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Social bonds & sperm storage

Eleanor Konik
Written by Eleanor Konik

I write stories & articles inspired by all eras of history & science... so I wind up putting notetaking software like Obsidian & Readwise thru their paces.

2 min read.
close-up of a bat's face
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.

Quick Facts

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.

Proximal Grooming

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.

Divergent Dispersal

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.

Salacious Storage

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 👀

💚 If you learned something from this overview, consider forwarding it to a friend and encouraging them to sign up for more overviews of my research into obscure history and science.

🦇 Do you know something particularly cool about bats? Please reach out — I'd love to hear about it, either via email or in a comment where other readers can see.

Note: There are a couple of affiliate links & codes scattered around, but these always come from links I was already recommending and usually I share them because they benefit you too (i.e. getting you extra time on trials).

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