Why do male rodents have huge testicles?

Why mice have longer sperm than elephants

(18.11.2015) If females mate with several males in the animal world, their sperm compete to fertilize the few eggs. In this sperm competition, longer sperm often seem to have an advantage.

A study by researchers from the University of Zurich and Stockholm now shows that the size of the animals also matters. The larger the animals of a particular species, the more important the number of sperm cells is and not their length. Elephants therefore have smaller sperm than mice.

Sperm are probably the most diverse cells in terms of shape and size and have not lost their fascination since they were discovered almost 350 years ago. But why are sperm so incredibly different between species? They all have the same task, namely to fertilize the eggs of the females.

As we know from many studies, the sperm competition plays a central role in the evolution of sperm. This competition arises when females mate with several males and their numerous sperm cells compete to fertilize the few eggs.

Longer sperm are often more successful in this. Interestingly, this is more the case with small rodents such as mice and rats than with larger animals. The sperm of rodents are often twice as long as those of larger predators, ungulates, primates or even whales. However, the reasons for this are controversial.

The number and length of sperm cells are taken into account

A new study could now provide clarity. Stefan Lüpold, new research member at the Institute for Evolutionary Biology and Environmental Sciences at the University of Zurich, and his colleague John Fitzpatrick, University of Stockholm, compared the influence of sperm competition on the evolution of sperm in 100 mammalian species.

In contrast to previous studies, they not only took into account the length of the sperm, but also the number of sperm cells per ejaculate. This is important because the resources available for sperm production must be divided between number and size. In other words, the longer each individual sperm is, the less of it a testicle of a certain size can produce.

Based on previous studies, it was believed that sperm count was as important, if not more important, than sperm length. Because the more sperm a male sends into the race against his opponent, the greater the probability that one of them will win.

The size of the animals is relevant

By looking at sperm size and number together and using new meta-analytical methods, the two researchers are now showing that species with intense sperm competition invest more on average in their ejaculates than those that tend to be monogamous.

They also found out that it depends on the size of the animals whether the length or the number of sperm is important.

With increasing body size, the selection pressure on the total investment in ejaculates increases, and the sperm count becomes more and more important than the sperm length. This is due to the more voluminous female genital tract, in which the sperm are more likely to be lost or “diluted”.

The sperm length or speed probably only has an effect in large species if enough sperm get near the eggs at all. In small species, the distance the sperm has to travel is shorter and the risk of loss is much smaller, so that large sperm are more of an advantage.

For this reason, the most complex forms of sperm are found in small and not large species. The small fruit flies have the largest sperm cells ever described and not whales, for example. In whales, the sperm are less than a tenth of a millimeter long and almost a thousand times shorter than in flies.

publication

Lüpold S & Fitzpatrick JL. Sperm number trumps sperm size in mammalian ejaculate evolution. Pro-ceedings of the Royal Society of London B. Doi: 10.1098 / rspb.2015.2122

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