Male testicle size often reflects the mating system of the species. A recent Science study found that in populations where females could breed any time of the year, female promiscuity is a successful defense against infanticide by males who aren’t the father. This leads to confusion about paternity, which leads to increased sperm competition down the line, and ultimately larger testes. The guy with the biggest balls likely has the highest chances of passing on his genes.
But larger testes don’t always produce more sperm, right? After all, changes in the organization of testes tissue may affect sperm production rates. Now, in lab experiments with mice, researchers discover that the descendants of males who mate with polygamous females have testicles capable of producing more sperm—even if testicle size didn’t change over time. The findings were published in Evolution this week.
A University of Western Australia team led by Renée Firman allowed six populations of house mice (Mus domesticus) to mate either monogamously or polygamously over 24 generations. Firman and colleagues previously found that males from polygamous populations produce greater numbers of sperm in the absence of changes in testes size. This time, "we were wondering how the mice had increased their sperm production in the absence of a change in testes size," Firman tells New Scientist.
When they examined the proportion of sperm-producing tissue (called seminiferous tubules) within the testes of males, they found that males who evolved in an environment with sperm competition had testes with a higher proportion of seminiferous tubules compared with males who had evolved under monogamy.
Sperm competition, they write, selects for an increase in the density of sperm-producing tissue, and consequently increased testicular efficiency. So sure, sometimes bigger is better, but it's not always size alone.