For Constipated Scorpions, Females Suffer Reproductively. Males, Not So Much.

Science
دوشنبه 11 اسفند 1399 ساعت 13:41
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Smith sonian mag
Faced with a predator, some animals choose to fight. Others flee. A select few fracture themselves into pieces.
Certain scorpions, like many lizards, are capable of breaking off part of their tail during a predatory attack. Unlike lizards, however, these scorpions have a peculiar anatomy in which their anus resides near their stinger at the end of the tail. Thus, when a scorpion breaks off its tail, it comes at a terrible cost. The scorpion loses its ability to defecate, ensuring a slow death by constipation over the ensuing months.
But while a stump-tailed scorpion’s days are numbered, a new study finds that losing the tail apparently imposes few reproductive costs on male scorpions. Females, however, aren’t so lucky.
In the paper, published online in January in American Naturalist , researchers examined the reproductive penalty paid by the scorpion species Ananteris balzani when it loses its tail. These small South American arachnids have a light brown color, delicate pincers and a sting a bit less painful than a bee, according to the paper’s lead author Solimary García-Hernández, who knows from experience. “They’re cute,” she says. “And they are really fast, which is not very common in scorpions.”
Lacking even an established common name, Ananteris has long been little-studied and poorly understood. García-Hernández first began studying Ananteris early in graduate school in 2011, and even found a new species in her parents’ backyard, which she named Ananteris solimariae .
It was a big surprise in 2015 when she, while working as part of a larger research team, found that Ananteris scorpions are capable of shedding their tails. “Autotomy”—the process of dropping a body part to escape a predator—was until then known to have evolved in only a handful of animal lineages like starfish, spiders and certain lizards. But while a lizard that sheds its tail pays a cost—it no longer has the appendage to store fat and locomotion is impacted—the mere act of self-amputation is hardly a death sentence. Not so, for the constipated scorpions of Brazil. Over a period of months, the scorpion’s tiny digestive tract fills with feces, causing the scorpion to become visibly swollen. About eight months after losing its tail, the scorpion dies.
The abdomen of a scorpion that has lost its tail showing the accumulation of feces (white material) in its digestive tract. This scorpion will eventually die of constipation. (John Alexander Uribe, @JohnUribePhotos )
“The behavior was so extremely weird that I thought I’d really like to better understand the implications of it,” says García-Hernández. So she designed an experiment to test what costs are imposed on a stump-tailed scorpion over the course of its post-tail life. Of particular interest was how losing the tail affected the scorpion’s reproductive abilities.
To test this, García-Hernández and her team at Universidade de São Paulo first collected nearly 150 scorpions from the Brazilian savanna. Next, she induced about half the scorpions to shed their tail. In the wild, the scorpions might lose their tail in an encounter with a hungry bird or rodent. But in the lab it was up to García-Hernández who tugged at them gently with a small pair of forceps.
The team then set up a series of matings between stump-tailed and intact scorpions. García-Hernández predicted that autotomized male scorpions would be less successful at mating than their fully endowed counterparts, since the tail plays an important role in their complicated mating ritual.
“To start the courtship dance , the males do like a tail-wagging,” García-Hernández explains. “If the female is receptive she lets the male grab her pincers and they start the dance.” During this promenade à deux , both scorpions face one another, pincers interlocked, and tango to and fro across the savanna, occasionally even interlocking mouthparts in a sort of scorpion kiss. The dance can take hours. Eventually, the male deposits a spermatophore on the ground and, using his tail as an anchor, abruptly shoves the female forward over the packet of sperm, which latches onto her genital opening.
But despite the apparent significance of the tail during this mating dance for both tail-wagging and shoving the female, García-Hernández and her team found that autotomized males were just as effective at mating as their intact compatriots. By analyzing video recordings of the scorpions mating in the lab, the team found that autotomized males took just as long as intact males to mate and were just as capable of depositing their sperm and transferring it to the female. “We thought after autotomy the males will have no chance to be as good as intact males,” says García-Hernández. But it turned out these male amputees were perfectly charming to the females. They simply wagged and shoved with their stump.
However, when the team explored the reproductive costs paid by stump-tailed females, the story was different. They found that tailless females, while able to mate successfully, went on to have 20 percent fewer offspring than intact females.
The reason for this difference? The five-month scorpion pregnancy provides a lot of time for females to get more and more constipated, says García-Hernández. She hypothesizes that the buildup of feces caused by the loss of the anus is either toxic to the embryos or that the feces simply crowds out the developing scorplings. This latter hypothesis is supported by the fact that a severely constipated scorpion can weigh 30 percent more than it did before it lost its tail. By comparison, that’s equivalent to a 150 pound person gaining 45 pounds of poop weight.
Zachary Emberts, an autotomy expert at the University of Arizona who was not involved in the study, says the paper is an important step forward for autotomy research.
“The question they’re asking in this study is how costly is this behavior,” says Emberts. “It seems really costly just on face value—you're losing part of your body.” But García-Hernández found that, as extreme as the behavior is, it makes sense in the dog-eat-dog world of evolution. A scorpion that drops its tail escapes being eaten and has a chance, if only for a few months, to pass on its genes.
Given the difference in costs between male and female scorpions, one would expect that females should be more reluctant to drop their tail. Indeed, in an earlier study , García-Hernández and her colleagues found exactly that. During simulated predatory attacks, male scorpions dropped their tails 88 percent of the time while females only dropped them 20 percent of the time. This difference between the sexes now makes sense: Females pay a higher cost for losing their tail.
Of all the forms of defensive behavior in animals, autotomy might be the most audacious. “If you think about it, it's a pretty extreme behavior,” says Emberts. “Organisms are sacrificing part of their body to survive.” For scorpions, tail autotomy is even more punishing. A scorpion, caught by death, sacrifices its ability to defecate for the rest of its life in exchange for a few months of borrowed time. But by extending its thread, however fleetingly, the scorpion has a chance to continue its lineage.
García-Hernández hopes to test new questions about autotomy moving forward, such as how the defensive behavior works under natural conditions with real predators. And she hopes this work inspires more research on this common genus of scorpion in Latin America. “There are a lot of things happening in our world that we never think about,” she says. “You can learn about science with animals you have in your backyard. You can be curious. You can look for new answers.”
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