Friday, May 27, 2016

Are Rattlesnakes Mindless Killing Machines? Dr. Bree Putman Says No Way.

   We all know that snakes have a bad reputation, especially rattlesnakes. Although many people think of rattlesnakes as mindless killing machines, I think they are one of the most fascinating group of animals in North America. Because rattlesnakes are so difficult to study, they likely have lots of secrets to reveal about how they find food, interact with each other, and thrive in many different kinds of habitats. All this was in the back of my mind when I read a recent scientific paper about how rattlesnakes may actually move vegetation around to create better spots to ambush prey. I recently spoke with the author of this research, UCLA Postdoctoral Fellow Dr. Bree Putman, about this work and other developments in the quest to better understand rattlesnakes.

When did you decide you wanted to study rattlesnakes? What makes them interesting and/or worthwhile study subjects?

    Ever since I was a little girl, I have loved reptiles. I think it was Jurassic Park that started my fascination because I was first obsessed with dinosaurs. This prompted my parents to buy me pet reptiles because, unfortunately, dinosaurs were not available for purchase. As an undergrad in college, I knew I wanted to become a wildlife biologist and so I searched for research credit my junior year. Most professors’ work seemed so boring to me at the time (who wants to work with fruit flies?!), but then I found Emily Taylor who studied rattlesnakes and I applied to work in her lab. She accepted me and I worked on a project radiotracking a population of rattlesnake for two years. This resulted in my first publication and I fell in love with the creatures. I found that they were much different than I had perceived. My first encounter with a wild rattlesnake was so uneventful; it just sat there staring at me from a burrow. This is what makes them my favorite study animal. Many people have these preconceived notions of how rattlesnakes behave and then when they actually go into the field and observe them in their natural habitat, they find that most of the myths are untrue. After tracking the same individual snakes for multiple years, I get to learn their personalities and quirks and it makes the snakes endearing (although they probably don’t find me endearing).

You’ve done a lot of work studying the foraging behavior of rattlesnakes. When I hear the word “foraging” I think of an animal moving around looking for prey. I thought rattlesnakes were sit-and-wait predators in that they typically stayed in one place and waited for prey to walk by; is this considered foraging?

    I would define “foraging” as the act of trying to acquire food, and animals can accomplish this task using several strategies. Many predators roam around looking for prey, but other predators are ambush hunters that use surprise attacks to capture prey. Rattlesnake use an ambush strategy where they select an ambush spot, wait there for several hours to days, and then launch a surprise attack on unsuspecting prey that wanders by.

What are the attributes of a good ambush spot for a hunting rattlesnake? How long will a rattlesnake stay in one place waiting for prey?

We are still trying to figure this out, but so far we know that rattlesnakes 
rely heavily on chemical cues (similar to scent) to select an ambush spot. They prefer spots that indicate prey presence, and also the presence of multiple types of prey (but this probably depends on the rattlesnake population as some are more specialized than others). Interestingly, they also prefer to set up ambush in spots where other rattlesnakes have been. Perhaps if it was a good spot for the previous snake, it will be a good spot for them too. Rattlesnakes also prefer to hide in some sort of cover: thick vegetation, under logs or rocks, or inside small mammal burrows. Rarely do we find snakes hunting out in the open. Rattlesnakes could wait for several hours to days for prey to wander by.

You observed that some snakes don’t just look for good ambush spots, they help create them! Could you elaborate on this and explain why it is surprising?

    For my doctoral research, I used security cameras to record the foraging behaviors of wild rattlesnakes. While reviewing hours of footage, I noticed two different snakes behaving strangely. These snakes suddenly jerked their heads upward while in ambush. They were hiding in thick grass and it appeared as if they were using their heads and necks to push vegetation away from the intended strike path (see video below). This was very exciting because first, rattlesnakes rarely do anything while in ambush so any movement excites me (even tongue-flicking), and second, this could suggest that rattlesnakes can learn from past mistakes or plan for future events. We’ve only recorded this behavior from two individuals, but I’ve spoken with other researchers who have observed the same behavior and so this may be more common than we think. However, more studies are needed to determine whether the snakes were actually manipulating their ambush site to increase their strike success. 

What happens after a rattlesnake bites a prey animal? How far away does the prey get before it dies and how does the rattlesnake find it? How long does it take to find it?

