A Little More About The Research

As promised, a more in-depth look at my research.

The Players: The Northern Pacific Rattlesnake and California Ground Squirrel

An adult California Ground Squirrel watches me closely while I snap its photo.

The pups of the California Ground Squirrel (Otospermophilus beecheyi) have to be some of the most carefree animals I’ve had the pleasure to observe. They peek out of their home burrows briefly, and then begin bounding playfully around once no hawks or coyotes are spotted.  The best word to describe their movements is “frolicking”. They really are quite cute. Even the adults have endearing qualities. They sunbathe with their bellies flat on the ground, chirp at passers-by, peek out of burrows at me, and are incredible acrobats. At the same time, they are the scourge of farmers and landscapers due to crop destruction and the giant burrow systems they dig, earning them frequent targeting by 22 mm rifle fire and poisons. I don’t mind those issues personally, but will say that the squirrels often growl quite ferociously at me when they are in the traps we set to capture them. That’s understandable.

While 22 cartridges and poisons likely claim many squirrels in California, they are chiefly the prey of a diverse group of predators, including bobcats, coyotes, all manner of predatory birds, and snakes. One study, produced by a herpetologist named Henry Fitch in the 1940’s, estimated that 60% of ground squirrel mortality results from predation by the Northern Pacific Rattlesnake (Crotalus oreganus oreganus), the sole venomous predator of these squirrels in the northern two-thirds of California. That’s a huge amount of squirrel meat converted to rattlesnake meat.  Also, this predator-prey relationship is an old one, reaching over 20 million years into the fossil record.  In short, those frolicking squirrel pups often frolic a bit too closely to their unseen and infinitely patient rattlesnake predators.

A large male Northern Pacific Rattlesnake hunts in the dappled sunlight under an oak tree on the Carrizo Plain Ecological Reserve, San Luis Obispo County, California. This guy never moved or rattled at me, even though I was right in his face shooting photos.

 Same photo but wider angle.  These sorts of photos are fun for playing “find the snake”. What awesome camo, even when sitting right out in the open! I found this guy  again two days later with an adult ground squirrel in his belly.

 This long evolutionary history has resulted in some incredible anti-predator adaptations in the squirrels. They confront snakes aggressively, throwing rocks and dirt at the snakes.  This behavior is always accompanied by stereotyped tail-flagging, as shown in the following video.

Squirrel Tail-Flagging Video by colleagues Rulon Clark & Bree Putman 

The tail-flagging is thought to signal the snake, telling it that it has been spotted and should go hunt elsewhere. To add a fascinating twist to this signal, Aaron Rundus and colleagues discovered that the ground squirrels shunt blood to their tails to heat them up when flagging down a rattlesnake, but do not do this when flagging at a gopher snake. Although many people have trouble telling the two snakes apart, it seems that the squirrels know, and send an infrared heat signal to the rattlesnake, which can “see” heat using their facial pits. 

This is a Black-tailed Rattlesnake from southeastern Arizona.  This is the best photo I had for showing the difference between the facial pits that give the pit vipers, such as rattlesnakes, copperheads, and cottonmouths, their group’s name. The ground squirrels have evolved to signal rattlesnakes through this channel with their heated tails.

So what if a squirrel gets bitten? Is it all over?

Yes, if you’re a young squirrel.  But, as they grow the ground squirrels begin to develop a remarkable level of resistance to the venom. It takes 40 times more venom to kill a ground squirrel than a lab rat. This isn’t immunity, because a snake can still kill a squirrel if it gets enough venom in, but small snakes don’t produce enough and large ones may have to land a full bite on the agile squirrels. Further complicating things is that venom, and thus killing capacity, can vary greatly from one snake to the next, and from one population to another. The resistance comes from proteins that specifically exist in the squirrel’s blood to bind and deactivate snake venom compounds.   Previous research headed by my close collaborator and secondary mentor Jim Biardi of Fairfield University showed that the resistance of the strength of selection for more resistant squirrels in areas with lots of snakes may jump to mind, which I believe to be the case and think it’s SO cool!

What more is there to know? And why?

There are many things we don’t understand about this predatory-prey system, and evolutionary processes in general, so the first question is easy to answer. Because snakes rely heavily on squirrels for food, and because resistance is important for squirrels defending their burrows from snakes, it seems plausible that this could be an example of a coevolutionary “arms-race” between the snake predator and mammal prey. Although we hear a lot about such arms-races, the only well substantiated instance involving two vertebrate species is the incredibly fascinating example of toxic newts and the garter snakes that eat them.

