More Testosterone Means More Foot Wiggling for These Frogs

Picture this: you’re out at a bar, and you’ve just met a real cutie. You stand up to order another round, but when you turn around, you see that some jerk has moved in on your date. The bar is so loud that yelling to get their attention won’t do you much good. What do you do? Well, if you want to take the tactic of a male Bornean rock frog, you should start aggressively waving one of your feet. Researchers have found that the frogs’ “foot-flagging” behavior is linked to testosterone levels. They published their research in the Proceedings of the National Academy of Sciences

The Bornean rock frog (Staurois parvus) is a small amphibian that makes its home in the island’s lush rainforests. S. parvus is especially fond of hanging out near thundering waterfalls, which makes for beautiful views but a whole lot of ambient noise. However, mating season necessitates certain types of communication. To work around this, S. parvus and a few other species have started using a sort of natural semaphore system, in which a male frog waves his specialized white-webbed back foot around like a flag. 

To other male frogs, a foot-flag is a clear sign of macho aggression. So researchers wondered if macho hormones—in this case, testosterone—might be involved in the process.

"We know that testosterone is an important regulator of many types of sexual behaviors, so it seems a natural hypothesis that this steroid might also influence waving by affecting the motor systems that control physical movement,” Matthew Fuxjager, Wake Forest University biologist and co-lead-author of the paper, said in a press statement.

To find out, Fuxjager and his colleagues selected 40 male Bornean rock frogs from a rainforest-like terrarium at the Vienna Zoo. The researchers looked for males who showed clear territorial behaviors, including foot flagging.

These males were split into experimental and control groups. Frogs in the experimental group were given a small injection of testosterone in saline solution. Those in the control group just got the saline. As soon as the frogs had gotten their treatment, the researchers paired them up: either two experimental-group frogs or two control-group frogs. Each pair was placed in a mesh container, where they were soon joined by a female frog. The mesh container was then placed in a large habitat with a waterfall, to replicate the environment the frogs find most romantic. To increase the realism, the researchers also played them audio of other frogs croaking.

The frogs were given an hour to get used to their new digs. Then, the researchers switched on a video camera and left it trained on the frog trio for seven hours. After all the frogs had gone through the experiment, a researcher, who didn’t know which frogs were which, watched the tapes and took notes on the frogs’ foot flagging and croaking.

The scientists killed six males, as well as six each of two related, non-foot-flagging species (Rana pipiens and Xenopus laevis). They then used RNA testing to examine the makeup of the animals’ thigh muscles, brains, spinal cords, and larynxes.

The results showed that testosterone did indeed affect S. parvus frogs’ bodies and their aggressive behavior. The thigh muscles of S. parvus specimens contained 10 times more male-hormone receptors than those of the other species. In other words, the Bornean frogs’ legs were ready to be pumped up. And the testosterone definitely pumped them up; the S. parvus males in the experimental group were much more likely than control-group frogs to start waving their legs around.

"These data therefore suggest that the evolution of the foot flag is associated with a dramatic change in how hormones act on specific muscles in the body, particularly those muscles that control the display in the first place," Fuxjager said. "These findings provide novel insight into how the evolutionary gain of a sexual display trait may be augmented by evolution of the hormone systems that control and refine adaptive motor skills."

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Big Questions
Why Do Dogs Crouch Forward When They’re Playing?
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Whether they're tilting their heads or exposing their bellies for rubs, dogs are experts at looking adorable. But these behaviors do more than elicit squeals from delighted humans; in many cases, they serve important evolutionary functions. A prime example is the "play bow": If you've ever seen a dog crouch forward with its elbows on the ground and its rear end in the air, wagging tail and all, then you know what it is. The position is the ultimate sign of playfulness, which is important for a species that often uses playtime as practice for attacking prey.

The play bow first evolved in canids as a form of communication. When a dog sees another dog it wants to play with, it extends its front paws forward and lifts up its behind as a visual invitation to engage in a friendly play session. Dogs will "bow" in the middle of playtime to show that they're having fun and wish to continue, or when a session has paused to signal they want to pick it back up. Play bows can also be a sort of apology: When the roughhousing gets too rough, a bow says, “I’m sorry I hurt you. Can we keep playing?”

Play between canines often mimics aggression, and starting off in a submissive position is a way for all participating parties to make sure they’re on the same page. It’s easy to see why such a cue would be useful; the more puzzling matter for researchers is why the ancestors of modern dogs evolved to play in the first place. One theory is that play is crucial to the social, cognitive, and physical development of puppies [PDF]. It’s an opportunity for them to interact with their own kind and learn important behaviors, like how to moderate the strength of their bites. Play also requires the animals to react quickly to new circumstances and assess complex actions from other dogs.

Shiba inus playing outside.
Taro the Shiba Inu, Flickr // CC BY 2.0

Another evolutionary explanation is that playtime prepares puppies for the hunting they do later as adults. Watch two puppies play and you’ll see them stalking, biting, and pouncing on one another—all behaviors canines exhibit in the wild when taking down prey.

Of course, it’s also possible that dogs simply play because it’s fun. This is a strong case for why pet dogs continue to play into adulthood. “Devoting a lot of time to play may be less advantageous for a wild species who spends much of its time hunting or foraging for food, searching for mates, or avoiding predators,” Dr. Emma Grigg, an animal behaviorist and co-author of The Science Behind a Happy Dog, tells Mental Floss. “Many domestic dogs are provisioned by humans, and so have more time and energy to devote to play as adults.”

Because play is a lifelong activity for domestic dogs, owners of dogs of all ages have likely seen the play bow in person. Wild canids, like wolves, foxes, and coyotes, tend to reserve this behavior for members of their own species, but pet dogs often break out the bow for their humans—or anyone else who looks like they might be up for a play session. Grigg says, “One of my dogs regularly play bows to her favorite of our cats.”

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Animals
Why Crows Hold Noisy Funerals for Their Fallen Friends
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The next time you hear a murder of crows cackling for no apparent reason, show a little respect: You may have stumbled onto a crow funeral. Crows are among the few animals that exhibit a social response to a dead member of their species. Though their caws may sound like heartbroken cries, such funerals aren't so much about mourning their fallen friends as they are about learning from their mistakes.

In the video below from the PBS series Deep Look, Kaeli Swift, a researcher at the University of Washington's Avian Conservation Lab, investigates this unusual phenomenon firsthand. She familiarized herself with a group of crows in a Seattle park by feeding them peanuts in the same spot for a few days. After the crows got used to her visits, she returned to the site holding a dead, taxidermied crow and wearing a mask and wig to hide her identity. The crows immediately started their ritual by gathering in the trees and crying in her direction. According to Swift, this behavior is a way for crows to observe whatever might have killed the dead bird and learn to avoid the same fate. Flocking into a large, noisy group provides them protection from the threat if it's still around.

She tested her theory by returning to the same spot the next week without her mask or the stuffed crow. She offered the crows peanuts just as she had done before, only this time the birds were skittish and hesitant to take them from her. The idea that crows remember and learn from their funerals was further supported when she returned wearing the mask and wig. Though she didn't have the dead bird with her this time, the crows were still able to recognize her and squawked at her presence. Even birds that weren't at the funeral learned from the other birds' reactions and joined in the ruckus.

Swift was lucky this group of crows wasn't particularly vengeful. Crows have been known to nurse and spread grudges, sometimes dive-bombing people that have harmed one of their own.

[h/t Deep Look]

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