It’s hard being a male. Whether through elaborate courtship dances, extravagant outfits or intimidating canine teeth, male animals have to be especially dominant or attractive if they are to beat their rivals and find favor with the opposite sex. But it doesn’t end there. When females accept more than one mating partner, the male’s sperm have to compete with the sperm of other males. To win this race and reach the egg first, the most important thing is for males to have plenty of good sperm – and large enough testes to produce them.
So what’s the best strategy to succeed? Should males focus on producing as many speedy sperm as possible? Are good looks and attractive ornaments more important? Or should males bank on weapons such as horns or long, sharp teeth to fend off the competition? These are the questions that UZH biologist Stefan Lüpold is exploring. His research focuses on sexual selection in the animal kingdom and how it shapes the evolution of species. Among other things, he is investigating which features increase the mating success of a species and why.
In a 2019 study, for example, his team analyzed and compared around 100 primates. The researchers based their study on the animals’ physical features – body size, canine teeth length and testes size, among other things. For every species in the study, they also determined how much the males invest in sexual ornaments that attract females..
Their study revealed that male primates have either large testes or conspicuous ornaments, but that it’s rare for them to have both. “It appears these specialized traits exclude one another,” says Lüpold. He describes this as an “evolutionary trade-off”. Good sperm production, exaggerated ornamentation or the hard-fought life as an alpha male – each specialized trait comes at a cost.
The strategies adopted by the different primates are linked to their species’ social organization and mating habits. For example, primate species where only one or a few dominant males can mate in a group, such as gorillas, are more likely to have distinctive weaponry and ornamentation – like the dominant silverback in gorilla groups. Their testes, however, are relatively small. Contrast this with bonobos and chimpanzees, where males and females are at it constantly with different partners. Male bonobos and chimpanzees may not have any fancy physical features, but their testes are huge. And this means that they invest more in the quality of their sperm.
A similar picture emerged from a second study by the research group, in which they also compared the traits of other mammals as well as birds, fish and insects. For each of these animal groups, the researchers found that the more females are monopolized and the more a dominant male can keep rivals away from females, the less the species invests in winning the sperm race. And the inverse also holds true: The less access to females is restricted, the greater the investment in sperm production.
Lüpold is investigating sexual selection with the help of a far less striking animal, which some would even describe as pesky – drosophila, or fruit flies. The biologist and his team breed thousands of these little insects in their lab. The researchers then use thin tubes to hoover them up, one by one, to use them in their experiments. “With a little practice, you can tell the males and females apart quite easily,” says Lüpold. The life cycle of fruit flies is only two weeks, so they can be bred and observed in great numbers in a short period of time. As it happens, they’re also remarkably versatile when it comes to their sex life.
One of the species the researchers are looking at is Drosophila prolongata. The males have enlarged front legs, which they use to fend off rivals and court females. In an effort to entice members of the opposite sex, they use their forelegs to wave and drum and vibrate the female’s abdomen. In about 10 to 20% of cases, however, this mating tactic fails to achieve its goal, the researchers observed. Sometimes a second male creeps up on the lovers-to-be and unceremoniously snatches the female away – leaving the first male to rue lost labors of love. “We observed that this sneaking tactic is often used by small males in particular,” says Lüpold.
The researcher and his team are currently taking a closer look at this unusual behavior. Since most female fruit flies mate with multiple partners, the males’ sperm have to compete against each other. As a result, many species have developed unusually long sperm. The longest are found in Drosophila bifurca. Measuring only three millimeters, these tiny flies produce sperm cells that are nearly six centimeters long. These are the longest sperm of any species on the planet – a thousand times longer than the sperm of elephants or wales. Coiled up like a ball of string, the sperm are released in the female reproductive tract, where they unfurl.
Biologist Lüpold reconstructed why these fruit flies have such long sperm in a series of experiments. The first step was to genetically mark the sperm cells and make them visible. To do this, the researchers implanted the DNA blueprint for specific red or green fluorescent proteins into the genetic material of the heads of the sperms. This allowed the scientists to see the sex cells under a fluorescence microscope and observe what happens to the sperm of different males, tagged either red or green, in the female reproductive tract. Using this approach, Lüpold and his team found that longer sperm are more successful than shorter sperm, because they can simply push aside their rivals and thus gain an advantage.
But it’s not only the sperm alone that get to decide who wins the race. Lüpold recently shed light on the role of female fruit flies during sexual selection. Females have the ability to store sperm in a specialized sperm-storage organ, a kind of closed-ended tube. There the sperm wait for the eggs to emerge from the oviduct. When they are later joined by the sperm of another male, most of the first sperm are displaced and ousted from the tube along with the excess sperm of the second male. The time at which this happens determines which sperm are in a better position to fertilize the eggs. The earlier a female stops this process and ejects the sperm, the more sperm of the first male remains, and the later she does it, the better the chances for the second male’s sperm.
“What’s interesting is that the female fruit flies can control when this happens,” says Lüpold. His research has shown that genetically identical females always give the advantage to the same males in this process. “The females select the males that are the best genetic match for them,” the researcher explains. In contrast, pre-mating courtship benefits the males that are especially fit and strong – the males that correspond to the fruit-fly beauty ideal, so to speak. “Female fruit flies first pick their mating partners and then also their sperm to increase the chance of having the fittest offspring.”