Tuleyome's Science Corner - Poison Oak

Geoffrey Benn • July 27, 2023

Poison oak (Toxicodendron diversilobum) with fall foliage by Frank Schulenberg

I had my first encounter with poison oak when I was working in a research lab at UC Davis. I had just taken off my gloves after doing an experiment, when I noticed blisters across the back of my hand. Had I spilled acid on my hand? I frantically checked my gloves and work area – no sign of a spill or damage to the gloves. Then I remembered that I had gone hiking near Lake Berryessa a few days earlier. A Google image search confirmed my suspicions – the rash was caused by western poison oak.


Western poison oak is not an oak at all, but is actually a member of the cashew family, which includes cashews, mangos, and sumac. Its closest relatives are the other members of the genus Toxicodendron (Latin for “poison tree”), which includes eastern poison oak, poison ivy, and the Chinese lacquer tree. Western poison oak is the only member of the genus present in California, where it can be found throughout the Coast Ranges, in the Klamath, and at lower elevations in the Sierra Nevada.


Western poison oak takes both its common and species name (Toxicodendron diversilobum) from its lobed leaflets, which resemble oak leaves, except that they typically occur in clusters of three, while true oak leaves are singular. It can occur as either a shrub or a climbing vine and can have quite a bit of variability in the shape of individual leaflets. The key identifying feature is its compound leaves, which each have three leaflets, with the center leaflet having a longer stem. These leaflets begin the year with a green color that will change to red in the fall. The leaves are lost for the winter, during which time the plant can be identified by its reddish-brown stems. It’s important to note that all parts of the plant can cause a rash, so you’ll want to be careful even in the wintertime when it is leafless.


Poison oak and its relatives produce an oil called urushiol, which is rapidly absorbed into our skin upon contact. Once inside our skin, urushiol binds to a protein called CD1a. This causes CD1a to be recognized by the immune system as foreign, leading the immune system to attack the skin cells, eventually causing a blistery rash. Interestingly, CD1a has also been shown to play a key role in causing psoriasis.


Despite the headaches it causes for humans, poison oak is an important component of the ecosystem of the inner coast range, where its leaves provide forage for deer and its berries provide food for a variety of birds. This raises the question of how these animals can contact and consume poison oak without developing rashes like we do. While this question has yet to be definitely answered, the recent discovery CD1a’s role in causing the human reaction to urushiol provides an important clue – the CD1a gene is primarily only found in humans and other primates, suggesting that its absence may contribute to other animal’s lack of sensitivity to urushiol. This is supported by a 2016 study which showed that mice that were genetically engineered to have the CD1a gene became susceptible to urushiol.


Given that there’s not an obvious evolutionary benefit for primates to produce such a strong response to poison oak, you may be wondering “why us?” The answer is unknown, but I suspect that primate sensitivity to urushiol may be an unfortunate evolutionary accident – CD1a likely evolved to recognize other substances similar to urushiol, such as compounds produced by infectious bacteria or fungi, and therefore triggers a defense response when urushiol is present. The good news is that with a better understanding of how the human body reacts to poison oak, scientists may be able to develop more effective treatments. In the meantime, it’s best to keep an eye on your surroundings and remember the mantra “leaves of three, let it be.”


-Dr. Geoff Benn (gbenn@tuleyome.org)


