Work on the novella starring the winged species of lactating egglayers continues apace, so it was time to do some hard thinking about some niche elements of their social structure. Since they were originally inspired by bat physiology, it occurred to me I should learn more about bat social structures.
- Female vampire bats have remarkably egalitarian relationships compared to other mammals that live in groups; their rank (insofar as it exists) is not correlated to size, age, reproductive status, kinship, grooming, or food sharing.
- Many bat species can live for over 30 years.
- Bats are one of the few animals that, like humans, can't synthesize their own Vitamin C and need to get it from their diets.
- It's usually the other way around, but spiders can prey on bats.
- Bat social structures have more in common with primates and octopuses than animals like mole rats, which means that how closely related individual bats are isn't a great predictor for how socially important or likely to cooperate they are.
Classification is Hard
There are two main groups of bats. For a long time, scientists divided them between the ones that rely on vision, and the ones that rely on echolocation. Now, scientists divide it up by genetics, and recently found that the different groups use different echolocation patterns. They either make continual calls on a constant frequency, or infrequent calls that vary a lot. The latter bats don't have bony ear canal walls, which makes them pretty unusual as far as mammals go.
Scientists studying the formation of social bonds stuck random bats together for a week, sort of like how college roommates are randomly assigned. Apparently, the vampire bats tended to maintain friendly relationships (as measured by grooming rates) with each other even months after they were released back into the broader group. This has interesting implications for arranged marriages (and proximity on relationship success) that weren't explored by the study, but it's still a pretty neat study.
As with most mammals, female bats tend to stick close to where they were born, while the males disperse across great distances. That said, all possible sex-biased patterns of dispersal occur in bats. A single male Molossus molossus might defend a group of unrelated females, while T. tricolor tends to roost in closely related kin groups where neither sex leaves their birth site. Female proboscis bats are the ones who leave, so interrelated males wind up roosting together.
Female bats can store fertile sperm for up to 200 days. Since bats tend to live in mixed-gender social groups, and aren't generally what anybody would call monogamous, this makes it hard for males to ensure paternity (and mixed-paternity litters are possible). Knowing that, I guess it's not surprising that there's a strong correlation between the size of a particular bat species' roosting groups and the size of the male testes. Body mass doesn't correlate, though, which leads to some funny mental images.
📗 If you found this interesting, you may also enjoy my very short story featuring a dad from this species defending his nest. You can also check out the collection of research papers I used to write this 👀
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