conditions (competitive vs. noncompetitive) and quantified sperm number and the proportion of sperm-producing tissue within the testes. Further, because sperm quality is a critical determinant of competitive fertilization success, we used computer-assisted sperm analysis to quantify six sperm motility traits. Our investigation revealed that males reared in an environment with a perceived risk of reproductive competition produced more sperm in the absence of changes in testes morphology. We discuss this result in relation to fixed and flexible phenotypically plastic responses to future competitive conditions, and conclude that adaptive adjustments in sperm number in response to the social environment are likely attributable to variation in sperm production rate. Further, we found no difference in in vitro sperm motility parameters among males from the different social environment regimes. Overall, this investigation improves our understanding of the mechanisms of male plastic responses to reproductive competition experienced during sexual development.

Exposure to male-dominated environments during development influences sperm sex ratios at sexual maturity

“…Male reproductive traits that are relevant to intrasexual selection have been shown to respond to the social environment in an adaptive manner (Firman et al. , ; Ramm et al. ; André et al.…”

“…; Firman et al. ) and sperm production (Firman et al. , ) compared to males reared under noncompetitive conditions.…”

“…) and sperm production (Firman et al. , ) compared to males reared under noncompetitive conditions. Whether the competitive environment influences variation in sperm sex ratios is yet to be investigated.…”

Spatially clustered resources increase aggregation and mating duration inDrosophila melanogaster

“…Several empirical studies have supported this prediction, including in crickets (Gage and Barnard, 1996), beetles (McCullough et al, 2018), bugs (García-González and Gomendio, 2004), platyhelminths (Giannakara et al, 2016), fish (Candolin and Reynolds, 2002), and rodents (Firman et al, 2018;Ramm and Stockley, 2009).…”

“…Demonstrations that male encounter rate can stimulate plasticity in sexual traits has generally been achieved by housing males at varying densities in the laboratory, with the most common treatment comparing a singly-housed male with a male housed with one or more conspecifics (Candolin and Reynolds, 2002;Firman et al, 2018;Gage and Barnard, 1996;Lizé et al, 2012;Moatt et al, 2013). This extreme manipulation of the total number of potential rivals is not intended to mimic the effects males experience in nature, but rather to demonstrate that such adaptive responses exist.…”