THE GIST

- At least five types of animals maintain relationships that are comparable to human friendships.

- While older animals may help to strengthen other friendships, these relationships are not solely based on size, age or other factors.

- Since even small-brained bats enjoy long-term relationships, large brains are not necessary for maintaining social connections.

As Valentine's Day cards attest, humans value love and friendship that aren't just forged by family ties, common interests or sexual attraction. Now researchers have determined that such human-like friendships exist among at least five different types of animals.

Prior studies determined that elephants, dolphins, some carnivores and certain non-human primates, such as chimpanzees, have the ability -- just as humans do -- to maintain enduring friendships in highly dynamic social environments. A new study, published in the latest issue of the Proceedings of the Royal Society B, adds bats to that list.

Female wild Bechstein's bats prefer to literally hang out with certain friends while they also keep loose ties to the rest of their colony. Lead author Gerald Kerth told Discovery News that these bat buddies mirror human ones. Despite all of their "daily chaos, the bats are able to maintain long-term relationships," he said.

"We do not work, play and live together with the same individuals all the time during the day and week," he explained. "But nevertheless, we are able to maintain long-term relationships with our friends and our family despite our often chaotic and highly dynamic social lives."

Kerth, a professor at the University of Greifswald's Zoological Institute, and colleagues Nicolas Perony and Frank Schweitzer monitored colonies of the bats over a period of five years. Male bats of this species are solitary, but females roost together in bat boxes and tree cavities. They preferred certain companions over the years.

In addition to resting together, "colony members exchange information among each other about suitable roosts, make flexible group decisions where to communally roost next, groom each other and profit from communal roosting through warming of each other," Kerth said.

The researchers determined that the female bats did not just select their companions based on size, age, reproductive status or relatedness, although older female bats often maintained links between friend subgroups.

Kerth believes the human-like friendships likely exist among other bats living in temperate zones, since these bats often live in colonies that also frequently split and merge, a phenomenon known as "fission-fusion."

"Fission-fusion dynamics with long-term social relationships are also found in elephants, dolphins, chimpanzees and humans," he said.

Jonathon Balcombe, an animal behavior research scientist and consultant for the Physicians Committee for Responsible Medicine, pointed out that female bats even sometimes serve as midwives.

In one documented case, a pregnant fruit bat in Florida was attended by an unrelated female fruit bat, according to Balcombe, who is author of the book "Second Nature." The helper bat repeatedly groomed and hugged the pregnant bat during the birthing process.

"Following birth, the helper groomed the pup, and she and a third female fanned the mother with her wings," added Balcombe.

Other animals with human-like friendships, such as elephants, tend to have large brains, but bats prove that even species with "peanut-sized brains can also have long-term relationships," Kerth said. "This for me suggests that it does not necessarily take large brains to keep track of relationships in highly dynamic societies."

The discoveries may negate prior theories on why humans evolved large brains and became such a dominant species. Previously, some anthropologists thought this was driven by our need to maintain relationships in an ever-changing social setting. Now it's suspected that other factors must have come into play. In fact, tiny-brained bats may even be better at maintaining friendships than we are.

Kerth said he admires "bats for doing things that I find difficult: making quick and efficient group decisions in a group of 20-plus individuals and keeping social relationships despite a regular mixing of social partners."

"I sometimes feel that this is cognitively demanding for me (since) we all know the embarrassing feeling when we forget the name/affiliation of somebody we haven't seen in a while but who remembers us well," he continued, suggesting that this recently happened to him at a conference.

In the future, he'd like to see if bats ever experience such social faux pas.