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Bat research is fascinating

my research project on sperm competition in bats

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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.

free love in the animal kingdom

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Like most women I have a keen interest in trying to understanding the dynamics of romantic relationships. I have never been particularly lucky in love and in trying to understand the reason for this have dedicated thousands, probably millions, of hours analysing, tearing apart, and seeking desperately to figure out what it is that makes for a lasting, nurturing, and healthy human relationship. As a University student I took a class by evolutionary psychologist Dr. Dave P Barash, author of “The Myth of Monogamy.” Dr. Barash’s class was one of my very favourites over the course of my degree but despite how much I enjoyed it, I did not enjoy the fact that it obliterated any hope I had for discovering in the animal kingdom an example of untarnished, fully-requited, monogamous love.

We’ve all heard stories of animals that “mate for life” – lobsters, swans, ducks, etc (check out “ducks, heartless rapists or evolutionary geniuses” if you are interested in a gigantic rant on this subject). The fact of the matter, though, is that of even the animals who form the strongest pair bonds, there are only a few examples of animals who have not been found to sneak away every once and a while to cheat – to form “EPC’s” or “extra pair copulations”. I learned from Dr. Barash, in fact, that the only species that we know for sure is fully monogamous is a species of tape worm that, at the time of sexual maturity, fuses its genitals to its mate. I suppose the praying mantis could also be considered monogamous as infidelity isn’t possible in a society where females eat the heads of their partners as a post coitus snack – to call that monogamy, though, seems a pretty desperate stretch.

If the quest is to understand human males, I suppose we have to look to primates, our closest relative being of course, the chimpanzee. In primates, interestingly enough, promiscuity of females can be predicted by looking at the testicle size of the males. Chimps have giant balls, the biggest of any primate and twice the size of the average man. To see what I mean, have a look at this post on DailyRandom.

They use these giant balls to produce in excess enough semen to flush out the lingering semen of any males to which their partners may have recently mated. In essence, male chimps have big balls because female chimps are whores. Ouch. To be fair, the males are whores, too though. In fact, chimpanzee society is reminiscent of 1960s style Haight Ashbury free love and males will stop nothing short of screwing a frog for the sake of a decent orgasm (look this up on YouTube if you feel the need to be disturbed for the rest of your life.) As human’s closest living relative, the love lives of chimps have done nothing to give me faith in the possibility of a future loyal and loving partner.

I was relieved slightly to find out about gorillas, a species whose testicles are microscopic in comparison to chimps and whose manhoods swell to a throbbing 1.5 inches in preparation for coitus. Their societies participate in a harem structure centred around a dominant male. The females tend to be extremely loyal and as a result, the male has no need for excessive genatalia. This is by no mean an example of monogamy but it is at least a stable and reliable social structure whereby the females are loyal to their partner and are in return, cared for.

Humans are about mid ranking across the primate testicle spectrum which suggests that our females have a tendency toward fidelity but that males still require a respectable volume of semen to flush out their women on the off chance that they have been acting lately more like a chimps than gorillas. There is significant variation, though, in testicle size amongst individuals of our humble species with some falling more toward the chimpanzee end of the spectrum and others toward the gorilla. Could it be that perhaps those men with large balls come from a genetic lineage of men who would be attracted to more promiscuous women and who, like chimps, would tend to be promiscuous, themselves? And could it be that men of smaller bits and pieces would tend to, like gorillas, attract more loyal women – or perhaps entire harems of loyal women?

Hmmm…

This would be a perfect campfire disucssion!