Sperm are an essential part of the reproduction process, but they vary dramatically in size. One tangled group of sperm recently fossilized in amber was as long as a human thumb.
Some species that use external fertilization eject tons of sperm into water to maximize the chances that one will reach an egg. This puts tremendous evolutionary pressure on those animals to produce large sperm.
1. Whales
If you’re looking for sperm, the most reliable source is probably a whale. These colossal creatures use internal fertilization to reproduce, carrying eggs inside their bodies. This is different from external fertilization, which is common among land animals and invertebrates. When males in these groups produce sperm, they eject it into the environment where it can compete with other males’ to reach and fertilize an egg. This forces animals to produce a lot of smaller sperm in order to increase their chances of one making it through the competition.
Because of this, whales have the largest testicles in proportion to body size. During the rut, a male blue whale’s two testicles can weigh up to a tonne together. They also produce a large amount of sperm, with each releasing more than 200 billion. They are the most prolific sperm producers in the animal kingdom.
2. Fruit flies
In some species of animals, sperm have to fight for the right to fertilize an egg. And in order to do so, they need to be bigger than their rivals’.
But this type of competition is rare. Most mammals are internal fertilizers, whose eggs stay inside their bodies. But other species are external fertilizers, whose eggs are fertilized by ejecting sperm into water. The sperm then rides the currents until it is picked up by a female.
This mode of reproduction is called spermcasting. And a new study shows that it puts the same evolutionary pressure on sperm as internal fertilization. That’s why sperm in external fertilizers tend to be smaller, because ejecting a cloud of them into water dilutes their chances of reaching an egg.
But the same forces that create sperm with shorter tails also cause them to be larger at the head. And it turns out that the bigger a sperm’s head, the better chance it has of shoving other sperm out of the way, regardless of whether they come from the same or different males.
3. Elephants
The largest land animal, elephants are a behemoth of strength and intelligence. They are keystone species, protecting their habitats and influencing the health of their environment.
In their native home of Africa, they live in family groups led by the oldest female, known as a matriarch. The animal is also often viewed as a symbol of luck, prosperity and wisdom.
In the wild, full-grown male elephants face a number of threats including natural predators (lions and other carnivores), habitat loss, poaching for their tusks, and conflict with humans who are encroaching on the animals’ wild territory, causing them to trample cultivated farmlands or settlements. However, elephants are able to defend themselves using their enormous strength and thick hides.
4. Norway rats
The Norway rat (Rattus norvegicus) is a highly adaptable rodent that has made itself at home in many urban settings. It is thought that the rats first arrived in America on ships from Europe in the 18th century and quickly spread across the nation.
These scaly creatures spend much of their time burrowing in soil along riverbanks, railway embankments and garbage dumps, but will readily move into structures for food and shelter. They can fit through openings 1/2 inch wide and will nest in basements, woodpiles and piles of debris, as well as attics and roofs.
The Norway rat is one of the oldest model organisms used in scientific research, but it is also the most invasive laboratory rat species. Because they are inbred (brother-sister or parent-offspring mating for multiple generations), they are prone to genetic variation and may display distinct phenotypes even within the same breeding colony (see Box 2). This variation hinders scientists’ ability to generalize results from one experiment to another.
5. House mice
A little mammal that can’t quite fit into any of the other categories, house mice are also among the animals with the highest sperm production. But this isn’t because of their super-sized testicles, which average about half a foot long, but rather because they engage in polygynandry, a pattern of sexual relationships between two or more males. This means that their sperm must be strong enough to bully competing sperm out of the way and quickly get to an egg to ensure fertilization.
House mice are members of the rodent genus Mus, and can be recognized by their pointed snouts and rounded ears, with long tails that are nearly hairless. Their distinctive call sounds similar to a bird song and is often emitted during courtship behaviour, as well as during mating. The odour of house mouse urine is known to trigger arousal in females, speed up sexual maturation in juvenile females, and terminate pregnancies (the Whitten effect). Individuals have varying odour characteristics depending on age, health, social group and hierarchy position, and diet.
6. Golden hamsters
The most common hamsters in pet stores are short-haired Syrian hamsters, also known as golden hamsters. They originally came in a mixture of gold, brown and black fur, but have since developed myriad other color mutations, including cream, white, tortoiseshell and calico.
These hamsters are also notable for their enormous testicles, which make up 3 percent of their body size. That explains why they rank fourth on the list of animals that produce the most sperm, along with Norway rats and house mice. Their sperm are long, at 189 micrometers, which helps them compete with longer sperm from other males to penetrate an egg and fertilize it.
It’s important to remember that semen quality depends on more than just the number of sperm and the morphological characteristics of those sperm. For example, dogs can have low semen quality if they have poor motility or morphology but high total sperm numbers.
