An analysis of 3,463 rabbit and hare bones – spanning 10 millennia – reveals when cottontails and hares were doing that for which they’re best known: breeding. Bunny booms occurred when prehistoric El Niño climate events drenched the Pacific Coast with heavy rainfall, encouraging the growth of delectable plants.
The complex weather pattern El Niño traditionally refers to a warm ocean current that appears off the coast of equatorial South America just after Christmas. It occurs every two to 10 years, and when it does, Mexico’s Baja California is inundated. “There weren’t many El Niños from 10,000 to 5,000 years ago,” perhaps between zero and two per century, University of Utah’s Jack Broughton explains. “After 5,000 years ago, there was a relatively dramatic increase in frequency of El Niños in Baja, and the rabbits go through the roof.”
Broughton and colleagues separated thousands of bunny bones from more than a million other small animal bits collected from an ocean-side rock outcrop 150 kilometers (95 miles) south of Tijuana, Mexico. It was called Abrigo de los Escorpiones, or Shelter of the Scorpions (pictured below), and it’s where raptors like hawks and eagles would sit and eat their prey. The bones would just fall to the bottom of the cliff. Researchers collected 15 boxes worth of bones – from birds to gophers to woodrats – that had accumulated over the course of 10,000 years.
By comparing skull and jaw measurements with museum specimens, the team identified three Baja bunny species: the brush rabbit (Sylvilagus bachmani), the desert cottontail (Sylvilagus audubonii), and the black-tailed jackrabbit (Lepus californicus). Brush rabbits eat grasses and forbs near dense brush, and they thrive when there’s more moisture; the desert cottontail has a similar diet but tolerates drier, more open conditions. Dry-loving jackrabbits, on the other hand, prefer sparse vegetation. Because there’s no direct record of ancient El Niños at the site, the team turned to Ecuadoran lake deposits.
The number of El Niños per century “correlates very strongly with the total rabbit population in Baja California, as well as relative abundance of the moisture-loving species,” says University of Utah’s Isaac Hart. That is, more brush rabbits, cottontails, and jackrabbits, in that order. Furthermore, warmer eastern Pacific seas meant more unfused, juvenile bunny bones relative to fused, adult ones. Bunny abundance overload!
Most work on long-term trends in small western North American mammals focus on interior sites where temperature heavily influenced their populations over the last 10 millennia. “This study is the most detailed of a coastal site over such a long period of time, and it shows precipitation is the dominant factor,” Broughton explains. Compared to the interior, coastal rabbits and hares are less harmed by temperature changes because the Pacific Ocean has a temperature-ameliorating effect. “The longer record we've provided here,” he adds, “should ultimately allow us to better predict how El Niño will vary in the future, and how animal populations will vary as a result."
The findings will be published in an upcoming issue of Quaternary Research.
Images: Lee J. Siegel (top), Isaac Hart (middle) / University of Utah