Sperm cells epitomise the Darwinian struggle for survival: every lone sex cell is part of a frenzied race to fertilise first. It must reach the egg before its competitors, or it forgoes the continuation of its genetic identity.
But from the male’s perspective, there is a higher goal – to maximise the number of eggs fertilised, regardless of which of his sperm achieve it. In organisms where numerous eggs are available for fertilization, cooperation is crucial. A fascinating example of this has recently been discovered in desert ants: Cataglyphis savignyi sperm cells can combine together to form enormous, super-fast, multi-tentacled bundles. These jellyfish-like creatures corkscrew their way through semen as a single cohesive unit, like a synchronised swimming team working together for a common goal.
The study, published in Biology Letters, doesn’t reveal the exactly how these sperms cooperate, but the cells in these spermatazoa bundles have been observed moving in sync. This results in total speeds one-and-a-half times greater than individual sperm cells working alone – enough to give an edge over competitors whose sperm travel solo.
Biologists Morgan Pearcy, Noémie Delescaille, Pascale Lybaert and Serge Aron from the Université Libre de Bruxelles stumbled across this highly unusual display of cooperation whilst researching sperm competition across numerous species of ants. They were examining the effect that competition had on typical traits, such as size, quantity and tail size. It’s not easy to breed ants in the lab, and lead author Pearcy said they were “unable to measure” fertility rates. “We can’t mate them in the lab – mating behaviour is determined by environmental conditions which we have no control over.”
To get their samples they had to squeeze the abdomens of dead males and collect their semen. The team examined the size and shape of each ant’s sex cells (or gametes) under a microscope, and to their surprise they found bundles and bundles of super-speedy sperm. They delved deeper into this phenomenon by looking at the gametes of Israeli desert ants.
Due to their high levels of promiscuity – a queen will, on average, mate with nine male ants in quick succession – there is huge competition between individual males. Queens of cataglyphis savignyi store the sperm in a storage organ (the spermatheca) where they can live for decades. On the other hand, the males themselves die almost immediately after mating. This creates an evolutionary imperative for male ants to maximise the number of gametes reaching the spermatheca. The sperm appear to achieve this by teaming up.
The study found that “males ejaculate bundles of 50–100 spermatozoa” whose heads are glued together by a sticky protein. This super-sperm propels itself by rotating the individual flagella (the tail bits) in-sync with one another, which significantly increases speed. This “optimise[s] the transfer of sperm to the storage organ”.
Ants aren’t the only species whose sperm are known to cooperate. Studies investigating wood mice have found that their sperm form sickle-shaped heads in order to ‘hook’ onto one another, “resulting in cooperation in distinctive aggregations or ‘trains’ of hundreds or thousands of cells”. Unfortunately, hooking onto one of these trains comes at a price: more than half of the cells involved undergo a reaction rendering them infertile.
Like in the desert ants, this cooperative behaviour occurs where a female mates with numerous males in quick succession, forcing each male to fight for a chance of fertilisation. As a result, many wood mice sperm forgo the opportunity to reach the egg themselves, and instead sacrifice themselves to help other sperm cells of high genetic similarity.
As touching as the idea of a ‘Good Samaritan’ sperm is, such altruism is very rare. Pearcy said: “Wood mice [are] the most striking example of this… They can form huge sperm trains. But in ants, and across the animal kingdom, such behaviour is almost unheard of.”
As a trait, it’s certainly beneficial for the male desert ants. However, the extent of the advantage conferred by cooperative sperm is still uncertain – especially the effect it has on ant populations as a whole. An individual’s faster sperm will increase his own chances of success, but at the cost of another male’s. Further studies are going to be needed to find out quite what the trade-offs are when it comes to cooperative ant sperm.
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