Patagonia’s Tuco-tuco, a case study in extinction
Bariloche, Argentina — In the foothills east of the Andes lies the Patagonian steppe, a transitional landscape between forest and desert in central Argentina that is dotted with hardy shrubs and dark green cypress trees. Cutting through this arid environment are icy blue rivers like the Río Limay, along whose banks live a species of social rodents named Tuco-tucos. Gregarious though this sounds, this Tuco-tuco is critically endangered and scientists are wondering if a behavior that has served humans so well may be deleterious to the Tuco-tuco.
A Tuco-tuco rustled from its burrow. Credit: Tatiane Noviski via Flickr.
Tuco-tucos are aggressive, territorial animals that spend most of their life underground and can weigh up to a pound. Most of the fifty other species of Tuco-tuco found in South America are solitary and can live anywhere from sand dunes to mountain meadows. But this Patagonian species has earned the name ‘colonial Tuco-tuco’ because of the social colonies it forms. Up to six adults, mostly females, can be found sharing a single burrow, sometimes holing up together for four years. The only known population of colonial Tuco-tucos is limited to the western bank of the Río Limay.
Oliver Pearson, a biologist from the University of California at Berkeley, first described social behavior in Tuco-tucos while on a field trip to Argentina in 1984, dubbing their species Ctenomys sociabilis. In 2001, a Berkeley biologist named Eileen Lacey who was interested in Pearson’s work looked at the social rodent’s genome. She found something that could help explain the species’ dwindling numbers—the colonial Tuco-tuco had very low genetic diversity.
Genetic diversity is important to any healthy, thriving population; more genetic variation means many more versions of genes. A large gene pool ensures the overall fitness of a population and is insurance against inherited diseases. Several royal families have lowered their genetic diversity with each generation of inbreeding, at times resulting in severe mental and physical disabilities——like King Carlos II of the Spanish Hapsburgs. The same occurs in captive animal populations; there simply aren’t enough individuals to maintain a genetically-healthy group.
For the colonial Tuco-tuco, reduced genetic diversity is the result of a historic decimation of their population size, thought to have occurred around 3,000 years ago. Since the colonial Tuco-tuco’s numbers fell so drastically and the current population is the progeny of those individuals that survived, the species experienced what evolutionary scientists call a population bottleneck. But what could have wiped out most of this animal’s population?
Mauro Tammone, a doctoral student at Argentina’s Centro Nacional Patagónico, has ventured a few guesses as to what may have caused this population crash. A volcanic event is one theory, albeit a persuasive one, he says; Patagonia has been plagued by volcanic eruptions since the uplifting of the Andes twenty million years ago. In fact, Tammone has found a layer of volcanic material while digging for rodent fossils in caves near the Río Limay. What’s interesting, he says, is that the older soil—below the layer of volcanic ash—has colonial Tuco-tuco fossils, while the soil after the volcanic event has none. The only thing Tammone can say with certainty is that C. sociabilis suffered a local extinction in this area.
Despite this geological proof, there is living evidence that contradicts the volcano theory as the cause of the bottleneck. On the opposite bank of the Río Limay lives a separate species of Tuco-tucos called Haig’s Tuco-tuco, or Ctenomys haigi. Though visibly almost identical to their social cousins 100 meters away, Haig’s Tuco-tuco is totally solitary. And they haven’t lost any genetic diversity. Surely a volcanic event would have affected them too, says Tammone, since they basically live in the same place.
How could two species that are so similar—same habitat, same appearance—have responded so differently to whatever event or series of events caused the loss of genetic diversity? The answer could lie in what distinguishes them: their divergent behavior.
“At the end of the day this is an inquiry into what extinction is,” says Ulyses Pardiñas, a paleontologist at the Centro Nacional Patagónico and an advisor of Tammone’s. “If it turns out that the reason that things are going so poorly for this species is its sociability, if that’s its Achilles’ heel, that’d be terrible,” says Pardiñas. “Right, Mauro?”
When I met them in the small city of Bariloche, a half-hour’s drive from the Río Limay cave, Pardiñas and Tammone were sorting through the week’s cache of owl pellets. Found within the range of the colonial Tuco-tucos, the researchers are using owl pellets to estimate which rodents are currently abundant in the area. While sifting through the bones of different rodents, Pardiñas tells me how the colonial Tuco-tuco’s sociability could be a disadvantage. What if their living in groups, as advantageous as that behavior has been proven to humans, was the reason for their populations’ demise?
Pardiñas offers up an anthropological cause for their population crash. Tuco-tucos have long been a local delicacy. The Selknam, aborigines who settled Patagonia more than 12,000 years ago, even preferred the Tuco-tucos to the llama-like guanaco, says Pardiñas, referencing the work of German ethnologist and missionary Martin Gusinde.
Tammone shows me a 5,000-year-old Tuco-tuco skull with evidence of fire damage to its incisors. “That’s what happens when you roast these animals whole over a fire,” explains Pardiñas. “If the aborigines knew this group of Tuco-tucos lived in colonies, they could easily polish them off, as compared to the other species of Tuco-tuco they knew lived in solitary burrows.”
In fact, this is what happened to the Magellanic Tuco-tuco (Ctenomys magellanicus) a couple thousand kilometers south in Tierra del Fuego. It’s one of the two other species of social Tuco-tucos in the world (the other lives in the far north of Argentina, in a desert region named Jujuy.) From 1930 to 1960, fed up with the Magellanic Tuco-tuco digging holes in their pastures—which left mounds of soil that dirtied their sheep’s wool—local sheep ranchers launched a Tuco-tuco extermination campaign. The ranchers outfitted their tractors with spikes and drove them through the pastures, impaling Tuco-tucos by the hundreds, recounts Pardiñas. This nearly drove them to extinction. Their numbers are just now starting to rebound. Whether their persecution has caused a bottleneck effect remains to be studied.
Tammone and Pardiñas are still looking for what caused the sharp decline in the colonial Tuco-tuco’s genetic diversity. The two scientists hope that contrasting these social rodents with their solitary neighbors across the river will generate more hypotheses. And Tammone’s investigation into Tuco-tuco fossils in the area may tell the story of changes in Tuco-tuco populations in the time since the last glaciers receded from Patagonia twenty-thousand years ago. Between the rodent fossils, layers of volcanic ash, and evidence of prehistoric campfires may lie an explanation as to why this social rodent is on a road to extinction.