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Cooperative Sperm Outrun Loners In The Mating Race

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Even sperm gotta stick together.

Bull sperm swim more effectively when in clusters, a new study shows, potentially offering insight into fertility in humans. In simulated reproductive tracts of animals like cattle and humans, the behavior increases the chances that groups of cooperative bovine sperm will outpace meandering loners as they race to fertilize a female egg cell, physicist Chih-kuan Tung and colleagues report September 22 in Frontiers in Cell and Developmental Biology.

The benefits of clustering don’t come down to flat-out speed. “They are not faster,” says Tung, of North Carolina Agricultural and Technical State University in Greensboro. “In terms of speed, they are comparable or slower” than sperm traveling alone. Like the sperm equivalent of herds of tortoises racing individual hares, the winners are not necessarily the swiftest but rather the ones that can stay on target.

On their own, sperm tend to follow curved paths — which is a problem, because the shortest distance between two points is a straight line. But when sperm gather in groups of two or more, they swim along straighter routes. It’s behavior that a couple of the same researchers noted in a previous study where they tracked sperm swimming in stationary fluids (SN: 3/17/16). Although that might give sperm clusters an advantage, it would only help if they happen to be going the right direction. Other benefits of sperm clustering weren’t clear until the researchers developed an experimental setup that introduced flowing fluid into their experiments.

In creatures like humans and cattle, sperm make their way to the ovum by swimming against a current of mucus that streams through the cervix and away from the uterus. It’s difficult to study what benefits clustering might confer while swimming upstream inside living beings. So Tung and colleagues created an analog in their lab: a shallow, narrow, 4-centimeter-long channel filled with a thick fluid that mimics natural mucus and flows at rates the researchers could control.

Whether alone or in groups, sperm naturally tend to swim upstream. However, clusters of sperm in the experiment did a better job heading upstream into the mucus flow, while individual sperm were more likely to head off in other directions. Despite the speedier travels of some individual sperm, a poorer ability to point upstream hampered the progress of sperm loners compared with slower moving clusters.

In a thick, mucuslike fluid flowing from left to right, clusters of sperm travel more consistently against the flow than individual sperm do on their own.

Clusters also stayed the course in the face of rapidly flowing mucus. When the researchers turned up the flow in their apparatus, many individual sperm were washed away. Sperm clusters were much less likely to get swept downstream.

While sperm in the study were bovine, the advantages of clustering should also apply to human sperm, Tung says. Sperm of both species have similar dimensions. The swimmers typically compete to fertilize a single ovum. And unlike pigs or other animals where semen is deposited directly in the uterus, both human and bovine sperm start out in the vagina and travel through the cervix to get to the uterus.  

Studying sperm in fluids that closely resemble the flowing mucus in reproductive tracts could reveal problems that don’t turn up in conventional observations of sperm swimming in stationary fluids, Tung says. “One hope is that this sort of knowledge can help us do better diagnoses” to provide clues to understand infertility in humans (SN: 3/31/03).

Subjecting sperm to realistic settings in the lab may soon offer practical help for people who have trouble conceiving, says fertility researcher Christopher Barratt of the University of Dundee in Scotland, who was not affiliated with the study (SN: 6/9/21).

“How a sperm cell responds to its surroundings and how that may change its behavior is a very important subject,” Barratt says. “This type of technology could be used, or adapted, to select better quality sperm,” for people in need of fertility assistance. “That would be a very big deal.”

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Mountain Lions Pushed Out By Wildfires Take More Risks

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Mountain lions have no interest in people, or the built-up areas we enjoy. But after a 2018 wildfire in California, local lions took more risks, crossing roads more often and moving around more in the daytime, scientists report October 20 in Current Biology. It’s another way the effects of human development could be putting pressure on vulnerable wildlife — in this case, potentially pushing them toward our bumpers.

