Scientists have overlooked a key part of animal reproduction: The microbiome, the word for the myriad bacteria that live within animals. The microscopic stuff may hold the key to better understanding not just how animals have sex, but how entire species evolve.
Typically, the microbiome refers to gut bacteria, which is the trillions of microorganisms that live in humans’ and animals’ gastrointestinal system. Stuff like viruses, fungi, and protozoa.
But the reproductive system has its own community of microbes, too.
Melissah Rowe is evolutionary ecologist at the Netherlands Institute of Ecology who studies reproductive biology and behavior. (Specifically, her work focuses on the powers of animal semen.)
Rowe tells Inverse that even though her research is still preliminary, the results already suggest that the microbiome is probably much more important than we currently think it is to animal reproduction.
In a study published in January, 2020, Rowe show that the reproductive microbiome plays a key role in animals’ sexual health and fertility. From ducks to primates to bedbugs to ants.
In a 2011 study Rowe reported on the surprising anti-bacterial qualities of mallard ducks' ejaculate. It could can kill bacteria, they learned.
At the same time, these antibacterial properties were linked to the color of the males’ beak — which it uses to attract females.
This is where it gets really interesting: The more colorful the bill, the better the male's sperm is at killing bacteria, and thusly, the more likely they are to bag a mate.
The results suggested that bacteria, reproduction, and mate selection may be working hand-in-hand. In humans, a similar relationship appears to play out, too — E. Coli bacteria, which can be harmful to human health, has been shown to damage sperm’s mobility and quality.
But Rowe was also interested in whether the relationship works the other way — could bacteria in animals' reproductive systems benefit them in some way, too?
In the 2020 study, Rowe and her colleagues investigate how the bacteria living in ejaculate and reproductive tracts and tissue may be a boon to different species.
“No one had come at this from a more evolutionary and ecological perspective, thinking about what those potential impacts on sperm function and performance might be for an organism’s fitness,” Rowe says.
“And, by extension, what that might mean for things like the evolution of ejaculate traits.”
Reproductive biomes: An evolving theory
Beyond mallard ducks, a number of animals and insects show traits suggesting that their evolution is shaped — at least in part — by the colonies of bacteria living inside them.
In the data, Rowe highlights four patterns that hint at this intimate relationship at work:
- Primates, deer mice, and lizards: The vaginal microbiome is more diverse in species where females have multiple mates.
- Bedbugs: During insemination, male bedbugs pierce the females' abdomen, sometimes resulting in deadly infections. But before mating, females increase their immune defenses, which may help to protect them.
- Ants: Black garden ants who have not mated foster the growth of more microbes — while females who have not mated inhibit microbes in their sperm-storage organs.
- Birds: In an ancient species of chicken, the red junglefowl, males make more antimicrobial proteins in their ejaculate as they progress from one mating to the next. This might act like a reproductive safeguard — keeping their sperm high quality even as it decreases over time.
These results are still preliminary. Some of the data are buried deep in domestic livestock literature, she says. The challenge is to pull that and new observations together to better characterize the relationship between the bacteria animals host and their evolution.
There are likely more links just waiting to be discovered, she says.
Sperm diversity and competition
Sperm come in all shapes and sizes — in fact, it is the widest-varying cell type we know of, Rowe says.
“The diversity is just incredible,” she says.
For one, sperm can be huge. In one species of fruit fly species, the fly itself measures just a few millimeters in length — but its sperm can be nearly 6 centimeters long, Rowe says.
Sperm shapes can get weird, too. Some animals' sperm is shaped like humans', but others lack the tail and flagella we associate with our own species' swimmers. In certain rodent species, sperm cells have “hooks” at the top of their head. Some, rather than swimming alone, even form clusters.
“They sort of stick together and swim as a group,” Rowe says.
Scientists are only beginning to understand the evolutionary processes that make sperm so diverse, but part of the reason is sperm competition.
In nature, females often mate with more than one male, Rowe says. So males have evolved strategies to make sure their sperm is successful above all others. A classic example — among evolutionary biologists, at least — is the damselfly penis. Its penis is barbed with spines that collect and dump out sperm from any previous mates.
“They basically win at the game of sperm competition by getting rid of all the other males’ sperm, and then inseminating with their own,” Rowe says.
“Evolution has led to this massive diversity of structures, and also sperm morphology.”
How that relates to individuals’ fitness, however, is a question Rowe and other scientists are yet to answer.
What about humans?
All of this, of course, begs the question: What about humans’ reproductive microbiome? Do we have sperm competition, too?
The most robust science so far focuses on the human vagina, and how its microbial tenants — particularly the bacteria Lactobacilli — affect birth. Studies suggests a mothers’ bacteria can have a positive effect on her offspring — but if the microbiome is disrupted, they may have a negative impact.
In humans, much of the research also focuses on the problems bacteria cause, Rowe says. Certain bacterial species are associated with lower sperm quality, for example, and having more bacteria than expected is linked to infertility, she says.
Rowe tends to avoid talking about humans, but she humored us anyway: What about sperm competition?
It’s “totally possible” that humans have some traits linked to competing, since sperm lives for several days inside the female reproductive tract, she says. If partners overlap, there is reason to believe their sperm would compete.
But compared to other primates, humans are probably pretty modest competitors, Rowe says. If we compare our species to chimpanzees, which mate rather freely and have lots of sperm competition, human testes just don't measure up size wise.
“That sort of suggests that humans are fairly moderate on the sperm competition scale, in terms of evolutionary reproductive traits,” Rowe says.
We can't come out on top at everything. But if we can take anything from Rowe's work, it is that the reproductive microbiome in both humans and animals is vastly underestimated and overlooked — perhaps it is time to get as acquainted with these microorganisms as we are with those in our gut.