Some magical moments in science involve the smell of dung.
Case in point: The discovery of the “unintentional rewilding” of bighorn sheep on Tiburón Island by a researcher who was simply probing woodrat middens for ancient plant specimens.
When desert ecologist Ben Wilder unpacked his collection of dung at the Tucson lab of Julio Betancourt, a former adviser who was helping him with identification, he had one sample that he knew wasn’t rat.
Betancourt took a whiff.
“I said, ‘That’s sheep’ before I even looked at it. I can smell it,” Betancourt said.
Betancourt, a paleoecologist who has held multiple appointments in environmental sciences departments at the University of Arizona, is currently working as a senior scientist with the U.S. Geological Survey in Washington, D.C.
He said he has dug through dung mats many times in his research life, and knew from the smell — or lack of it — that this sheep dung was not recent.
Which presented a mystery.
There are bighorn sheep on Tiburón Island, but they were taken there by Mexican wildlife conservationists in 1975 to breed and serve as a feeder population for threatened bighorn sheep herds on the mainland.
It proved to be a good spot for them, said Wilder, who has spent a lot of time on Tiburón and other islands in the Sea of Cortez as part of his study of the evolution of plant communities in the Sonoran Desert. Wilder is finishing up his doctoral degree at the University of California, Riverside.
Tiburón has no mountain lions or other large predators, and has adequate food and water for the bighorns. The introduced herd of four males and 16 females has grown to more than 600.
The Mexican government and the Seri Indians now allow a hunting organization to raffle hunting trips, promising trophy-sized rams, for bids starting at $80,000.
Proceeds go to the Seri tribe, who administer the island, along with the Mexican federal government, as an ecological preserve.
What the researchers did not know when they began the program was that they were not introducing sheep to the island, but rather reintroducing them, according to a paper Wilder and his co-authors published last month in the journal PLOS One.
The sheep dung that Wilder found in a shallow cave on Tiburón was not from those reintroduced herds. It was about 1,500 years old, according to DNA analysis done for the study by bighorn sheep expert Clinton Epps, of the Department of Fisheries and Wildlife at Oregon State University.
“It was a really magical moment,” said Betancourt. “Science is about moments like that when all of a sudden you say ‘Holy crap, you’ve got something there.’”
It allowed Wilder, Betancourt, Epps and the co-authors to introduce a new term to the scientific literature.
“The discovery presented here refutes conventional wisdom that bighorn sheep are not native to Tiburón Island, and establishes its recent introduction as an example of unintentional rewilding, defined here as the introduction of a species without knowledge that it was once native and has since gone locally extinct,” the journal article says.
Wilder, a former Tucsonan who is finishing his doctorate in botany and plant sciences in Riverside, is co-author with Richard Felger of a book on the native plant populations of islands off the coast of Sonora.
Tiburón is the largest Mexican island, just two kilometers from the mainland near Kino Bay, Sonora, at its closest point.
It was connected by land bridge to the mainland at the end of the last Ice Age, 5,000 to 6,000 years ago, Wilder said.
Wilder wasn’t looking for evidence of bighorns. He was investigating ancient deposits of woodrat dung to determine what plant species they ate thousands of years ago.
He said he thought the pellets of dung he found might be sheep, but assumed they were recent when he gathered them up.
Carbon dating produced a date of 1,576 to 1,632 years ago. Epps’ DNA tests confirmed the pellets came from bighorn sheep.
It put the bighorn introduction in a possibly better light, Wilder said.
Wilder came to realize that the plants he studies on the island had evolved with bighorn browsing, although predators, who may also have disappeared, could have kept the population in check.
The discovery also introduced a note of caution. If these sheep had colonized the island before, how did they go extinct?
The paper suggests a number of possible reasons: isolation reduced the tribe’s genetic diversity and overall fitness, hunting thinned the herd, or drought robbed them of water and food sources.
Similar things could happen in the future, the paper warns.
“It’s a case-by-case story when you are talking about reintroductions and bringing back lost species,” Wilder said. “It’s really critical to have a baseline — ‘Were these species here before?’ and ‘Where do you go from that?’”
Betancourt said the discovery “brought home to me something I live by — the power of a single historical fact. Something as simple as ‘This animal was here before’ can stimulate all of this discussion and all these questions.”
For now, Wilder is sticking to his plant studies, but said he’s tempted to further pursue the story of bighorn sheep on Tiburón.
He is intrigued, especially by the relationship between scientific study and cultural knowledge in the region. Wilder said the Seris who accompany him on his field trips have taught him much about the region.
Their place names, for example, often refer to species that no longer exist on a particular island, but did in the past.
“This is a situation where science is finding something that traditional knowledge had not preserved.”