The Woolsey Fire began near Los Angeles on November 8, 2018, and burned more than 36,000 hectares in the Santa Monica Mountains. Nearly 300,000 people evacuated, and three people died. Animals fled the fire too, including the local mountain lions (Puma concolor). The fire was a tragedy, but also a scientific opportunity, says Rachel Blakey, a global change biologist at UCLA. Many of the lions wore tracking collars, allowing scientists to study how the fire changed their behavior.

Of the 11 collared cougars in the area at the time, nine made it to safety during the fire itself. “They have really large home ranges, so it’s nothing to them to be able to cover many kilometers in a day,” Blakey says.

No matter how much they moved, the mountain lions avoided people. One collared cat, P-64, initially fled the fire — until he got close to a developed area. Given the choice between fire and people, the lion retreated back into the burning area. “That’s where his movements stopped,” Blakey says. The park service later found P-64’s remains. He’d burned his paws, and it’s possible that he was unable to hunt and starved to death.

Using data from the nine lions that survived the fire and others collared after, the scientists showed that the cats generally avoided the severely burned areas of their territories. With vegetation gone, the cats had little cover for stalking and ambushing prey.

Instead, the cougars stuck to unburned areas, and continued to avoid people. But they took more risks around human infrastructure, increasing their road crossings from an average of about three times per month to five.

A mountain lion seen running across a paved road, away from the camera
After the Woolsey Fire in 2018, mountain lions in the Santa Monica Mountains crossed roads more often, a risky move that could put the cats’ lives in danger.National Park Service

These weren’t all two-lane country highways. The first collared lion to successfully cross Interstate 405, which has 10 lanes in places, did it after the Woolsey Fire. And the big cats crossed U.S. Route 101 once every four months, whereas before the fire, they’d crossed only once every two years. Their territories also overlapped more often, increasing the potential for deadly encounters between the solitary cats. And the generally nocturnal animals increased activity during daytime hours from 10 percent to 16 percent of their active time — boosting a lion’s chances of potentially bump into a human.

Road crossing is what Blakey calls a “risk mismatch.” Lions in areas with lots of people appear to weigh the risk of encountering humans as more dangerous. But “running across a freeway is a lot more likely to be fatal,” she says. That risk, combined with the risk of running into other cats, can be deadly. One young, collared male ended up dead on a freeway in the months after the fire. He was fleeing a fight with an older, uncollared male.   

Intense burns like the Woolsey Fire highlight the resilience of mountain lions, says Winston Vickers, a wildlife research veterinarian at the University of California, Davis who was not involved in the study. “They have amazing mobility, they mostly can get away from the immediate fire, they mostly survive,” he notes. The changes in risk-taking, he says, could reflect how confined the population is, hemmed into the mountains by human development.

Wildlife crossings, such as the new Wallis Annenberg Wildlife Crossing over the 101, will hopefully give the mountain lions a safer option for roaming, though the main goal is to promote gene flow between lion populations, Blakey says (SN: 5/31/16). In a landscape where fire, humans and highways combine, it’s good to have somewhere to run.  

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A Parasite Makes Wolves More Likely To Become Pack Leaders

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A parasite might be driving some wolves to lead or go solo.

Wolves in Yellowstone National Park infected with Toxoplasma gondii make more daring decisions than their uninfected counterparts, researchers report November 24 in Communications Biology. The wolves’ enhanced risk-taking means they are more likely to leave their pack, or become leaders of their own.

“Those are two decisions that can really benefit wolves, or could cause wolves to die,” says Connor Meyer, a field biologist at the University of Montana in Missoula. The findings reveal a parasite’s potent ability to influence a wolf’s social fate.

Disease is often considered important for wildlife, mostly in the context of killing its host, Meyer says. “We have evidence now that just being infected with a certain parasite — Toxoplasma — can have pretty major implications for wolf behavior.”

A microscope image of the Toxoplasma gondii parasite
The single-celled parasite Toxoplasma gondii is known to alter the behavior of its warm-blooded animal hosts in ways that help complete the microbe’s life cycle.Todorean Gabriel/iStock/Getty

Single-celled T. gondii has a track record of altering animal behavior. Its most important hosts are cats, which provide a breeding ground for the parasite in their small intestine. The parasite offspring hitch a ride on feline feces. Other animals then ingest the parasite, which then manipulates its new hosts’ behavior by tweaking certain hormones, making the hosts bolder or more aggressive. Infected mice, for example, can fatally lose their fear of cats, allowing the parasite to infect more hosts once the mice are consumed (SN: 1/14/20). 

