"A male's penis may be all that a female has to go on when assessing the quality of her suitor."

Golden moles - small, blind, burrowing mammals from southern Africa - are more closely related to aardvarks, manatees and elephants than to true moles.

But one feature that is more mole- than elephant-like is the size of their penises, which on average are just a couple of millimetres long, says Stuart Blackman. Yet research suggests that even this can be enough to impress a female.

Biologists have found that some male Hottentot golden moles Amblysomus hottentotus are far better endowed than others.

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Crucially, the mole's penis is much more variable in size than other parts of its body, a sure sign that the organ is used by males to compete for mates.

One of the researchers, Bill Bateman of South Africa's University of Pretoria, told BBC Wildlife that, during the moles subterranean sexual fumblings, a male's penis may be all that a female has to go on When assessing the quality of her suitor.

"We know how many animals choose their mates, but this is mostly based on species that use cues we are familiar with, such as sight and sound," said Bateman. "Golden moles can't do this, and neither can bats in caves nor seals in murky water - they have to find other ways to judge potential mates."

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The biologists suspect that females will start mating with any male, but then quickly reject those that don't measure up, on the basis that males who can afford to invest in a longer penis are higher-quality mates.

The length of the females reproductive tracts also varies, and this may represent a process of co-evolution between males and females. "Deep vaginas make it more difficult for males to deposit sperm close to the site of fertilisation, which would also benefit females as only the best sperm from the best males would do," Bateman said.

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Top image: suncana, CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons