Penis size does matter for bank voles, says study

Bank vole (c) Dr Mike Thom Bank vole males have a lot of competition for mating

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The secret to social dominance for bank voles appears to be the size of their genitals, according to scientists.

The link was made by researchers from Europe who were studying the small brown mammals' reproductive behaviour.

The study, in Behavioural Ecology and Sociobiology, found dominant males had wider penis bones, also called baculum.

Although not present in humans, these bones are found in many other species of mammal but their exact function has not been confirmed.

A scan of the baculum bone (c) Jean-Francois Lemaitre The bank vole's penis bone is trident-shaped with a wide base

The study was conducted by Dr Jean-Francois Lemaitre from the University of Liverpool with colleagues in France and Switzerland.

Bank voles live for a maximum of 18 months and females give birth to four or five litters per year.

"This species is particularly interesting for study... because females mate with several males during a single reproductive bout," explained Dr Lemaitre.

Researchers suggest that this competition may have driven evolutionary adaptations in genital anatomy to improve males' chances of reproduction.

To test their theory, the team collected wild bank voles in Cheshire and studied their lab-reared offspring to understand which were dominant and which were subordinate.

PENIS BONE FACTS

  • Humans do not have bones in their penises but many other primates do, including gorillas and chimpanzees
  • All male rodents have bones in their penises
  • The fossilised baculum of an extinct walrus was discovered in 2007 and measured 1.2m

Pairs of males were then exposed to nesting material from a female and the scent mark patterns they made were recorded.

The males that left more scent marks were classified as dominant, as they made more attempts to attract the female.

Researchers then scanned the baculum of all the males and compared the images.

Width or length?

Previous studies have focused on the length of penis bones in rodents but in this instance, the team discovered that differences in the width held greater significance.

"Our main result is that dominant males have larger, but not longer, baculum than subordinate males," said Dr Lemaitre.

The largest baculum found in the study measured 1.7mm wide and 4.5mm long.

Microscope photo of bank vole penile spines (c) Meg Stark Bank voles also have microscopic spines on their penises

"Although studied for a long time, the exact function of the mammalian baculum is still unknown," said Dr Lemaitre.

He theorised that as female bank voles require physical stimulation to release eggs, wider baculum bones may achieve this more often, leading to greater reproductive success.

"Our study is a first step in the understanding of the relationships between genital [structure] and reproductive success in mammals," Dr Lemaitre told BBC Nature.

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