In Yellowstone National Park, many wolves are also infected with T. gondii, recent research has shown. So Meyer and colleagues wondered if gray wolves (Canis lupus) in the park showed any parasite mind-bending of their own.

Wolves were reintroduced to Yellowstone in 1995. Ongoing study of the park’s packs meant that the researchers had access to about 26 years’ worth of blood samples, behavioral observations and movement data for 229 of the park’s wolves.

The team screened the wolf blood for antibodies against T. gondii parasites, which reveal an infection. The researchers also noted which wolves left their pack — usually a family unit consisting of a breeding pair and their offspring — or became a pack leader. 

Both are high-stakes moves for a wolf, Meyer says. 

Infected wolves were 11 times as likely as noninfected wolves to disperse from their pack, the team found, and about 46 times as likely to eventually become leaders. The findings fit in with T. gondii’s apparent ability to boost boldness across a wide range of warm-blooded life. 

The study fills a crucial gap in the Toxoplasma pool of knowledge, says Ajai Vyas, a neurobiologist at Nanyang Technological University in Singapore, who was not involved with the study.

“Most of the earlier work has been done in the lab,” Vyas says. But there are limitations to that approach, especially for re-creating how animals experience the effects of the parasite in their natural environment. Such research has “become almost like studying whale swimming behavior in backyard pools; [it] does not work very well.”

Wolves’ enhanced boldness may even form a feedback loop, the team proposes. The researchers found that not only do cougars (Puma concolor) in the park carry the parasite, but wolves’ infection rates were highest when the animals’ ranges overlapped with the park’s densest aggregations of cougars. Infected wolf leaders may be more likely to bring pack members into riskier situations, including approaching cougar territories, making additional infections more likely. 

The feedback-loop idea is “very fascinating,” but more research is needed to confirm it, says Greg Milne, an epidemiologist at the Royal Veterinary College in London, who was not involved with the study. Such research may involve determining if infected wolves are more likely to migrate into an area with more cougars. 

“I think people are just starting to really appreciate that personality differences in animals are a major consideration in behavior,” says study coauthor Kira Cassidy, a wildlife biologist at the Yellowstone Wolf Project in Bozeman, Mont. “Now we add a parasite-impacting behavior to the list.”

Next, the team is interested in examining the long-term consequences of a T. gondii infection, and whether infected wolves make better leaders or dispersers than uninfected wolves.

 It’s also not known how infection impacts survival and reproduction rates, Cassidy says. “Infection may very well be detrimental in some ways and advantageous in others.”

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A New Book Asks: What Makes Humans Call Some Animals Pests?

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Pests
Bethany Brookshire
Ecco, $28.99

We spend so much time making sure wildlife stays away from us, whether that’s setting traps, building fences or putting out poisons. Sure, unwanted guests are annoying. But why do we consider some animals “pests”? It’s all about perspective, says science journalist Bethany Brookshire. “We can put poison out for rats and protest their use as laboratory animals. We can shoot deer in the fall and show their adorable offspring to our children in the spring,” she writes in her new book, Pests: How Humans Create Animal Villains.

Brookshire argues that we deem animals “pests” when we fear them (like snakes). Or when they thrive in a niche we unintentionally created for them (think rats in the New York subway). Or when they find a way to live in a habitat now dominated by humans (all those deer in the suburbs). Sometimes we demonize an animal if we feel like it’s threatening our ability to control the landscape (like coyotes that attack our livestock, pets and even children).

Through the lens of science, history, culture, religion, personal anecdotes and a big dose of humor, Brookshire breaks down how our perspective shapes our relationships with our animal neighbors. She also goes into the field — trailing rats, hunting pythons, taming feral cats, tracking drugged-up bears — to see firsthand how pests are treated.

