Male pygmy hippos control sex ratio with their sperm

You're still my mum

6 month old pygmy hippo Monifa snuggling up to her keeper at the Taronga Zoo. Image by myopixia via Flickr.

Okay, I’ll admit it: I’m using this paper as an excuse to post pymy hippopotamus pictures. They’re so cute! It’s also a nice distraction from what I am actually supposed to be working on: my thesis. My posts will continue to be sporadic for the next couple of months while I focus on, finally, getting this Ph.D.

Besides the cuteness of miniature hippos, the science in this paper is also really cool. Joseph Saragusty and colleagues at the Leibniz Institute for Zoo and Wildlife Research, along with Tim Bouts of the Zoological Society of London discovered that male pygmy hippos can control the ratio of female-to-male offspring they father. The paper was published in the February 28 issue of Nature Communications.

The pygmy hippopotamus (Choeropsis liberiensis) is a critically endangered species native to western Africa. Less than 3,000 remain in the wild, due to habitat loss and hunting. The pygmy hippo looks a lot like common hippos, but they’re about a fifth the size (and much less aggressive, too). Unlike the common hippo, pygmys are solitary and nocturnal. They also spend less time in the water, instead preferring to root around in the forest.

A pygmy hippopotamus mating pair at the Duisburg Zoo in Germany. © Raimond Spekking

Despite their reclusive nature, pygmy hippos breed well in captivity. In mammals, the number of daughters and sons should average out to be equal, but captive populations of pygmy hippos are skewed towards having more females. Of all live births recorded between 1919 and 2008 (1,089 births in total), less than 43% of those were male. This is much lower than the expected 50%. The researchers of this study wanted to know why so many more daughters are born than sons.

Skewed sex ratios often pop up in animals, but how and why it happens is a mystery. When sperm are being made in the male, there should end up being an equal number of sperm with X and Y chromosomes, and therefore an equal probability of fathering a daughter or son. But when one sex is produced more than the other, scientists have generally assumed it’s because the mother is somehow skewing the sex ratio in a way that will benefit her more–because mothers have to invest more in pregnancy and raising the babies, so they should be pickier about such things.

So dads have been left out of the equation in studies of skewed sex ratios in mammals. When looking at the pygmy hippo population, however, the authors of this study decided to keep an open mind. If the male is in control of the offspring sex ratio, what would be the best way to find that out? Look at the sperm and literally count the X and Y chromosomes!

The scientists went to zoos and collected ejaculated sperm (by electroejaculation) from 10 males–10% of the male world captive population–to count up their sperm. The sperm were “painted” with dyes that stuck specifically to either the X chromosome or the Y chromosome. Then, the number of each color sperm was counted by eye.

Some poor scientist counted about 2,000 sperm from each sample (talk about microscope neck. Ouch!). Surprisingly, the numbers of X and Y chromosomes were not equal. In fact, the Y to X ratio (43.4%) was almost exactly the same as the ratio of sons to daughters in the population (42.5%).

Pygmy hippo and baby

Mama and baby at the Edinburgh zoo. Photo by Lynne Mellstrom via Flickr.

What about mom? Is she involved in the decision to produce more daughters than sons? The researchers didn’t do any experiments to test for sex selection on the female side, but they did compare the probability of three different scenarios.

First, males and females might be in conflict over the optimal sex ratio, and this conflict averages out to 42.5%. Second, females might be neutral, and only males have an effect on sex ratio. The final possibility is that males and females share the same goal, to have more females than males born in the population. Based on the statistical analysis done in the paper, the third possibility seems the most probable (though they couldn’t rule out the second). Isn’t it nice when mom and dad can agree?

Unfortunately, there’s no data on wild pygmy hippopotamus populations. Nearly nothing is really known about their breeding behavior in the wild. However, it is known that males need to control relatively large territories for breeding purposes. The authors speculate that perhaps it’s in the father’s best interest to produce fewer sons, because there’s only so much territory to go around. Investing more in daughters increases his ability to pass on genes to the next generation. But we have no way of knowing if this is true.

Another thing we don’t know is how males can selectively get rid of Y-bearing sperm. Each round of cell division during sperm production has to create an equal number of X and Y-carrying sperm. What happens to all those Y-chromosomes? And how do the males only get rid of 7% of them–instead of all of them? I can’t wait for that paper…

In the meantime, we’ll have to console ourselves by looking at more adorable pygmy hippopotami! Check out the links below:


Saragusty, J., Hermes, R., Hofer, H., Bouts, T., Göritz, F., & Hildebrandt, T. (2012). Male pygmy hippopotamus influence offspring sex ratio Nature Communications, 3 DOI: 10.1038/ncomms1700

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