The newts produce tetrodotoxin in their skin, the same compound that makes eating poorly prepared puffer fish potentially deadly to humans. The toxin block ion channels that are key to muscle activity, paralyzing the victim.  The ion channels of some western garter snakes show adaptations that prevent ion channel blockage and allow the snakes to eat the newts.

It seems to me that something so intellectually exciting and intuitive to the mind as a coevolutionary arms-race should be well-studied for its own sake, just because it is awe inspiring, and because we are naturally curious about such things as beings who can critically think. Therefore, I see my study as a way to substantiate knowledge gained during the amazing newt-garter snake studies, while adding new knowledge about how coevolution operates.  For instance, venom toxicity is much more complex than tetrodotoxin action, as a cocktail of up to 50 different proteins can be found in the venom of a single Northern Pacific Rattlesnake.

Is ground squirrel resistance equally complex?  We have no idea. I plan to find out.

Is there matching in the amount of paired venom and resistance proteins among isolated populations of squirrels and snakes? Who knows? We don’t even know much about the variability of venom among different geographic regions in these snakes.  I plan to find outDoes high squirrel resistance to a specific subset of venom proteins favor a heightened role for other venom proteins? In other words, do the important “arms” in the arms-race vary from place to place?  Let’s find out!

I will be flat out honest with you, it’s my basic curiosity about nature and its inner working, and my love of all things scaly (and especially rattlesnakes) that drive me to do this work.  For those that value ways to apply knowledge gained, here are a few possibilities:

In the short-term, this work employs and trains me, a future college professor, and will train many undergraduates that I will mentor.  They will learn research skills they can apply to diverse scientific fields, while I get better and better at being a mentor.  Currently, I am mentoring 2 Ohio State  undergraduates, and more will be needed for future laboratory analyses on venom, resistance, and genetics.

Complex, multi-chemical arms-races between venomous snakes and resistant prey may provide an example of how natural enemies stay one step ahead of one another.  Bacteria and antibiotics represent a complex evolutionary interaction where resistant bacteria are fought with a battery of drugs.  Have you ever been sick? I’d love to provide knowledge of how these complex problems sort themselves out in nature for use in medical treatment options.

Speaking of medicine, a whole bunch of medicines have been developed from compounds first found in snake venom.  Diabetes, heart, and headache medications are included in the list.  The next useful venom protein is out there awaiting collection and characterization.

Oh yeah, unfortunately people do get bitten by snakes. Many have been drinking, and most are harassing the snakes or trying to kill them, while some bites are “legitimate”, meaning the snake was unseen by the person, stepped on, or prodded by a hand reaching into a dark space. Research has led to powerful antivenins that alleviate rattlesnake bite symptoms and save many lives and limbs.  Anti-venin research is just beginning to take notice of geographic variability in venoms, and thus bite symptoms. Knowledge of venom variations can lead increased anti-venin potency. Venom variability leading to ineffective anti-venins is a big problem in the tropics.  Still, knowledge gained here in the U.S., where I can catch 20 rattlesnakes in a day to make precise and accurate assessments of venom, can be applied to snake research elsewhere. Venom variation can impact bite symptoms of rattlesnake bites as well, as is evidenced by wide variation in the presence or absence of potent neurotoxin, Mojave Toxin, in Mojave Rattlesnakes and the Southern Pacific Rattlesnake (a close relative of my species).

How to Find the Answers

In short, we have to get venom samples from lots and lots of snakes, and blood samples from lots and lots of squirrels.  Then, we can do lab-based analyses that allow us to figure out venom composition differences among rattlesnake populations, blood composition differences among squirrel populations, and specific venom toxicity and squirrel resistance for a number of venom activities (e.g. neurotoxic, hemotoxic, tissue degrading). Over the next three months I will drive my team all over California, where we hope to capture and sample 20 snakes and 20 squirrels from each of 20 field sites. Doing the math, you’ll find that’s a heck of a lot of snakes and squirrels.  They are both really common, so we’ll get them!
I hope that I’ve peaked your interest and that you will continue to follow my adventures studying these animals.  Brief autobiographies on the three members of my research teams for the current field work (March-June, 2013) in California are coming soon, and I promise a post detailing the highlights of each of our 4 day trips to nearly 20 different sites across California during the coming months.  These will be accompanied by lots of photos and videos of what we see and do, including rattlesnake capture and venom collection.
Sincerely,
Matt
P.S.  Have questions about this work or other questions relating to reptiles and amphibians?  Email me at matthewholding28@gmail.com