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.
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.