Nature Notes | The Rio Grande Cooter

Photograph courtesy Ivana Mali. A juvenile Rio Grande Cooter at Black River, a spring-fed oasis near the Guadalupe Mountains. The cooter once flourished throughout the Rio Grande watershed, from Big Bend to the Gulf, but it's unclear today whether the turtle is successfully reproducing anywhere but Black River.
Photograph courtesy Ivana Mali. A juvenile Rio Grande Cooter at Black River, a spring-fed oasis near the Guadalupe Mountains. The cooter once flourished throughout the Rio Grande watershed, from Big Bend to the Gulf, but it's unclear today whether the turtle is successfully reproducing anywhere but Black River.

The Rio Grande Cooter: A Desert Turtle is the Lovable Face of an Endangered Watershed

By Andrew Stuart

There are few things as magical as flowing water in the desert. Such places refresh both the body and the spirit, and in a land like ours, their presence can feel like an unearned gift. We immediately sense such places are special, and important.

These places – where springs create pools and streams – are also sanctuaries for some of the most remarkable, if unlikely, desert creatures. Behold the Rio Grande Cooter – scientific name Pseudemys gorzugi. This turtle wasn’t recognized as a distinct species until the 1980s, and scientists have only begun to unlock its secrets. Its future, however, is as tenuous as the watershed it calls home.

Below the Guadalupe Mountains, in New Mexico, Black River is the sort of place one is tempted to keep to oneself. It’s a storybook oasis. It flows for miles amidst white-sand hills, its turquoise waters dappled by the shadows of giant cottonwoods. Vermillion flycatchers dart from bank to bank, as wild turkeys chortle in the brush and ospreys wheel overhead.

But the river itself belongs to one Chihuahuan Desert swimmer.

Dr. Ivana Mali is a professor at Eastern New Mexico University.

“It’s a very charismatic species,” Mali said of the Rio Grande Cooter. “They’re just pretty. They’re in the same family as the red-eared slider, and the painted turtle. To tell the species apart, you would have to take a look at the head markings, and to a degree, the top shell markings.”

Born in Serbia, Mali moved to the U.S. in 2004, and studied wildlife biology at Texas State University. Since 2016, she’s been trapping and studying Rio Grande Cooters at Black River.

Cooters – turtles of the genus Pseudemys – are unique to North America. The majority are found in the southeastern U.S. The Rio Grande Cooter is the westernmost species. As Mali suggests, our desert cooter is easy on the eyes.

It’s adorned in lines and whorls of yellow or red. It has “c”-shaped markings on its olive shell, and distinctive yellow markings on the side of its head. Its looks once made it a target for the pet trade – in a way that likely negatively impacted populations.

The cooter’s haunts are remote, and until recently, it was one of the least-studied turtles in the U.S. and Canada. But Mali and her colleagues have gained new insights into its ways.

Her team typically traps between 60 and 100 cooters at Black River each summer. They bait the traps with sardines – and romaine lettuce. Cooters in general are thought to be omnivorous when young, but strictly herbivorous as adults. Yet, from analyzing fecal samples, and from their success with sardines, Mali’s team has found that our species is different.

“What we found is that they are truly, to various degrees, omnivorous,” Mali said. “We have found algae. We have found cottonwood leads. But we did find that they do eat invertebrates that, for example, fall into the water – you’ll have dragonflies, even ants. But not fish.”

Basking is a crucial part of a turtle’s life. As ectotherms – so called “cold-blooded” creatures – turtles need heat from the environment to warm themselves for the day’s work, of mating or foraging. Mali placed a game camera near a fallen tree – a popular basking spot. The cooters, she found, don’t hibernate – they bask on the log even in the cold of winter. The basking intensifies in spring, and then declines as the waters of Black River warm in summer.

One remaining mystery is the cooter’s nesting habits. Mali’s team has yet to find a nest at Black River. But the general pattern for cooters is known. Leaving her natural element, and risking predation, the female takes to land. After locating a spot to her liking, she uses her hind legs to dig a hole. Then she lays and buries her eggs.

Mortality at this stage is high. Eggs, of course, are tasty, and raccoons and skunks are adept at locating buried nests and feasting on what they find. It’s likely that less than 5 percent of eggs survive to become mature turtles.

Yet if a Rio Grande Cooter does survive infancy, it’s apt to live a long life. Mali’s findings suggest these turtles live up to 60 years.

And though Mali hasn’t documented nesting – it’s certainly happening.

“We know that they’re quite successful,” she said, “because we catch a lot of juveniles – 30 to 40 percent of our captures are juveniles, and we always catch a few really, really small ones, so we know that they’re reproducing.”

In this respect, it turns out, Black River is an unsettling anomaly.

The evidence suggests that the cooter once flourished throughout the Rio Grande watershed, from the Big Bend to the Gulf, and up the Pecos River, and was the dominant turtle species in much of that watershed. Then came damming, depletion, and pollution from oil-and-gas activities. Today, long stretches of the Rio Grande and Pecos no longer provide the water quality, or quantity, aquatic creatures need.

Spring-fed tributaries – places like the Devils River, Independence Creek and Black River – have become “strongholds.” Native wildlife persists in these places, as well as in the Lower Canyons of the Rio Grande – where springs replenish the river. In these spring-fed streams, the Rio Grande Cooter, and other native river creatures, may be making their last stand.

Mali’s mentor, Dr. Michael Forstner, is researching cooters on the Devils River. The Devils is the most pristine river in Texas, and in some respects cooters seem to thrive there. Forstner finds turtles with shells a foot and a half long – significantly larger than their Black River kin. Yet he’s found virtually no juveniles.

The extent to which the Rio Grande Cooter is successfully reproducing outside of Black River is now an open question.

Mali said that – because of their longevity – turtles are a “delayed indicator” of the health of river ecosystems.

“I tell people, it’s kind of hard to kill a turtle,” she said. “They will be there, and they will try. But once you see that the turtle population is going down, it’s kind of too late for all the other taxa out there.”

The Rio Grande Cooter is recognized as a “species of greatest conservation need” in both Texas and New Mexico, and the turtle is being considered for listing as an endangered species. Yet whether or not that designation is made, there’s no doubt that the Rio Grande watershed itself – and the very presence of fresh, flowing water in our arid region – is endangered.

The cooter puts an undeniably lovable face to that dilemma.

Nature Notes is supported by the Shield-Ayres Foundation and produced by Marfa Public Radio with the Sibley Nature Center. The program can be heard each Tuesday and Thursday, at 7:45 a.m. and 4:45 p.m., Central time, on KRTS Marfa, 93.5 FM, and KXWT Odessa/Midland, 91.3 FM. This episode was written by Andrew Stuart.


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