Naked mole rats have the worst sperm in the animal kingdom

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Often called "sabre tooth sausages," naked mole rats are among the most bizarre members of the rodent realm. They live for 20 years — an enormous amount of time compared to other rodents — and exist entirely within a network of underground tunnels. They are blind, feel no pain on the surface of their skins, can run equally fast backwards and forwards, and recognize each other by smell alone. Naked mole rats exist in a social structure that's very insect-like, with one fertile queen who has a few fertile male consorts. The rest of the colony gathers food, builds out the tunnels, and defends the colony.

Now a group of scientists have discovered that the naked mole rats have really crappy sperm.

The group tested sperm from males who breed with the dominant queen rat, as well as lowly worker males. Only 1-15% of the sperm could swim at all, and all were randomly formed. According to a release about the study:

The sperm in all cases would be considered abnormal for any other mammal. The sperm head is irregular, often lobular, and the chromatin within the nucleus is dispersed (see image). The neck is poorly developed and the sperm have the smallest mid-piece of any known mammal, with randomly arranged mitochondria. The most degenerate feature of naked mole rat sperm is the tail which is missing the fibrous covering thought to be essential for swimming through the female reproductive tract.


Prof Gerhard van der Horst from University of the Western Cape explained, "Once the queen has picked her consort(s) she keeps the other females and males subordinate by using physical aggression. It seems that the resultant lack of competition between breeding males for the colony queen contributed to an overall decrease in sperm 'fitness'. Surprisingly, despite the low motility and dismal features of their sperm, these naked mole-rat males are fertile and fathered a number of healthy offspring per litter. We are very excited about our results because this would be the first example of degenerate orthogenesis occurring at a cellular level."

The best part of all this is the way the scientists keep calling these sperm "degenerate" and "dismal." In the absence of competition, it's a spermy race to the bottom. Or something like that.

Read the full scientific article via BioMed Central.

Photos of naked mole rats and sperm by Liana Maree.