Abstract
Sperm competition leads to increased sperm production in many taxa. This response may result from increases in testes size, changes in testicular architecture or changes in the kinetics of spermatogenesis, but the impact of each one of these processes on sperm production has not been studied in an integrated manner. Furthermore, such response may be limited in species with low mass-specific metabolic rate (MSMR), i.e., large-bodied species, because they cannot process energy and resources efficiently enough both at the organismic and cellular levels. Here we compare 99 mammalian species and show that higher levels of sperm competition correlated with a) higher proportions of seminiferous tubules, b) shorter seminiferous epithelium cycle lengths (SECL) which reduce the time required to produce sperm, and c) higher efficiencies of Sertoli cells (involved in sperm maturation). These responses to sperm competition, in turn, result in higher daily sperm production, more sperm stored in the epididymides, and more sperm in the ejaculate. However, the two processes that require processing resources at faster rates (SECL and efficiency of Sertoli cells) only respond to sperm competition in species with high MSMR. Thus, increases in sperm production with intense sperm competition occur via a complex network of mechanisms, but some are constrained by MSMR.
Highlights
Sperm competition takes place when the sperm of two or more males compete to fertilize the ova of a female [1,2,3]
We found that the percentage of testicular tissue comprising seminiferous tubules increased with relative testes size (PGLS: P=0.0002) and with mass-specific metabolic rate (MSMR) (PGLS: P=0.0001) (Table 1)
Neither relative testes size nor MSMR had any effect on the diameter of seminiferous tubules, the height of the seminiferous epithelium, or the number of Sertoli cells per gram of testis (PGLS: P>0.05; Table 1)
Summary
Sperm competition takes place when the sperm of two or more males compete to fertilize the ova of a female [1,2,3]. In species that experience high levels of sperm competition, males increase their sperm production [5], which allows them to ejaculate more sperm in competitive contexts [6]. The most straightforward way to increase sperm production is by augmenting the size of the testes [7,8,9], but males could maximize their rate of sperm production by adjusting either the architecture of the testis or the kinetics of sperm formation [10,11]. Sperm are produced in the testes through the process of spermatogenesis that takes place within the seminiferous tubules [12], which are separated by interstitial tissue [13]. The seminiferous epithelium contains germ cells and their supporting and nourishing cells, the Sertoli cells [13,14]
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