The naked mole rat must be one of the strangest mammals in existence. They live in underground colonies like those of ants and bees, with a fertile queen lording over sterile workers. They feel no pain in their skin, they live unusually long lives, they can cope with chokingly low levels of oxygen, and they seem to be immune to cancer. Their sight is poor, they can’t control their body temperature very well, and their teeth jut out beyond their lips. And they look like wrinkled sausages.
Now, just when you thought they couldn’t get any weirder, we can add another bizarre trait to the naked mole rat’s extensive list: they have really rubbish sperm. Gerhard Van der Horst form the University of the Western Cape, South Africa, sampled the sperm from a wide variety of male naked mole rats. For a start, he found that they are smaller than those of other mammals, and produced in lower amounts. Rather than the sleek, smooth tadpoles that other mammals have, the mole rat’s sperm had all manner of abnormally shaped heads.
Some are squashed, others long. Some have bulbous lobes all over them. There are sperm with shrunken heads, two heads, conical heads. The DNA inside them isn’t packaged properly. Most are asymmetrical. And all the males – whether breeding or not – had the same misshapen sperm.
Behind the heads, things don’t get better. In most mammal sperm, there is a distinctive midpiece, loaded with rings of energy-producing structures called mitochondria. These power the cells’ frenzied swimming. Humans have around 15 of these rings, and some rodents have up to 300. By contrast, the naked mole rat’s midpiece is short and disorganised, with just 7 rings of mitochondria. Behind the midpiece lies the tail, which is coated with a sheath of fibres that provide support as it beats back and forth. Even mammals with more primitively shaped sperm, like the platypus, still have sheathed tails. The naked mole rat? No sheath.
With all these deviations, it’s no surprise that the naked mole rat’s sperm won’t be winning any races. Only between 1 and 15 percent of them can actually swim, and just 1 percent of these can swim quickly. The vast majority move at a slow crawl; they might even be the slowest sperm of any mammal. Still, that seems to be enough.
All of van der Horst’s breeding males had raised healthy litters of pups. Their sperm may largely consist of deformed weaklings, but there are clearly enough normal ones to make more naked mole rats. Van der Horst thinks that the rodent’s odd social structure is responsible for its malformed sperm.
In many other animals, females mate with many males, or can store sperm inside their bodies. In these cases, the sperm do battle inside the female’s body for fertilisation rights. This “sperm competition” is the norm in the animal kingdom, and it has driven the evolution of longer and ever more elaborate sperm. Some have chemicals that incapacitate their rivals, some have barbs, and some team up with their brothers. Others win the competition by simply being very long and fast – some fruit flies, for example, have sperm cells that are a couple of inches long. When competition is intense, every sperm counts.
But the naked mole rat has no sperm competition at all. In any single colony, with around 40 to 90 individuals, the only ones that can reproduce are the queen and her male consort (with a couple of possible affairs on the side). With such fixed fates, there is no reason for the other males to compete for mating rights, and no reason to have Olympic-level sperm.
Evolution prunes wastefulness and over time, the naked mole rat’s sperm degenerated to the point where most are useless. They’re like the wings of island birds that often shrivel in the absence of land predators, or the eyes of cave-dwellers that deteriorate in the absence of light. The rats won’t be lovers, so there’s no point for their sperm to be fighters.
Reference: Van der Horst, Maree, Kotze & O’Riain. 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 in press.
More on sperm competition: