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Sexual orientation in males and relationship to sperm size November 15, 2009

Posted by Geekgirl in anatomy, genetics, heredity, sexual orientation.
Tags: , ,

Are you gay because you came from a large, fast swimming sperm?

Here is the question strictly from a biological point of view. If gay males do not procreate, why doesn’t being gay die out? We all know that gay male humans can and do procreate. But gay animals? For the purpose of a biological argument, let’s assume gay males do not procreate.  This has often been called a Darwinian paradox in the research. Biologists answer this question differently from everyday people. The gene(s), or whatever the complex biology is behind being gay, doesn’t die out because there must be some advantage. Make sense? I came across a recent publication that presents a novel prediction in the research on sexual orientation and why being gay doesn’t die out.

Dr. Reed at Harvard presents an interesting correlation and proposes an interesting prediction. Please note. Correlations are NOT proof of a direct cause and effect. Also, a prediction is just that. A theory that is waiting for further experiments.

The article’s abstract and reference appears below. Since it’s a little hard to read, I’ll do my best to translate into every day words.

A bit of background and vocabulary to help with the abstract.  From a biologist’s perspective, we know that homosexual behavior exists across many species. As Dr. Reed notes in his article, and he draws on many prior publications, 96 to 98% of men are straight and this is a stable number. This means the number of gay men is also a stable number. He also makes the point that same sex behaviors have been noted in nearly all groups of animals, totaling nearly 450 different species. This behavior has been well studied in fruit flies, a common laboratory model. The genus name is Drosophila.

We also know that the size of the sperm compared to the size of the egg varies between species. It also varies between males of the same species. When a sperm is close to the same size as the egg, this is called isogamy. Gamy is short for gamete, iso means the same. When a sperm is different in size from the egg, usually much smaller, this is called anisogamy. An is used to contradict the iso. In other words, different.

So what did Dr. Reed find? Dr. Reed studied publications regarding different Drosophila species, the length of the sperm and the frequency of homosexual males within these species. This is what he found:

Drosophila hydei have a nearly 1:1 size ratio of sperm to eggs while Drosophia melanogaster have a ratio of 0.08. That means the sperm are 8% of the size of the egg. These numbers mean the actual length of the sperm compared to the eggs. In addition, D. hydei males are significantly more interested in other males than were D. melanogaster males.

At this point, you might be thinking – what does this mean? Glad you asked. This is where the big question comes in, that question of why doesn’t being gay die out?

There are already reports that women with gay sons and siblings are more fertile. We don’t know why. There are already several confirmed studies that the more sons a woman has, the odds of a gay son goes up. Theories have been proposed but those theories need research to prove them.

Dr. Reed’s theory, a prediction, is new for me. It goes like this.

We know that when a man makes small sperm, more sperm are made. We know that when a man makes large sperm, fewer sperm are made. It’s a matter of how much energy and resources it takes to make sperm.

Making more sperm  increase the odds of fertilizing an egg.  But – and here it comes – it’s the large sperm that can swim faster. The sperm that gets there first gets the egg. In other words, both reproductive strategies have advantages and are in competition with each other from a biological point of view.

So  the fruit flies with longer sperm have more homosexual males. What is it about longer sperm that causes this? We don’t know. Dr. Reed doesn’t speculate. But clearly, if we can see a difference in sperm size, we know the sperm must be different.

What does this mean for humans? Sperm size varies within an individual of course. But more importantly, it varies more between individual males. Dr. Reed quotes a paper from Morrow and Gage that shows a variation of 48 to 60 microns across a sample of only 10 young healthy men. In addition, he makes the point that sperm morphology is highly heritable. What does that mean? One can theorize that men with greater sperm lengths will be more likely to exhibit same-sex behaviors. Dr. Reed is careful to point out that no studies have been done to test this theory.

For me, and these are my points, this theory leaves me with many questions and a new avenue to explore. If it starts with sperm size, what is it about big sperm? It could explain why women with gay sons are more fertile. It isn’t their fertility after all. It’s being fertilized by a fast swimming large sperm. Does sperm size change across a man’s lifetime? Does sperm size change if sperm count decreases? Sperm counts among men are decreasing worldwide and the cause, or causes, are unknown.

Does it all start at conception and all the other theories about what happens in the womb are a consequence, not the start? Makes some sense, doesn’t it? Does big sperm equal gay? I think this is research worth pursuing. For now, don’t let it go to your head.


Dr. Reed’s paper will be published in Medical Hypotheses. It has been electronically published ahead of the print version and can be found through Science Direct or PubMed Central.

Sexual orientation in males and the evolution of anisogamy

Dr.  Reed *
McLean Hospital, 115 Mill Street, Belmont, MA 02138, USA
Harvard University
a r t i c l e i n f o
Article history:
Received 27 August 2009
Accepted 10 September 2009


How might homosexual orientation have evolved and been maintained? Several adaptationist explanations
have been examined in attempt to reconcile the presence of same-sex sexual behaviors with traditional
selection-based theory, showing little empirical support. The current paper presents a novel
adaptationist explanation for the evolution and maintenance of same-sex sexual behaviors in males, both
between- and within-species, related to the evolution of anisogamy.

Under conditions of isogamy, sexual reproduction occurs between individuals with gametes of similar morphology. With the evolution of anisogamy came greater specificity on the types of individuals that would produce offspring when mated with (i.e. those with opposing gamete sizes). It is suggested that with this evolutionary change, a specified psychological adaptation orienting individuals primarily towards mating partners with newly opposing gamete sizes was then selected for. It is thus hypothesized that sexual orientation will vary along the anisogamy–isogamy continuum, with homosexual orientation being associated with closer approximations towards isogamy.

This hypothesis leads to two specific predictions. First, in comparisons between species, the presence of same-sex sexual behaviors will be more likely to occur as sperm to egg ratios approach 1:1. Second, in comparisons within species, those individuals with greater sperm lengths will be more likely to exhibit same-sex sexual behaviors than those with lesser sperm lengths. Examination of the present hypothesis stands to greatly increase our knowledge of the selective forces shaping both biological and psychological evolution.

_ 2009 Elsevier Ltd. All rights reserved.

Conflicts of interest statement

None declared.


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