Education Associate

RECENT ARTICLES

May 11, 2026
 At Tuleyome, we support Public Lands Advocacy, Conservation, Stewardship and Education. Our commitment to these three pillars of action means that we must encourage our supporters to vote and to invest the time to learn how candidates for public office view public lands and public access to those lands. Public lands are a central part of our national heritage and have been so since the very founding of the Republic. The first shots of the American Revolution were fired on public land (the Lexington Common) in 1775. The practice of managing lands for the common welfare is itself tens of thousands of years old and is central to the traditions of indigenous people across all of the Americas. The relatively novel notion of private ownership of land has become prevalent in our society, but that in no way should blind us to the centrality of public land to the health and welfare of all. And yet there are those who would turn every scrap of public land over to private ownership for exploitation if they could. Efforts to privatize public lands or give Federal lands to states, where political pressure to privatize is stronger, have taken on a new urgency. They represent a significant threat to the public’s right to enjoy public lands. These efforts represent an existential threat to nature, wildlife and the land itself, as exploitation of the national legacy for private gain becomes the dominant ideology of certain elements in our society. Fortunately, as citizens, we have the opportunity to educate ourselves about who supports public lands and the public’s right to benefit from public lands. If you want to be a public lands voter, here are the three questions you need to ask everyone who is asking you for your vote: Do you support keeping public lands in federal public ownership, and will you oppose efforts to transfer them to state or private control? What specific actions will you take to improve public access for hunting, fishing, and outdoor recreation? What is your plan to ensure that public lands are managed for a balance of conservation, restoration, recreation and responsible energy development? Public lands, which include our national forests, lands managed by the Bureau of Land Management and Bureau of Reclamation, as well as the national park system, are managed for all Americans. Proposals to sell or transfer these reduce public access and jeopardize wildlife habitat. Ask for a clear commitment to keeping land access public rather than transferring it to state-level management. Ask what your representatives will do to eliminate private land bottlenecks. Ask for concrete plans to improve trails, access points, and habitat connectivity. Public lands are at the center of the climate crisis debate and many voters prefer conservation and renewable energy over increased oil and gas drilling. We support a balanced approach that prioritizes habitat protection and sustainable use, rather than maximizing extraction. This is the kind of long-term legacy that we owe to the people whose land we appropriated. It’s a legacy we owe to the generations of Americans before us who fought to establish the rule of law and the values of republican government. Public lands are the embodiment of the notion that we are a nation of equal citizens, indivisible, and not servants to the economic interests that extract private profit from public resources while socializing private losses on public accounts. If you want to be a public lands voter, take the time to educate yourself by visiting https://www.tuleyome.org/ and then ask those who want your vote the hard questions about where they stand on public lands. Then, Vote! Register to Vote here: https://registertovote.ca.gov/ The last day to register to vote for the June 2, 2026, Primary Election is May 18, 2026. Find your district and voters guide here: https://www.sos.ca.gov/elections Voting by mail in California is safe and secure and really easy to do. You can track the status of your ballot here: https://california.ballottrax.net/voter/ Craig Perrin Tuleyome Volunteer Board Member
By Kristie Ehrhardt May 11, 2026
Maybe you’ve seen some roadkill while driving through the Berryessa Snow Mountain National Monument, along the freeway, or even in your neighborhood. Or maybe you’ve been hiking and caught a whiff of something really, really foul. Or maybe you’ve seen ants swarming on another dead insect or even come upon an animal skull during a day on the trail. Have you ever thought about what happens when an animal dies in nature and how does it “go away”? Enter nature’s clean up crew! Carrion eaters are organisms that eat dead and decaying animals. This can be roadkill, leftovers from other carnivores’ meals, animals that simply expire from old age or injury or those that have been hunted and unfortunately never recovered by their pursuer. It sounds pretty gross but carrion eaters play a critical role in ecosystem nutrient recycling. Sometimes they’re loosely called “scavengers” and one of our most valuable scavenger species is pretty common and can be seen almost anywhere in our region from urban to rural areas. Do you want to take a guess at what species it is? I’ll give you some clues… They have an exceptionally fine-tuned sense of smell and can detect the scent of rotting flesh from over a mile away. Are you jealous? Their stomach acid is ultra acidic and allows them to eat bacteria and virus-laden decaying flesh and still remain healthy. When they feel threatened they will vomit in self-defense. Considering what they eat, it’s pretty clever and effective. It also makes them weigh less so they can escape quicker. To cool off in hot weather they will defecate and/or urinate on their own legs. This is called urohydrosis. They lack vocal cords and cannot make vocalizations, they can produce only grunts and hiss sounds. Did you guess what species I’m talking about yet? Of course, it’s the Turkey Vulture! Their scientific name, Cathartes aura , literally means, “cleansing breeze”. It seems appropriate when you consider their role in removing carcasses that spread disease such as anthrax, botulism and cholera from the environment. Turkey Vultures do not have feathers on their heads and resemble wild turkeys which is where their common name came from. Although their bald, red heads don’t help endear them to those that like “pretty birds”, it does keep them clean while they jam their naked heads into sticky, stinky dead animal carcasses. Although Turkey Vultures are classified as a bird of prey, they do not have strong talons like other raptors and are not able to kill their