Genetic analysis of ancient poop found off the coast of Mexico suggests bighorn sheep may be native to Tiburón Island, the largest island in the Gulf of California and Mexico.

Bighorn sheep were not thought to inhabit Tiburón Island prior to their introduction in 1975 but scientists discovered fossilized dung in the mountains of Tiburón Island that challenges that assumption. Scientists compared the pellet-shaped poop to fecal pellets of other large mammals and extracted DNA to sequence and determine the origin.

Carbon dating suggests the poop originated from 1470-1630 years ago. Genetic analysis confirmed that the dung belonged to bighorn sheep, similar to those found in southern Arizona and California, but different from the extant Tiburón population.

The identification of ancient bighorn sheep on the island that the species was native to Tiburón Island.

A bighorn sheep (Ovis canadensis) skull on Tiburón Island. Skull from bighorn sheep population introduced on Tiburón Island, Gulf of California, in 1975. Radiocarbon dating and ancient DNA analysis from a local cave now show that this introduction unintentionally re-established bighorn sheep that had gone extinct on the island. Credit: Photo by B.T. Wilder

"This finding raises a host of fascinating questions", says Wilder, "Are bighorn sheep on Tiburón Island a restoration or a biological invasion? This extended biological baseline confirms that the Tiburón bighorn sheep went extinct before.

Given the cultural and conservation significance of the unintentionally rewilded population, actions can be taken to avoid the same fate."

The authors suggest that native desert bighorn sheep may have previously colonized this island when lower sea levels connected Tiburón to the mainland, most likely during the Pleistocene. They were likely eliminated within in the last ~1500 years, probably due to inherent dynamics of isolated populations, prolonged drought, or human overkill.