These bats uniquely mate like birds among mammals.

 The male serotine bats have penises seven times longer than the females' vaginas, rendering penetration impossible due to the size difference.

Male serotine bats (Eptesicus serotinus) possess penises that are seven times longer than the vagina of a female from the same species, rendering traditional penetrative mating unfeasible. In a unique departure from typical mammalian behavior, these bats employ a strategy observed in birds but never before documented in mammals.

Bats, the sole mammals capable of flight, distinguish themselves as oddities in the mammalian realm. Serotine bats, in particular, attract attention for a remarkable feature—when erect, the male's penis can expand to nearly a quarter of its body length. Researchers, as reported in the November 20 issue of Current Biology, detail an unprecedented mating method observed in these bats, where the disproportionately large penis, over 16 millimeters when erect, cannot fit into the female's 2-millimeter-long vagina. 

Biologist Nicolas Fasel's curiosity led to the discovery, documented through videos captured at the Ukrainian Bat Rehabilitation Center and the attic of St. Matthias Church in the Netherlands, unveiling the unique mating behavior of these bats without penetration.

Rather than inserting the penis into the vagina, male serotine bats employ a unique method: they use it to displace a membrane covering the female's genitals and then press the penis against the vulva, maintaining contact for extended periods, sometimes exceeding 12 hours. After mating, the dampness observed around the female's vulva is suspected to be semen, resembling patterns found in other ². This unconventional mating behavior, resembling avian practices, marks a novel phenomenon in mammals, according to observations by biologist Nicolas Fasel and colleagues. 

Evolutionary biologist Patricia Brennan notes the intriguing departure from the conventional function of a penis, emphasizing the eccentric reproductive strategies seen in bats, known for sperm storage and unique penile structures. Fasel concurs, dubbing bats as the outliers in the mammalian realm with their peculiar behaviors.


N.J. Fasel et al. Mating without intromission in a bat. Current Biology. Vol. 33, November 20, 2023. doi: 10.1016/j.cub.2023.09.054.


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