Competitive Advantage Sperm Count Beats Sperm Length

Sperm count often more important than sperm length
No sperm is like the other: that is not only the case in humans, but also in the animal kingdom. In addition, the sperm shape in the different species is very variable. The question of why this is so far, the science could not really answer. However, researchers now seem to have found an explanation.

It does not always depend on the length
Apparently it does not always depend on the length: Many small sperm race in the race to the egg under certain circumstances rather to success than a few large. As two scientists report in the journal "Proceedings B" of the British Royal Society, this is true for larger animals, for example, when the sperm of one male in the female must prevail over that of another. Thus, their study also provides a reason why mice and other rodents have larger sperm than, for example, elephants or whales.
The number of sperm is more crucial than its length. (Image: atiana Shepeleva / fotolia.com)

Sperm vary in shape and size
According to the researchers, sperm are among the most diverse cells of all and can differ considerably in shape and size in different animal species. For example, fruit flies with six-centimeter-long giant sperm have the largest sperm at all, reports the University of Zurich in a press release on the current study results. For giant whales, however, the sperm are less than a tenth of a millimeter long and almost a thousand times shorter than the flies.

Sperm shape influenced by competition
The fact that sperm competition in particular has influenced the shape of sperm in different animal species is considered to be certain among scientists. Competition arises when a female mates with several males. However, it is not yet clear why this competition leads to the formation of particularly long sperm in some species and to the development of rather smaller sperm in others. The two researchers Stefan Lüpold from the University of Zurich and John Fitzpatrick from the University of Stockholm have now shown in their study that the body size of the animals plays a crucial role in this. For their study, they had collected information on the size and number of sperm from 100 species of mammals in the literature and analyzed this information statistically comprehensively, taking into account body size and sexual behavior.

Larger animal species invest more in the number of sperm
First, the scientists showed that the investment of a species in the ejaculate increases with increasing pressure of competition. The more promiscuous one species is, the more or the larger sperm it produces. "That was not particularly surprising, that is in line with the previous theory," Lüpold is cited by the agency NA dpa. "In addition, we have shown that larger species, in contrast to smaller ones under competitive pressure, invest primarily in the number of sperm and not in their length," the scientist continues. This is explained by the researchers with a dilution effect in the female genital tract: the larger the genital tract and the longer the way to the egg, the more likely the sperm were lost on the way. "It would make more sense to just send more sperm into the race and save on the sperm size," explains Lüpold.

Oldest sperm in the world
Sperm have been known to science for about 350 years. Researchers reported in the summer of this year on the discovery of the world's oldest sperm. These were discovered in the fossilized cocoon of a gill-worm in the Antarctic. They are 50 million years old. The international research group led by Benjamin Bomfleur from the Natural History Museum in Stockholm presented the find in the journal "Biology Letters" of the British Royal Society. (Ad)