South and Central America are hotbeds of animal diversity and home to a dazzling array of species, especially mammals (Wikipedia’s incomplete list of South American mammals, for example, contains a whopping 1331 entries). Biologist Rita Gomes Rocha calls the region a “natural museum,” where new species like the famed olinguito are discovered with increasing regularity. 

Finding new animals or studying the ones we already know is not often an easy task. Most South American mammals are small—rodents, bats, and marsupials that tip the scales at a couple of pounds at most—and excel at hiding from biologists or avoiding traps. Others are easily confused for something they’re not because of physical similarities (the National Zoo actually had an olinguito in captivity long before the species was named and described, mistakenly believing that it was a closely-related olingo). Either way, finding them, naming them, and learning about them often requires the time, funding and manpower to spend hours and hours wandering around the rainforest.

To fill in some of the gaps in our knowledge of tropical mammals, Rocha and other researchers in Brazil recently wondered if they could get some help from another group of animals: owls. 

They didn’t enlist the birds as field assistants, but rather looked at their leftovers. Owls often swallow their prey whole and later regurgitate the bits and pieces they can’t digest, like bones and fur, in masses called “pellets.” Since barn owls are found all over the tropics in different habitats and specialize in preying on small mammals, Rocha figured that scouring their pellets for bones and DNA could be an easy, low-cost way of detecting and inventorying an area’s mammals and deducing their ranges. 

Other researchers have had success doing this in other parts of the world, but DNA degrades more quickly in a tropical climate, so Rocha first had to see if pellet bones could be a viable source of genetic information. After collecting pellets in Central Brazil, picking out mammal teeth and jawbones and extracting DNA from them, Rocha and her team were able to get viable sequences from nearly three-quarters of their samples and positively identify 11 different mammal species. 

Among these were a few surprises. Two species—Emilia's Gracile Opossum and Pinheiro's Slender Opossum—had never before been recorded in the area that Rocha was working in, and their presence there extends their ranges hundreds of miles farther south than thought (a barn owl’s territory is typically much smaller, so this likely wasn’t a case of an owl picking up a far away meal). 

The researchers also found DNA from two animals in the genus Oecomys, also known as the rice rats, that were genetically distinct from the rest of the group and couldn’t be further ID’d, suggesting they may be undescribed species new to science.

Owl pellets clearly have a lot to tell biologists (even those that aren’t that interested in the owls themselves), and Rocha encourages mammalogists working in the tropics to make searching for and collecting them a regular part of their fieldwork. It’s low cost, and requires no special equipment like camera traps, and the pellets not only provide a non-invasive source of DNA, but also skulls and other bones for anatomical study and museum specimens.