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Bat research is fascinating

my research project on sperm competition in bats

Hi folks – long time, no blog!

I have written before about the relationship between testes size and brain size in micro bats (aka echolocating, insect-eating bats as opposed to fruit or nectar eating bats). Many of you have also emailed me information about the scientific study that was published in 2006 by research team Pitnick et al. that first demonstrated this relationship. Thank you very much for that and in exchange I thought I might give you a bit of an update on what I am in the middle of doing to further our understanding of the brain-testes relationship in bats.

For my masters degree I am looking at whether or not there are any differences in the brain volume of male and female micro bats. I have been wondering if there might be ever since reading about Pitnick et al.’s study. My thinking was that if there is, indeed, some sort of inverse relationship in bats between brain size and testes size whereby the two metabolically expensive organs trade off against each other then it stands to reason that perhaps females could have larger brains than males since they don’t have testes competing with their brains for dominance in the body. At least, I thought, this might be the case in species where males have very small brains due to their enormous testes which hog all the body’s resources.

My study is fairly small in scale but I am hoping that it will still prove interesting. I am comparing averages of male and female brain volumes in 9 Australian micro bat species – some that have mating structures where females are promiscuous and some where females are loyal to a single mate (a dominant male harem leader). To do this we are CT scanning a collection of skulls and then using computer technology to create 3D models of the brains as a way to ascertain volume.

Here’s a picture of the first skull we scanned. It makes me laugh every time I look at it – such a big machine to scan such a tiny object (that little spot on the blue towel is the skull):
going through the CT scanner

And here is a model I am putting together of that same skull. It needs work but it’s still pretty cool:
model of a bat skull - in progress!

Interestingly, the factor that influences testes size in micro bats in female sexual behaviour. When females are highly promiscuous and take many sexual partners the males tend to have much larger testes than species where females are loyal to a single mate. It is assumed that the large testes trait develops in species where females are promiscuous because there is greater competition among sperm for access to the eggs in these individuals and so those males with more sperm available tend to be selected for over time. It is similar to how if you are really keen to win a meat raffle you will buy more tickets.

The same relationship between female promiscuity and testes size occurs in primates, including humans and for that matter, basically all animals. You can look at most any wild animal and get a good idea of the level of female promiscuity by looking at the testicles of the males. The difference with bats is that because bats weigh very little (as little as 2 grams!!) and have an extremely tight energy budget they cannot afford to have both heavy brains and heavy testes and so they trade one tissue type against the other.

Pitnick and his colleagues in their landmark study originally predicted that the opposite relationship would occur – ie that large brains would equal large testicles. They postulated that species with promiscuous females would have larger brains as a way for the males to avoid being cuckolded. They were surprised when they discovered that the opposite was true and remarked that “perhaps monogamy is more neurologically demanding.” I think many of us would attest to the truth of this statement!

If my study shows that there are any size differences in male and female bat brains as a function of testes size it may potentially mean that female promiscuity is a force strong enough to influence male intelligence in a measurable way. An intriguing notion but at this stage very much a theoretical one.

*Sidenote: the boys on the construction sites where I have been working have heard me talk about animal testicles so much that they have become terrified I’m going to show up at their bedroom door one day with my calipers! Actually that’s not completely true .. some have actually volunteered for the job! .. all in the name of science, right? 😉

** Oh and yes the bat in the picture above is alive! It was just very very cold because I pulled it from a tree which had just been buldozed on a construction site in the very early hours of a winter morning.

Men are competing for us even after they have cum and gone (sperm competition in bats and people)

It’s true – even after a man manages to woo us, fend of the competitors and get us into the sack, the job still aint done. At this point he basically just relays the responsibility onto his sperm who continue to compete in what reproductive biologists call a “sperm competition”. For sexually monogamous couples there are no sperm competitions but when a woman cheats on her partner or if she goes on some kind of unprotected Girls Gone Wild SOCal spring break, there can be stiff competition (pun sadly intended) for sperm.

I have just begun research into sperm competition for my master’s degree. I’m studying it in regards to bats. Bats have always been of fascination to me but it wasn’t until I started learning about their sexcapades that I decided to go into major student debt over it!

You see, because many species of bat live in gigantic social groups (sometimes numbering in the hundreds of thousands) there are near unlimited opportunities for them to sleep around (or “hang out” as we Gen-Y’ers call it ..hey maybe we get this term from the bats!). Most bats also don’t believe in monogamy. Only 12 of the over 1,200 species of bat are believed to be monogamous – and with a little genetic testing, I’m convinced it’s even less than that. They tend to live either polygamously or, more commonly, like baboons where any male can mate with any female with the exception of some direct family members. For bats, every day is a SOCal spring break! At least in the breeding season, anyway.

Even more ridiculous, female bats can distend their reproductive tracts to hold huge quantities of semen which they can keep alive in their bodies for up to 200 days!! All of this adds up to a situation where sperm are competing with each other like gang busters.

What happens, then, when the sperm have to compete so much is that natural selection favours males with larger balls (that produce more sperm) in the way that if you want to win a raffle you buy more tickets. Over time, then, bat balls have evolved to absolutely vulgar proportions! In one species, the Rafinesque’s big-eared bat, the combined testes weight is 8.4% of total body weight. That’s the equivalent of a 180lb man walking around with 20lb balls! IT’S INSANITY!!

The craziest part of it, too, is that because bats have to maintain low body weights in order to fly they can’t just carry around these gigantic balls without having to pay for it somewhere else and so not surprisingly – the brains take the hit. They have actually found that the larger the balls of a bat, the smaller the brain*.

I was just thinking about all of this in the shower (as you do) and realised how ridiculous it all is. I mean in an effort to be the winner of the sperm competition, testicles only think to do one thing: produce more sperm. Even if the end result makes them dumber. Does it never occur to them to produce quicker, faster, more efficient sperm and not have to sacrifice grey matter? Nope. Well, sometimes. Occassionally, rather than producing more sperm to win the sperm competition they’ll produce bigger sperm but this really isn’t much better.

And doesn’t this just sum up men perfectly?? I mean the male agenda is always about bigger, better, more. Women on the other hand, understand value. That’s why we produce one perfect, valuable, egg rather than a million crappy ones. Not to mention why we fantasize about one perfect partner instead of a threesome …

*This doesn’t apply to flying foxes

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