Upon reading Nellie Bowles great piece “The Dawning of Sperm Awareness” in the New York Times about the conundrum of our dropping sperm counts, I asked myself: “Am I coming across as a disruptor by not being particularly worried that we are headed toward extinction?” I thought it a good opportunity to unpack the beef behind this belief.
Sperm Aren’t People
One reason that I am not terribly worried about our sperm counts is that they are a poor measure of a species’ fertility. Except when absolutely zero, sperm counts don’t correlate well with actual fertility. When someone tells me that human sperm counts have fallen 50% (from 100 million/mL to 50 million/mL) over 40 years, I say “So what, they’re still normal! If you believe that God or Darwin has given up on us as a species, then yes, we are headed the way of dinosaurs.
We Will Survive!
Another reason I am not concerned about a trend toward human extinction is that there is no hard evidence that our “fecundity” (defined as our biological capacity to reproduce irrespective of pregnancy intentions) has changed. And to your retort, “Well, surely our fertility is decreasing” (defined as demonstrated fecundity measured by live births) you are heading into perilous waters! Our fertility is influenced by a whole host of human behaviors, including lifestyle and contraceptive choices, family planning, jobs, divorce, war and natural disasters. One thing that impressed me the most at the NIH workshop on Human Fecundity held 3 years ago was the mathematical argument that if one simply figures in the delayed parenthood elected by many couples in developed nations, this alone could account for the perceived decline in birth rates (a measure of fertility not fecundity) in these countries. And reproductive choice, my friends, has nothing to do with fecundity.
Let Sperm Speak
Aside from obtuse epidemiological arguments, what does biology teach us on this matter? One thing that I learned while consulting with zoos over the past several decades is how much more sperm and how much better swimming sperm other mammals have compared to humans. Yes, our sperm really does look “like crap” as I stated to Nellie Bowles in the Times.
For the sake of evolutionary proximity, take the mammals considered most closely related to us. Chimpanzees ejaculate 10 to 20-fold more sperm, a billion or more per ejaculate, than do humans. But, gorillas and orangutans have sperm counts very similar to humans, on the order of 20 million to 120 million/ejaculate.
Why the difference? Most reproductive biologists agree that it’s because gorillas and orangutans, like humans, exist in a one-male mating system in which several females mate with an alpha-male. Reproductively, this is a low competition state for males. With chimps, however, the opposite is true: females copulate with multiple male partners. Bottom line, when males compete for females, sperm counts need to be higher. When they don’t compete, sperm counts don’t need to be so high.
It appears that the same thing is true for sperm motility. Among species in which male reproductive competition is stiff, sperm swim much faster and harder, a feature that is evolutionarily favored with polygamous mating patterns. Indeed, chimp sperm (the fastest) is a whole lot more lively than human sperm (OK) and also gorilla sperm (lowest). So, maybe there is a good biological reason why humans don’t need super sperm anymore.
Human evolution is certainly occurring, but maybe not along the same trajectory as it has in the past. It is also not happening in any way similar to that of wild animals or even our ancestors. How could it? We’ve created environments that have little or nothing to do with nature. High sperm counts may not be a feature of humans in the future, but we may not need as much sperm as we used to anyway. It’s an evolutionary adjustment, not extinction.