    This depends on the prey and probably on the population of rattlesnake as each population seems to have a unique relationship with the local prey species. In the foothills of San Jose, California (where I work) rattlesnakes specialize on California ground squirrels, although they are opportunistic and will capture other prey such as mice, lizards, and birds. If not targeting a squirrel, snakes are more likely to hold onto the prey item during a strike. Then, it just waits for the venom that it injected to kick in before consuming the animal (rattlesnakes will not eat something unless it is completely dead). However, squirrels are bigger beasts to battle. They are resistant to rattlesnake venom (but NOT IMMUNE!) and so it can take a very long time for them to succumb to the venom (sometimes a couple of hours!). Rattlesnakes tend to release squirrels after a bite because the likelihood of a squirrel retaliating and hurting the snake, if held in the snake’s mouth, is extremely high. Snakes imprint on the squirrel’s scent during the strike and then they try to follow the scent trail to the presumably dead prey animal that they released. However, because of their venom resistance, squirrels often run very far away from where they were attacked and the snake has to spend considerable time searching. We’ve recorded several instances when snakes were unable to locate their released squirrel. These snakes spent more than 5 hours searching for their prey and never found it. This suggests that the benefit of eating a large juicy squirrel outweighs the costs of prolonged searching. These missing squirrels may have had enough resistance to fight off the venom, thus leaving behind very confused rattlesnakes.

I think some people perceive snakes as mindless killing machines, does your research suggest we should be thinking about them differently?

    Like any predator, rattlesnakes must overcome prey defenses across a series of stages that occur during the predator-prey encounter. First, they must not be discovered by the prey. Then, they must launch a successful attack on unsuspecting prey. Then, they must successful kill the prey. And finally, they must be able to find and consume the killed prey. Because prey have evolved defenses at each of these stages, predators (including snakes) are unsuccessful most of the time. This puts tremendous pressure on them to evolve counter-offenses (aka better hunting tactics) because if a rattlesnake is unable to consume enough energy, it will not be able to mate and pass on its genes (because reproduction is dependent on body condition), or in the worst case scenario, it could starve to death. Thus, rattlesnakes need to process multiple sets of information to make the “right” decision at every stage of the prey encounter or they won’t be successful. As mentioned above, rattlesnakes integrate several types of information when selecting their ambush site. We also found that they only strike prey during approximately 20% of all encounters, suggesting that are waiting patiently for the “perfect” moment to strike. However, we are still trying to figure out what factors influence the strike. Even in the rare instance when a rattlesnake decides to strike, it is only successful about 50% of the time. It’s a tough life for a hungry rattlesnake!

Will snakes behave differently depending on how easy it is to find food? Are there any other features of the environment that might affect a snake’s foraging behavior?

    First, I don’t think it is ever easy for a rattlesnake to find food (see above). Second, I believe that a rattlesnake’s hunting success depends less on environmental factors and more on the local prey populations’ anti-snake defenses. Many people think that temperature could have a huge influence on hunting success because as ectotherms, rattlesnakes’ activity is dependent on ambient temperature. Temperature could influence how long a snake is able to stay at an ambush spot; it will need to seek shelter if it gets too hot. Many laboratory studies have found that strike speed and presumably success increases with temperature. However, we examined this on our wild rattlesnakes and found little support. Snakes were able to launch successful strikes (where they successfully killed and consumed the prey item) across a wide range of ambient temperatures, even below 14°C! This would never be considered an optimal temperature for rattlesnakes. Snakes are basically successful when prey are completely unaware of the presence of the snake and do not perform an evasive maneuver and snakes are unsuccessful when the opposite is true. However, I am interested in understanding if the environment affects the anti-snake defenses of prey, thereby influencing rattlesnake strike success.

You have helped pioneer the use of video cameras to monitor rattlesnakes while they are hunting. Have you recorded anything surprising not directly related to your research questions?

    OMG, yes! I have seen so many behaviors considered unusual for rattlesnakes. We published a note on a non-rattling tail display that we recorded three individual snakes performing. The snakes slowly raise their tails and wave it from side to side. We have no idea what the function of this behavior is, but it is so distinct and interesting! 

    We also find that rattlesnakes often use the same refuges and interact with each other. They will follow each other into squirrel burrows or hang out together under a log. It’s interesting because we only observe them during the summer foraging season (no mating or overwintering in hibernacula) so why are they hanging out? You would think that this would lower their hunting success if they are competing for the same resource. I’m very interested in understanding the social lives of rattlesnakes.

What are some interesting questions that emerged while you were conducting your research that you hope to answer in the future?

    I would love to further understand the function of rattlesnake-rattlesnake interactions. Some research has been done using social network analysis to investigate associations between snakes, but this work is really in its infancy. It’s just a tough topic to study because rattlesnakes are so secretive and it takes a lot of time, money, and effort to follow them around and observe their behavior. It’s also hard to get funding for something like this. I’m also interested in understanding what limits the venom resistance of California ground squirrels. In other words, what prevents squirrels from evolving full immunity? There are many other questions that intrigue me and I hope to continue to work on this system in the future, but now I’m starting a postdoctoral research position working with urban lizards in Southern California (in collaboration with the Los Angeles Natural History Museum). Eventually, I hope to become a professor, and when that time comes, I will continue to understand the secret lives of rattlesnakes.

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