The vast majority of bats strongly depend on, but do not make, shelters or roosts. We investigated Lophostoma silvicolum, which roosts in active termite nests excavated by the bats themselves, to study the relationship between roost choice and mating systems. Due to the hardness of the termite nests, roost-making is probably costly in
terms of time and energy for these bats. Video-observations and capture data showed that single males excavate nests. Only males in good physical condition attracted females to the resulting roosts. Almost all groups captured from excavated nests were single male-multifemale associations, suggesting a harem structure. Paternity assignments based
on ten polymorphic microsatellites, revealed a high reproductive success of 46%by
nest-holding males. We suggest that the mating system of L. silvicolum is based on a
resource-defense polygyny. The temperatures in the excavated nests are warm and stable, and might provide a suitable shelter for reproductive females. Reproductive success
achieved by harem males appears to justify the time and effort required to excavate the nests. Reproductive success may thus have selected on an external male phenotype, the
excavated nests, and have contributed to the evolution of an otherwise rare behavior in bats.