Science News spoke with Brookshire, a former staff writer for Science News for Students (now Science News Explores), about what we can learn from pests and how we can co­exist with them. The following conversation has been edited for clarity and brevity.

SN: What inspired you to write this book?

Brookshire: I wrote a news story that was about mice living with humans (SN: 4/19/17). [It was based on a study] showing that we’ve had house mice since we’ve had houses. I love the fact that humans have had these other animals taking advantage of the ecosystems that we create basically since we started living settled life. Every location that has humans has their “rat.” Sometimes that’s a rat, and sometimes it’s a pigeon or a cockatoo or a lizard or a horse. It’s not about what these animals are doing. Animals live in ecosystems that we create, and we hate animals that live too close.

SN: What surprised you during your research?

Brookshire: The reflexiveness of people’s responses [to pests]. People respond emotionally. When you make them pause and think about it, they go, “Oh wow, that doesn’t make any sense. I should not be caught trying to kill a raccoon with a sword.” But in the moment, you’re so wrapped up in the violation of what you see as your personal space.

The other thing is the extent to which our disdain of pests is wrapped up in social justice. A lot of times we see this hatred and disgust for animals that we see as “low class.” High-class people don’t have rats. And that’s really about social justice, about infrastructure and the ability of people to live in clean houses, store their food properly or even have a house at all.

Also, the way we deal with these animals often has vestiges of colonialism, as in the chapter on elephants. [In Kenya, European colonists] made people grow corn and sugarcane, which elephants love. Colonization created national park systems that assumed that humans had no place in wilderness, shoving out Indigenous pastoralists. Colonization created the market for poached ivory. And colonizing people assumed that Indigenous people did not like elephants or know their benefits. We are living with the consequences. Many modern efforts at elephant protection are spearheaded by Western people, and they assume the biggest issue with elephants is poaching and that Indigenous people don’t know what’s best for themselves or the elephants. In fact, human-elephant conflict [which includes elephant crop raids] is the far bigger problem, and Indigenous people have a long history of coexisting with elephants.

SN: In the book, you looked at many different cultures and included Indigenous voices.

Brookshire: It’s important to realize there’s more than one way to look at the world. By learning from other cultures, it helps us understand our biases. It’s only when you get outside of your own beliefs that you realize that’s not just the way things are.

SN: That shows up when you write about the Karni Mata Temple in India, also known as the Temple of Rats. Temple rats are not treated as pests, but a rat in a house would be.

Several rats sitting on the rim of a bowl of milk sitting on the ground
India’s Karni Mata Temple is home to thousands of black rats, which devotees believe are people reincarnated as rats by the goddess Karni Mata. The rats are given meals and milk in large bowls. In other contexts, rats are usually unwelcome visitors.Credit: Günther Jontes/Wikimedia Commons (CC BY 4.0)

Brookshire: That’s the result of context. And you see that in Western cultures all the time. People love squirrels. Well, they’re basically rats with better PR. Then you have people who have pet rats, who would probably scream if a sewer rat ran by.

SN: Are there any animals that you consider a pest?

Brookshire: No. The animal that I’ve probably come away with the most negative impression of is humans. It’s funny because we think we can extinct anything. And I love how these animals have gone: “Oh, poison? That’s cute.” “Oh, a trap? You’re funny.” We’ve tried to use electric fences on elephants [to stop them from eating crops]. And elephants are like, “Guess what? Ivory doesn’t conduct electricity.” Even if they don’t have tusks, elephants just pick up a log [to destroy the fence].

SN: Are you hoping to change people’s minds about pests?

Brookshire: I hope that they will ask why they respond to pests the way they do. Instead of just going, “This animal bothers me,” ask why, and does it make sense. I also hope it opens more curiosity about the animals around us. I learned from Indigenous groups just how much knowledge they have of the animals in their ecosystem. I hope more people learn. A world that you know a lot about is just a better world to live in.


Buy Pests from Bookshop.org. Science News is a Bookshop.org affiliate and will earn a commission on purchases made from links in this article.

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