Two heads are not always better than one, it seems.

These conjoined twin male bats were found in 2001 under a mango tree in southeastern Brazil. The person who found the already-deceased animals donated them to collections at the Federal Rural University of Rio de Janeiro. Last month, they were described in a publication by Marcelo Nogueira at the State University of Northern Rio de Janeiro.

“We believe the mother of these twins was roosting in this tree when she gave birth,” Nogueira explains.

View Images An x-ray of the conjoined twin bats shows where their spinal columns converge. Photograph by Laboratório de Radiografias, Divisão de Vertebrados, Museu Nacional - UFRJ

Surprisingly, this specimen is the third example of conjoined bats found, but the phenomena is still poorly known outside of human beings. This is likely because so few of the animals probably survive. In people, twins being conjoined is typically a fatal condition over 80 percent of the time—and it may be higher among animals without medical or social support.

In people, where the phenomenon is well-studied, conjoined twins are still quite rare and only happen in 1 in 200,000 births in the United States. (Read more about two-headed sharks.)

The researchers believe these bats are newborn Artibeus bats from their physical characteristics, and also surmise they died at birth or were stillborn as their placenta is still attached.

An x-ray shows these male bats have separate heads and necks, but their spines eventually converge. They also have two similarly sized but separate hearts. (See a rare two-headed porpoise.)

Beyond just being an oddity, Nogueria explains, studying these bats can tell us more about their development.