June 21 (UPI) -- For every animal, sperm perform the same function -- egg fertilization. Yet sperm size varies dramatically from species to species.
Most studies on the topic have focused on how sperm compete and win the race to fertilize an egg, but scientists have struggled to explain the dichotomy.
New research, published Monday in the journal Nature Ecology and Evolution, however, suggests sperm evolution is heavily influenced by the competitive environment -- the location where sperm and eggs meet.
"There is a missing piece of the puzzle -- the location where sperm and eggs meet can also influence sperm size," lead study author Ariel Kahrl, researcher and zoologist at Stockholm University in Sweden, said in a press release.
To measure the influence of fertilization location on sperm size, scientists analyzed the sperm size and fertilization dynamics of more than 3,200 animal species.
The researchers divided animals into different groups based on where the sperm meets the egg.
"In species with internal fertilization -- like mammals, birds and insects -- sperm fertilize eggs inside the female's body, while in species with external fertilization -- like sea urchins and many fish species -- sperm and eggs are released into the water and fertilization happens outside of the female's body," said Kahrl.
The data showed animals that practice internal fertilization have developed both bigger, longer sperm and more quickly evolving sperm. Species that practice internal fertilization also possess greater sperm variability.
The findings echo the conclusions of a previous study that identified the female reproductive tract as the primary driver of sperm evolution.
"When sperm are released externally, selection keeps sperm size small to allow males to produce a lot of sperm," study co-author Rhonda Snook said in the release.
"But when sperm are transferred to the females in internal fertilizers, males may compete better with bigger sperm and females may prefer to fertilize eggs with bigger sperm," said Snook, a professor of zoology at Stockholm.
Researchers also analyzed the reproductive dynamics of a third group of animals, invertebrates that practice spermcasting.
Spermcasting sees male invertebrates release their sperm into the water where it is filtered out by females before internally fertilizing the eggs.
"Spermcasting represents a mix of internal and external fertilizations, which gave us the opportunity to see what part of the fertilization process influenced sperm evolution," senior author John Fitzpatrick, an associate professor in zoology at Stockholm, said in the press release.
Spermcasting animals feature relatively small sperm, like external fertilizing animals, but showcase faster sperm evolution, like internal fertilizing species.
"Our results clearly show that interactions between sperm and females help generate the tremendous diversity in sperm size we see in animals today," said Fitzpatrick. "The greater the potential for interactions between sperm and females, the faster sperm evolve."
The authors of the new study acknowledged that not all internal fertilizing animals have large sperm.
Human sperm, for example, is about the same size as the sperm produced by spermcasting species.
"In animals with large bodies, like humans, sperm are diluted inside the female's reproductive tract," Kahrl said.
"From the sperm perspective, it doesn't matter if dilution occurs inside a female or in the ocean -- dilution keeps sperm small. It's only when sperm are confined in small spaces within the female that sperm become supersized," Kahrl said.