Why Do Some Animals Have Seemingly Disproportionate Sperm Cells?

Some animals have sperm 20 times the size of their bodies.
Loukia Papadopoulos
The photo credit line may appear like thisRost-9D/iStock

Did you know that a fruit fly's sperm is 20 times the length of the animal's body? It is indeed and the animal kingdom has a wide variety of sperm sizes.

A new study is now researching how and why these sizes differ so much. Lead author Ariel Kahrl, a postdoctoral researcher in evolutionary biology at Stockholm University, and her colleagues looked at 3,200 species to understand what determines the size of an animal's sperm.

What they found was nothing short of fascinating. "We show that sperm are shorter in species whose sperm are diluted in aquatic environments (that is, external fertilizers and spermcasters) and longer in species where sperm are directly transferred to females (that is, internal fertilizers). We also show that sperm length evolves faster and with a greater number of adaptive shifts in species where sperm operate within females (for example, spermcasters and internal fertilizers)," write the researchers in their study.

"Our results demonstrate that fertilization mode is a key driver in the evolution of sperm length across animals, and we argue that a complex combination of postcopulatory forces has shaped sperm length diversification throughout animal evolution."

Basically, the researchers found that external fertilizers produced lots of smaller sperm because this sperm needed to go further faster in order to have a chance at fertilizing an egg. This animal only has a limited amount of energy to make his sperm and therefore focuses it on making plenty of smaller sperm.

Internal fertilizers however don't encounter that issue and can therefore produce less and longer sperm. The sperm of these species of animals are produced in a very tight space and have to fight each other to get to the egg. This is where having a bigger size probably helps a lot in fighting off other sperm. Internal fertilizers use all their energy therefore to make less but bigger sperm. Pretty interesting huh?

Did you also know that competitive sperm can swim faster and poison peers? Read the study here.

The research is published on the journal Nature Ecology & Evolution.


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