For more than a decade, Friends of Griffith Park (FoGP) has sponsored the testing of raptors and mammals found dead in and around the Park for exposure to anticoagulant rodenticides. These results, in combination with studies across California, have demonstrated that dozens of mammal and raptor species are routinely ingesting rodenticides by eating poisoned rodents. Most research on the impacts of rodenticides on non-target wildlife has focused on mammals and raptors.

Although snakes are major rodent consumers, only a few studies around the globe have examined whether snakes are also ingesting rodenticides. An ongoing study conducted at the Natural History
Museum of Los Angeles County, and partially funded by FoGP, is helping to address this lack of information. Early results demonstrate that multiple snake species are also routinely exposed to these toxins.

What are Anticoagulant Rodenticides and How Do They Impact Non-target Animals?

A common way to try to limit rodent populations in agricultural areas or around structures is to use anticoagulant rodenticides. Once ingested, these toxins prevent blood from clotting. Exposed rodents die due to internal bleeding several days after ingestion. Non-target animals that prey or scavenge upon these dead or dying rodents can then get exposed as well (termed secondary poisonings). Many types of anticoagulant rodenticides will bioaccumulate, or build up in a predator or scavenger over the course of multiple exposures, and can then cause illness or death.

The above scenario should sound familiar to those who followed Griffith Park’s resident mountain lion, P-22. As a four-year old, P-22 was found in 2014 suffering from mange and with high levels of anticoagulant rodenticides. Exposure to rodenticides is known to weaken the immune system, with mange being a common outcome for coyotes, bobcats, and mountain lions. Following his death, P-22 was once again found to have been exposed to multiple anticoagulant rodenticides and was again suffering from mange.

Are Snakes Also Getting Exposed to Rodenticides?

Southern California has been a hotspot for research into secondary poisonings from rodenticides. Research results have caused the California Department of Fish and Wildlife to enact bans on multiple rodenticides, sometimes banning public use and other times banning use by both the public and by professional applicators. Nevertheless, some anticoagualant rodenticides are still available for use in California. Increased awareness of the species impacted by rodenticides could help to influence future policy on rodenticide use.

Because rodenticide exposure was common in studies of SouthernCalifornia mammals and raptors, I began wondering whether snakes might also be consuming poisoned rodents. About a decade ago, I started keeping salvaged snakes from areas where rodenticide exposure seemed possible.

As Curator of Herpetology at the Natural History Museum of Los Angeles County, I have developed a network of people who salvage roadkilled snakes. Over the years, freezers at the museum started
filling up with roadkilled snakes that might one day be used for rodenticide testing. The challenge was to find funding to test these specimens. In early 2023, Gerry Hans, on behalf of the FoGP Board, reached out to the Museum to suggest a partnership to fund rodenticide testing of local wildlife. Around the same time, the Museum was awarded a research training grant from the National Science Foundation that included funds to support research by early-career scientists. With funding now available, we could finally defrost and sample the frozen snakes for rodenticide testing.

Over the past few months, I worked with museum researcher Taylor Vasquez to do exactly this. Early results are now in, and over 30% of the snakes tested positive for rodenticides, including the following species: California kingsnake, gopher snake, coachwhip, Southern Pacific rattlesnake, and Southwestern speckled rattlesnake. Four snakes tested positive for multiple rodenticides.

This ongoing study highlights the negative impacts of using anticoagulant rodenticides to control rodents—numerous native species, including coyotes, bobcats, hawks, owls, and snakes, also end up being poisoned. As native predator populations decline from unintentional poison exposure, pest populations are likely to increase making people even more dependent on pest control efforts.
The best way to simultaneously control rodents and also protect the wildlife that would otherwise help limit rodent populations is to use mechanical traps specifically designed for mice and rats. Avoiding rodenticides will help our native predatory species and also decrease the chance that a family dog or cat is also exposed to rodenticides by consuming a poisoned rodent.

The snake study will continue through this summer. We will increase the number of specimens tested, the number of species examined, and the geographic coverage of the sampling. Stay tuned for an update in a future issue of this newsletter.

~Dr. Greg Pauly, Natural History Museum of LA County

Dr. Greg Pauly is Curator of Herpetology and Director of the Urban Nature Research Center at the Natural History Museum of Los Angeles County. He studies the ecology, evolution, and conservation of reptiles and amphibians, including the Blainville’s horned lizards found in Griffith Park. He also leads the Reptiles and Amphibians of Southern California (RASCALS) project on the iNaturalist community science platform.

SEE ROADKILLED SNAKES?

Through Fall 2024, we are looking to add more snakes to the rodenticide study. With temperatures warming, snakes are on the move. Unfortunately, this means that some snakes will be found dead along roads. Fresh roadkills can be salvaged and potentially used for rodenticide testing.

HOW YOU CAN HELP
If you can safely approach a roadkilled snake (please be careful around traffic), take a photo of the snake, and email that photo to Gerry Hans (Gerry@friendsofgriffithpark.org) and Greg Pauly (gpauly@nhm.org) with a detailed description of the location (best if you can provide an address or allow your phone to log latitude/longitude coordinates with your photo).

If the snake is not venomous, you can also move it to a shady spot so it doesn’t continue to get run over. If the snake is venomous, please do not handle it in any way. We are looking for snakes from the Griffith Park area; we also encourage reports of roadkills elsewhere in our Greater Los Angeles area. When possible, Gerry, Greg or a Museum staffer will attempt to salvage the snake for study use.

To include in the study, salvaged snakes must be:
• A species that eats rodents.
• Within a half mile of houses, other buildings, or agricultural lands.
• In relatively good shape. If the snake is more than a day old, smells, or has been run over numerous times, its liver is probably too damaged for testing.
• Snakes cannot be juveniles. A rattlesnake needs to be at least 20 inches long. If any other species (for example, a gopher or king snake), the snake needs to be at least 24 inches long. In smaller snakes, the liver will be too small for testing.