own food which makes them reliant on carrion left behind by other carnivores, mother nature, or vehicles. Turkey Vultures also do not build nests like most other birds. Instead they lay their eggs on the ground in a sheltered place like a cave, hollow tree, rocky crevice or cliff or even in an abandoned building. I see a lot of Turkey Vultures so apparently that nesting strategy works well for them. California Condors are also carrion eaters and use the same nesting strategy but struggle with success in the wild. Unfortunately poor nesting success and other identified dangers continue to keep their free flying population fairly meager. Turkey Vultures are widely distributed and occur in nearly all of the Americas from southern Canada to the southern tip of South America. It is a permanent resident in the southern United States but northern birds may migrate south. It is found in a diverse range of habitats including deserts, grasslands, farm and rangeland, wetlands, foothills and mountains, as well as suburbia. They prefer open county without dense tree cover where they can easily take flight and enjoy the thermals. While soaring they resemble a dihedral (shallow V) and although they are master fliers, rarely flapping their wings, they appear unsteady by rocking and tilting in flight. Their slow, teetering flight allows them to fly low aiding them in detecting their next meal. Their body feathers are dark iridescent brown-black with highly contrasting silvery-gray flight feathers. Their short, hooked beaks are ivory-white against their bright red heads, juveniles’ heads are black. Their wing span is similar to Bald and Golden Eagles at around six feet and they can weigh over five pounds. The oldest wild Turkey Vulture was 16 while captive birds have lived to be 51 years old. Lead poisoning and outright killing of Turkey Vultures due to the false belief that they spread diseases (in reality they clean up the ecosystem!) are their main conservation concerns. Their population numbers remain strong with a global population estimated to be 18,000,000 individuals. Other interesting Turkey Vulture fun facts include: In the air a group of Turkey Vultures are called a “kettle” but on the ground they go by “committee” or a “wake”. Large groups of them roost together in trees and although I'm not sure what that’s called but that is an impressive sight! Gas and oil companies have been known to employ Turkey Vultures to detect pipeline leaks because the scent added to natural gas is similar to that of decaying flesh that they are so fond of.  Next time you see a Turkey Vulture soaring over our favorite monument, on the way to our favorite monument or even on your way home from school or work, give them a nod for doing a bang-up job of waste management!
By Kristie Ehrhardt May 11, 2026
Did you know that Tuleyome has a hiking equipment lending program for kids?! The idea began as a Boy Scout project back in 2016. Tuleyome collaborated with Davis Boy Scout Troop 111 and so began our Boot Bank! The Tuleyome Boot Bank is a lending program that provides gently used, high quality hiking boots and other camping equipment to kids up until age 18 for FREE! The program was founded as a way to help solve the financial burden on families who wanted to enjoy the outdoors but simply couldn’t afford to continuously buy good quality, well-fitting hiking boots for little feet that just keep growing. As a mom I understand how hard it is to part with a pair of perfectly functional little hiking boots that never fit long enough to get worn-out, just out-grown. One of Tuleyome’s aspirations is to help families enjoy and explore nature and sharing well-fitting footwear is just one of the ways that we can help do that. Initially the boots were loaned out of the back of an antique milk truck that was generously donated to Tuleyome, along with operating costs to help encourage the young program, by one of our favorite supporters. The yellow truck, affectionately nick-named “The Wonder Truck” would visit community events filled with boots of various sizes for various sized little feet. But, unlike the boots, the truck began to wear out and unfortunately parts for a 1967 Divco step van were getting harder and harder to come by. We’ve since retired our “Senior” Boot Bank truck and it can be viewed free roaming at the Woodland Regional Park Preserve where it will live in peace surrounded by nature. Our “Junior” Boot Bank is a little more spry and able to travel at freeway speeds so you might see it around town or even visiting the Berryessa Snow Mountain National Monument. The program began as a kid’s hiking boot “lending library” but has since grown into a program that now includes gear that is available for short-term loan to participants in Tuleyome events such as backpacking or camping trips or to youth-serving organizations that are hosting outings. The gear library includes tents, sleeping bags, hiking poles, backpacks, headlamps and other camping and hiking equipment. The goal of the program is to encourage youth and their families to experience the pleasure of exploring the outdoors and the recreational opportunities offered by Mother Nature. We are hopeful that by participating in positive hiking and camping experiences that kids and their families will develop a love and respect for nature, conservation and public land enjoyment. Thanks to our remarkable volunteers, the boots that Tuleyome loans out are always inspected and freshened up before they are made available for borrowing. Over the years, the collection of boots have been provided through donations by individuals and corporations such as Big 5, REI and Target. If the young members of your family are in need of boots or if your organization needs gear for a trip, visit the Tuleyome office at 622 Main Street, Suite 200 in Woodland during our monthly Youth Boot Bank Office Hours, 2:30pm-5:30pm on the first Wednesday of each month. During office hours you will be able to try on several pairs of boots and check them out, return a pair you’ve already borrowed or donate boots to the program. The Boot Bank is open to anyone 18 and under and provides free loans for up to six months; at which point borrowers can renew or trade up for a larger size if necessary. *User Agreement: Boot users sign an agreement in which they agree to use the boots only for hiking, outdoor adventures and inclement weather; not for everyday use (the purpose of the Boot Bank is to encourage participation in outdoor recreational opportunities, not to meet basic shoe needs). Users agree to return the boots in good condition except for reasonable wear and tear.