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.

Size matters – and if it didn’t, you’d be smaller

Does size matter? It’s the age old question. Some say that it’s the ‘motion of the ocean’ that matters while others have pointed out that ‘they don’t make small dildos’. Still others have pointed out that, actually, they do make small dildos! If you ask me, I’ll tell you that size does matter. But I’m not just speaking of pers

onal preference, here. The question of whether or not length and/or girth is important becomes obvious if you look, as I always do, to the animal kingdom.

Male humans, you see, have penises far larger than any of the other primate species around the world. Even Silverback gorillas, as massive as they are, only manage to achieve erections of about 1.5 inches. Many theories have been put forward for why humans have such large dingdongs. Originally they wondered if a big penis might have evolved to make insemination easier but that didn’t make sense since the less endowed primates are just as good at breeding as we are. They then thought a big wang might be useful in male/male competition (aka showing off) but then they realized that most indigenous cultures have coverings over their genitals so that didn’t seem like a good explanation either. They then wondered if a big cock might have evolved to help us to perform some of the more adventurous kama sutra style position that many of us are quite partial to but that also didn’t make sense because orangutans, chimps, bonobos and many other primates are into all the same crazy shit that we are – and then some!

Finally someone wondered

if female choice might have been involved but this was dismissed almost immediately after a few studies showed that women don’t really like looking at penises. This isn’t surprising considering what a huge epic fail playgirl magazine was!

So if women don’t even like to look at penises could female selection really be involved in the evolution of the huge human wang? Well as Meredith Small points out in her book ‘Female Choices: Sexual behavior of female primates‘ .. and as what virtually all women already understand .. a woman doesn’t have to love the sight of a cock to appreciate how it feels. Hence it is entirely possible and indeed very likely that the reason men have such large penises is that women enjoy them so much that over time they mated more often with these big boys than with the little smokeys and essentially bred the big cock gene into the species. Women, therefore, are amazing designers. Not just of fashion and homes but of men – and their penises!

Of course female don’t’ always have choice over who they mate with. There are arranged marriages and tribal system where a dominant male type character has rights to females and of course there has always rape. Also women will also make mate choices based on things other than cock size (shocking, I know). But generally speaking it can definitely be argued (and I do) that there have been enough women choosing mates based on their cock size over the last few thousand years to increase the size of human penis beyond what would ever be necessary on a purely functional basis.

So does size matter? Of course it does .. but don’t worry. You’re already doing way better than almost any other primate on Earth.

If you enjoyed this you might also enjoy my post on primate testicles