Sperm are incredibly varied and have evolved in many different shapes and sizes. They are the result of a trade-off between investing in traits that are useful for competing with rivals and increasing the chance of fertilizing an egg.
Male harbour porpoises use their huge testicles to produce a lot of semen. But it isn’t their size that makes them top of the list.
Humans
In the animal kingdom, there is a lot of variation in sperm length and shape. For example, Norway rats — a species that engages in polygynandry and has multiple males per female — have very long sperm (each sperm measures 189 micrometers). But house mice also produce very short sperm, at just 124 micrometers. The longest sperm of any animal, however, belongs to tiny fruit flies. These tiny sperm are tightly-coiled and have hooks on their heads, which allow them to connect with the heads of other sperm in groups called sperm trains, increasing the speed at which they can swim and thus increase their chances of successfully fertilizing eggs.
If size mattered, larger mammals would produce longer sperm. But sperm production and storage are limited by a male’s body size, so the size of the testicles limits how big the sperm can be. If an animal’s sperm is too large, it could be diluted or lost in the much bigger female reproductive tract. As a result, it makes evolutionary sense for smaller species to have the biggest sperm that can bully competitors out of the way and get to an egg more quickly.
Elephants
When you think of elephants, the first thing that comes to mind might be their impressive size. The world’s largest land mammal is a powerful symbol of Africa, and these giants are also incredibly smart. They use long-term memories to find water and food in their vast home and can communicate with family members miles away.
Despite their massive bodies, elephants release an astounding amount of sperm during ejaculation: a single male may produce up to 300 million tiny sperm cells. These sperm cells have to fight their way through three layers of defence to reach the egg and complete the fertilisation process.
The size of a species’s body is thought to have a direct impact on its sperm production levels. Males of smaller species produce fewer and larger sperm, while those of larger species produce more but much smaller sperm. This is a result of sperm competition: in species where females mate promiscuously, there’s strong pressure on males to invest in their sperm so that one of them will be the lucky one to reach an egg.
Fruit flies
Imagine a sperm the size of an apartment block, and you’re getting close to how big some male fruit flies’ reproductive cells are. These gargantuan sperm, which are coiled up into knotted balls that unravel once inside the female reproductive tract, are a product of sexual selection driven by competition.
Male fruit flies lay about 500 eggs in their lifetimes, and each of those eggs can be fertilised in just eight to ten days. That means a single male produces hundreds of millions of sperm each year.
These sperm are the longest of any animal studied so far, and they measure up to nearly five millimeters in length. They’re also oddly shaped, with hook-like attachments on their heads that enable them to connect with other sperm and swim in groups—sometimes together called sperm trains—faster than individual sperm.
The reason that the tiny fruit fly ranks so high on this list is clear: Males of many species engage in polygynandry, where they mate with multiple females in order to improve their chances of fertilising an egg. This puts intense pressure on each male to produce a lot of sperm, and to make it as large as possible so that it’s more likely to get through the three lines of defence protecting an egg.
Whales
Whales are the largest animals on Earth, ranging in size from the dwarf sperm whale to the colossal blue whale. They live in every ocean and communicate with complex vocalizations. Their thick layer of blubber insulates them from the cold ocean water.
They are members of the Cetacean order and share a number of characteristics: they breathe air, are warm-blooded, nurse their young and have hair. They also have specialized paddlelike flippers that encase hand bones and free floating vestigial pelvic bones.
Whales can be divided into two groups: baleen and toothed whales. Baleen whales (Mysticetes) have bristle-like plates in their mouths called baleen, which sieve krill and other tiny crustaceans from the seawater. Toothed whales (Odontocetes) have teeth and hunt individual fish, squid and other marine mammals for food.
Male sperm whales produce enormous amounts of semen – their testicles can weigh up to one tonne. But this is of low quality, as many of the sperm cells are dead. So, sperm whales focus on producing fewer but higher-viability sperm cells. Their sperm is used to fertilize females’ eggs.
Mice
Scientists have long known that animals produce tons of sperm to try to fertilise eggs. But how they do that is a mystery. Sperm are incredibly diverse and come in all shapes and sizes. They have to fight their way past three layers of defence in an egg. To do this they use chemicals from their acrosome and a spike on their head to punch through the egg’s membrane. If all goes well, the single sperm cell can make it to the egg and do its job.
Sperm can also be poisoned, which could lead to miscarriages and other problems. A protein called RAC1 is one such poison that kills sperm by blocking its movement. But scientists have now found a gene variation that can protect sperm from this death.
Researchers are trying to use this knowledge to help endangered species. They have shown that it’s possible to create rat sperm in sterile hybrid mice, and they are hoping that creating sperm for species in decline could help them increase their numbers. It’s an exciting development, and the team has published their work today in Stem Cell Reports.
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