It's Not Without Caws That Crows Desecrate Their Dead

Science
جمعه 29 تیر 1397 ساعت 14:37
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Smith sonian mag
Kaeli Swift needed bodies.
Not real ones, mind you. After all, “I don’t want to haul around rotting animals,” the graduate student at the University of Washington says bluntly. “With a fresh carcass, longevity in the field in the height of summer is short. Those won’t last very long.” So for her unorthodox experiment, she set about getting her hands on some taxidermied crows. She was trying to figure out why—and how often—crows tried to have sex with crow corpses.
Scientists know that these crafty gossamer birds are among the few animals who won’t just take notice of their dead, but ritualize the occasion. When chancing upon a slain comrade, most crows take understandable offense. They’re loathe to touch the corpse—which could be a surefire way to contract a deadly infectious disease or expose themselves to predators—and they’ll even sound a vocal alarm to alert the masses to the calamity.
But one afternoon, Swift noticed a crow do something out of the ordinary. The bird swaggered brazenly toward one of the taxidermied bodies she’d planted, then arranged itself into an expectant position —wings flared and drooping, erect tail bobbing up and down. To Swift’s disbelief, the live crow, upon making contact with the dead one, was anticipating sex.
Crows, which enjoy puzzling through brain-teasing games and cobbling together new tools, belong to an elite club of exceptionally clever animals. Swift has previously shown that they’re wily enough to hold a grudge, and will avoid people and places they’ve learned to associate with friends who have been felled. They’re so attuned to the threat of a dead body that they’ll gather around them to signal to each other that danger is afoot.
But touching a corpse—let alone copulating with it—sends a far different message. So why are some crows taking the risk with behavior that is, often literally, not (re)productive?
Crows touch their dead only a minority of their time, but the interactions can get intimate. (Kaeli Swift, University of Washington)
Other examples of intense physical contact with the dead have been documented across the animal kingdom, from dolphins to elephants to non-human primates —all animals Swift also considers to be “intelligent and social.” However, despite anecdotal evidence, the drivers behind these rituals remain elusive. Animals could simply be trying to learn more about the deceased individual or the nature of its death, Swift says. Or perhaps prolonged contact could be a misalignment of instinct—an inappropriate urge to mate, acquire food or defend one’s territory.
Due to understandable logistical and ethical constraints, this sort of behavior had never been systematically explored in animals, says Swift. But if crows were indeed a part of this odd cult of the macabre, this presented the unique opportunity to characterize the phenomenon in a relatively simple system—and maybe uncover some of the motivations behind it.
Earlier this week, in the journal Philosophical Transactions of the Royal Society B , Swift and her supervisor John Marzluff published a study detailing their findings, describing the behaviors of 309 breeding pairs of crows. Crow necrophilia, it turns out, isn’t quite the norm—but it’s also not as unusual as you might think.
The utility of using taxidermied crows for the experiment was twofold: to avoid the impractical burden of toting perishable crow corpses around downtown Seattle, and to remove any concerns about familiarity (or lack thereof) between the living and the dead, which could have complicated the crows’ reactions. All of Swift's specimens were perfectly preserved strangers.
As expected, most crows were wary of the bodies, scolding the corpses with caws or mobbing it in groups. Only 24 percent of the time would the crows physically engage their taxidermied counterparts with prods, pecks and tugs. And just 4 percent of encounters were attempts at copulation.
Yet this behavior didn’t seem to be random. When Swift swapped in taxidermied pigeons and squirrels—common nosh for crows—they were regarded differently than the bodies of crow brethren. These other animals were fussed with more often, but set off fewer crow alarm bells: Live crows weren’t just recoiling in the face of death as a whole. Additionally, the interactions didn’t seem to be a way for the crows to glean information—such a process wouldn’t involve violence or fruitless attempts at reproduction.
Next, when Swift mounted dead crows in different positions mimicking alive and dead, the two conditions evoked vastly different responses. Lively look-alikes were repeatedly dive-bombed as if they were potential intruders. The ones that looked as dead as they were, on the other hand, enjoyed more cawing and mobbing—social signals that danger is afoot. In other words, the crows weren’t mistaking the taxidermy for living threats.
It was clear to Swift that touching dead bodies is not the norm for crows; it’s simply not worth the risk. But this study is the first to describe American crows regularly making physical contact with their dead, and opens up the potential for future investigation into this behavior. Christian Rutz , a professor at the University of St. Andrews in the United Kingdom who studies crows, calls the study “an important experimental investigation of how crows interact with their dead.” Rutz, who was not affiliated with the study, adds that “it is fascinating to see that physical contact appears to be relatively rare.”
Strangely, where attempts at necrophilia occurred, they were often paired with other aggressive behaviors: Some crows went as far as mutilation, tearing at tissue and even occasionally dismembering the corpses. It was a dizzying mix of reactions, especially in living crows attuned to the concept of the deceased. What could be driving these birds to canoodle their cadavers, sometimes even violently?
A final observation may hold the answer. Swift conducted her studies from the beginning of April through the end of August, bookending the typical crow breeding season. Corpse quarrels were most prevalent in late spring, tapering off as summer progressed. Though Swift was unable to directly sample her subjects, this pattern coincided surprisingly well with what is known about the ebb and flow of sex hormones in crows. According to crow researcher Douglas Wacker , a professor of neurobiology at the University of Washington at Bothell who often collaborates with Marzluff, crow testosterone is much higher in April and May. After this, crows begin to enter a period of “reproductive quiescence.”
This early window of hormonal frenzy could put crow cognitive abilities on the fritz—including when it comes to reconciling with the dead. “Early in the [breeding] season, when they’re really pumped up, maybe hormones are downplaying their ability to process information,” says Swift. “Most birds can respond [to corpses] with the appropriate reaction. But maybe in a minority, they can’t, and they respond with everything.”
Martina Schiestl , a crow researcher at the University of Auckland and the Max Planck Institute for the Science of Human History who did not participate in the research, supports the idea that a surge in hormones can compromise rationality. “I think the hormones are something we can’t leave out of the equation,” she says. Schiestl suggests that another iteration of the study completely outside the breeding season—perhaps in the winter months—could be informative.
The intersection of aggression and sex may not be entirely incongruous. For instance, according Wacker, testosterone is elevated in displays of both territorial defense and mating rituals in crows.
Additionally, research by David J. Anderson , a professor of biology at the California Institute of Technology not affiliated with this study, has shown that the brain regions involved in these behaviors in mice and flies are closely related. However, he cautions that it’s difficult to extrapolate these findings to Swift’s experiments, due in part to the fact that taxidermied crows can’t fully approximate true corpses.
Future work, including direct sampling of crow hormones, will be necessary to put the matter to rest. Swift hopes to eventually explore the nature of the interactions between live crows and corpses of their kin—a difficult study to design due to the ethical concerns of killing or sedating live crows. But, according to Rutz, a well-executed setup might yield strikingly different results: After all, a dead family member could plausibly elicit less aggressive reactions than a taxidermied stranger.
Until then: where do we stand on the case for crow necrophilia? It’s no accident—but only a minority of crows dabble with the dead. On the rare occasions they do, however, it’s likely not without caws.
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