Female distribution has an effect on the size evolution of testes in Australian rats.

Plains mouse (Pseudomys australis) at Mac Clark (Acacia peuce) Conservation Reserve. Credit: Tim Bawden/Wikimedia Commons, CC BY 2.0

A new study discovered that the distribution of females in 33 Australian rodent species had an effect on the evolution of male testes size in these social animals.

According to evolutionary biologists at the University of Western Australia (UWA), Flinders University, and Columbia University in the United States, the research, which was recently published in Biology Letters, confirms that female availability has an effect on the strength of sperm competition in these rodent species.

Researchers’ understanding of reproduction and male fertility will aid in the conservation of Australia’s rodent species threatened by introduced predators, habitat clearance and degradation, and climate change, according to lead author Dr. Renée Firman, an ARC Future Fellow at the University of Western Australia’s Center for Evolutionary Biology.

“We performed phylogenetic studies on 33 rodent species to deduce the relationship between female spatial and temporal dispersion and sperm competition, or more precisely, male-male competition following mating,” she explains.

“We discovered that males of these social species had smaller testes than males of solitary species, implying that social male dominance hierarchies result in reduced levels of sperm competition.”

Additionally, the study discovered that testes size increased as mating season length decreased, showing that “temporal synchrony of female receptivity predicts the intensity of sperm competition in Australian rodents,” Dr. Firman notes.

Males may be effective in monopolising reproduction within social groups, resulting in lower levels of sperm competition than in non-social species with free-ranging females.

“Overall, our findings indicate that patterns of female distribution, both spatially and temporally, can have an effect on the strength of post-mating sexual selection in species,” the researchers write.

Finally, female distribution patterns will influence the severity of sperm competition by determining the amount to which males can prevent female remating.

“This study contributes to our understanding of how the spatial and temporal distribution of mating opportunities shaped the evolution of sperm competition,” says Dr. Bruno Buzatto, a lecturer in terrestrial ecology at Flinders University who is also affiliated with the University of Western Australia and Macquarie University.

Further information: Renée C. Firman et al, The spatial and temporal distribution of females influence the evolution of testes size in Australian rodents, Biology Letters (2022). DOI: 10.1098/rsbl.2022.0058

Journal information: Biology Letters

Source: Flinders University