Sperm are pretty cool. Some are tiny, some are huge. Some have hooks, some crawl, and some can’t actually fertilize an egg. But all of them are awesome products of evolution. That’s why I want to start a collection of posts dedicated to quirky sperm.
For the first post, a relatively recent paper (about a year old) on naked mole rat sperm.
Do you know naked mole rats? They are weird, ugly (in my opinion) little mammals that live more like ants than rats. They freak me out (see picture). But apparently, their sperm are nothing to be afraid of.
Scientists Gerhard van der Horst and colleagues in South Africa studied the structure and swimming ability of naked mole rat sperm to see if there was anything special about them. What they found was a jumble of poorly assembled, lazy-swimming sperm. The conclusion: competition really is healthy (especially for sperm).
Why did the scientists think there might be something funky about naked mole rat sperm in the first place? Well, aside from being totally freaky-looking, naked mole rats have other quirks. They behave more like social insects than almost all other mammalian species. Just like ants or termites, naked mole rats live in colonies with a single queen who has all the babies. There are a few (1-3) breeding males, and the rest of the rats are workers.
So what does this have to do with their sperm? In most mammalian species, males have to compete for females. In this scenario, several males will often mate with a single female, allowing for competition between the sperm of the different males. As you might imagine, faster sperm with structures that allow them to efficiently fertilize an egg would have a better chance of getting to an egg first. This allows males with faulty or slow sperm to be weeded out of the population. In the naked mole rats, there is no such competition.
There is also no competition between females. In evolution, he or she with the most babies, wins (simplistically speaking). Naked mole rat queens don’t have to worry about that. Their genes will be passed on to all the rats in the next generation. So if she has a few less babies because the sperm she’s using to fertilize her eggs are kind of crap, that’s okay.
The researchers’ predictions were that naked mole rat sperm would swim slower and be shorter than sperm from species with more competition. They also predicted that breeder males would have better sperm than the non-breeders.
The truth turned out to be far worse than they could have imagined. Naked mole rat sperm are sorta screwed.
The sperm were not only slow, and not only short, they were a mix of normal-ish and totally mashed up sperm. There was no real difference between the breeders and non-breeders, since each male had a mixture of sperm types.
From the article:
…sperm structure varies markedly with sperm head shape, including round, oval, elongated, slightly lobed, severely lobed and asymmetrical heads. There are several further variations that could only be described as irregular or amorphous.
That’s science speak for “their sperm is wack”.
Just how wack is it, compared to the sperm of other mammals? Unlike normal mammalian sperm, naked mole rat sperm does not have a smooth, normally-shaped head. Instead the sperm have irregular edges, sometimes have multiple lobes, and can often be fragmented.
Even the sperm tail is degenerated compared to other mammals (even the more ancient marsupials). The tails are missing a part called the “fibrous sheath” which is thought to give structural support to the sperm tail so it can quickly swim through viscous liquid. This may be part of the reason that naked mole rat sperm swim really, really slowly (compared to, say, human sperm). Another reason is that the sperm contain fewer mitochondria, which are needed to produce energy for swimming.
On top of the severely degenerate sperm shapes, naked mole rat sperm often had messed up DNA, too. Instead of being tightly packed together, as in normal sperm, the DNA was more loosely packed and sometimes even broken up.
The authors point out that while many of the defects are likely due to the fact that males in this species don’t have to compete with other males for fertilization, some defects (like the DNA) could be due to inbreeding.
In other words: naked mole rats, WTF?
Other weird things about naked mole rats: 1) They can’t regulate their body temperature like other mammals. 2) They need very little oxygen to survive. 3) They don’t itch and their skin can feel no pain. 4) They’re naked and blind.
Other articles on this blog about wacky sperm:
- Diatom sperm go fishing for eggs
- The making of a toad, through sex and cloning
- Sperm to spare
- Sperm toxins in worms
van der Horst G, Maree L, Kotzé SH, & O’Riain MJ (2011). Sperm structure and motility in the eusocial naked mole-rat, Heterocephalus glaber: a case of degenerative orthogenesis in the absence of sperm competition? BMC evolutionary biology, 11 PMID: 22142177