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Scientists explained why small animals tend to have longer sperm cells

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Posted November 26, 2015

In animal kingdom size sometimes means everything. But sometimes size is hard to explain. For example, elephants have smaller sperm than mice and although sperm has been known for 350 years, science still is not completely sure why. At least it was not until now, because now scientists at the Universities of Zurich and Stockholm revealed that the size of the sperm depends on size of the animal and number of sperm.

Smaller animals tend to have longer sperm. For example, sperm of mice is often twice as long as those of the considerably larger carnivores, ungulates, primates or even whales. Image credit: Pogrebnoj-Alexandroff via Wikimedia, CC-BY-SA-3.0

Smaller animals tend to have longer sperm. For example, sperm of mice is often twice as long as those of the considerably larger carnivores, ungulates, primates or even whales. Image credit: Pogrebnoj-Alexandroff via Wikimedia, CC-BY-SA-3.0

Sperm properties, despite its sole purpose, vary significantly between species. It is said that in species where number of males mate with the same female, longer sperm has a competitive advantage, which would explain why mice has longer sperm. In fact, sperm of rodents often is twice the length of the sperm of larger carnivores, ungulates, primates or even whales. But scientists now say that explanation must be broader.

Scientists performed a research, comparing the influence of sperm competition on the evolution of sperm in 100 mammalian species. There have been similar researches before, but what makes this one unique is that researchers took into consideration not only length of the sperm, but also the number of sperm per ejaculate.

This measurement is important, because it correlates with the size of the animal – the longer every individual sperm is, the fewer of them one testicle of a certain size can produce. In fact, some scientists argued that number of sperms is more important than the length, as it gives better odds of fertilization.

Based on this research scientists concluded that species that have to compete for one female for mating, invest more into their ejaculates on average than their monogamous counterparts. Scientists also say that there is no direct answer whether the size or the number of sperms is more important, as it mostly depends on the size of the animal. They explain that the bigger the animal, the greater the selection pressure the male faces. This means the number of sperm compared to sperm length becomes more important. In fact, in these larger species, length of the sperm only comes into effect if a sufficient number of sperm manage to get near the egg.

Because in smaller species the distance for the sperm to travel is shorter, it allows advantages of larger sperms to manifest themselves better.  In such short distance, the number of lost sperms is lower; therefore, the number of sperms in general is less important than the length. This is why the most complex sperm forms are found in small species. For example, the longest sperm was found in small fruit flies, while whales have sperm a thousand times shorter than those of the flies.

Source: UZH

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