A pointed leathery nose and powerful forelimbs make for a proficient diggerAmong Hottentot golden moles, penis size, it seems, does matter to the female of the species.
Female Hottentot golden moles use the size of male golden moles' penises as a guide to their suitability as a mate, scientists have discovered.
A long penis makes a male mole more attractive and raises his chances of fathering offspring.
That suggests that penis size is a sexually selected trait in this species.
Scientists have published details of the discovery in the journal Mammalian Biology.
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Prof Nigel Bennett, University of Pretoria, South AfricaWe know very little about their mating strategy. Suffice to say it's probably every mole for himself”
Golden moles are a group of small, blind, highly specialised mammals endemic to sub-Saharan Africa.
They live underground in networks of specially dug tunnels, feeding on earthworms and the larvae and grubs of invertebrates such as insects.
"Golden moles are ideally suited to their ecosystem, they are cylindrical in body shape, have a pointed leathery nose shield that they use to push through the clay soil and two modified forepaws that resemble picks," said Prof Nigel Bennett of the University of Pretoria, South Africa, who conducted the study with Ms Tarryn Retief and Dr Bill Bateman of the same institution.
Though they appear similar, golden moles are not close relatives of true moles. They are more closely related to tenrecs, small insectivorous mammals.
The Hottentot golden mole (Amblysomus hottentotus) lives in South Africa, Swaziland and possibly Lesotho.
The moles are solitary and polygamous, mating at any opportunity. Females reproduce all year round, giving birth to litters of one to three babies.
"We know very little about their mating strategy," said Prof Bennett. "Suffice to say it's probably every mole for himself."
The researchers suspect the animals rely on smell and touch to find and gauge the attractiveness of partners.
To investigate in more detail they examined the body measurements of male golden moles.
Measuring up
Their study revealed that male Hottentot golden moles actually have very small penises, ranging from 1.2 to 2.5mm long. The animals themselves are between 74 and 97mm long.
"They probably have one of the smaller penises per length of body in the animal kingdom," explained Prof Bennett.
However, penis size varies significantly in male Hottentot golden moles.
Crucially the size of the penis varies much more than other body dimensions, including body length and testes size.
A rare surface glimpse of a Hottentot golden moleThat is a strong sign that penis length is a sexually selected trait in Hottentot golden moles, with females preferring males with longer genitalia.
The scientists already knew that the genitalia of some animal species is influenced by sexual selection; for example, many insects evolve elaborate-shaped penises to most effectively deliver sperm.
"The basic role of a penis is to put sperm as close to the site of fertilisation as possible, and perhaps to get it as far ahead of sperm of other males as possible," Dr Bateman told BBC Nature.
"Therefore it makes sense for males to invest, in the evolutionary sense, in big or long penises."
From the female perspective, attractive males are those which are able to invest heavily in certain traits, such as brains, muscles, foraging ability or penis size.
"So females can use penis size as a measure of how good - how high quality - the males are."
Penis size may be a useful trait in environments, such as dark underground tunnels, where females cannot spot other attractive traits.
"We think that penis size matters for these essentially solitary, more or less blind, underground dwellers. Judging penis size when mating may be one of the only criteria they have available to them," explains Dr Bateman.
"The mating is probably pretty rough and ready, and unromantic to say the least, so a physical assessment of a penis in situ, as it were, may allow females to reject males."
Other animals in dark environments, such as bats in caves or seals living in murky waters, might also use penis size to gauge the attractiveness of their mate, the scientists speculate.
"It's for these reasons that it is unlikely that penis size is used in this way in, say, humans," adds Dr Bateman.
"We tend to have a whole lot of pre-copulatory mechanisms to choose mates and [in humans] penis size really is something that is only discovered once a mate choice has already been made."
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