Male rodents with shorter breeding seasons tend to have larger testes, probably because the time pressure increases sperm competition between males.
Studies have found that males typically have bigger testes when the females of their species have multiple mating partners. This is because their sperm must compete within the female reproductive tract to fertilise the eggs, meaning it is advantageous to have bigger testes that can produce more sperm.
鈥淚t鈥檚 like a raffle 鈥� the more tickets you have, the more likely you are to win,鈥� says at the University of Western Australia in Perth.
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Firman and her colleagues wondered if testes size may also be influenced by length of breeding season, since less time for mating should create more intense competition between males.
They studied the relationship between the length of breeding season and testes size in 33 Australian rodent species that have females that mate with multiple males.
Breeding season length was estimated using data from that showed when the habitats of each species produced enough food for them to reproduce. Testes size was measured by weighing museum specimens.
As predicted, the results showed that species with shorter breeding seasons tend to have bigger testes.
For example, the plains mouse (Pseudomys australis) has a relatively short breeding season because it lives in the desert region of central Australia where food is limited for much of the year. Males have large testes that are 3.5 per cent of their body weight.
The western mouse (Pseudomys occidentalis), in contrast, can breed all year round because it lives in areas of high vegetation near the south coast of Western Australia. Males have smaller testes that are 0.5 per cent of their body weight.
鈥淵ou can imagine if there鈥檚 a shorter breeding season, individuals are mating in a more intense period so there should be greater levels of sperm competition,鈥� says Firman, who will present the results at the later this month.
at the University of Adelaide in Australia agrees that breeding season length could affect testes size, but says there are probably additional factors at play. For example, sperm competition and testes size could also be influenced by population density, she says.
The relationship between breeding season length and testes size hasn鈥檛 been studied in other mammals, but聽聽found that male mammals with shorter breeding seasons were more likely to mate with females outside their normal social group, which is also an indication of increased sperm competition, says Firman.
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