Sunday, September 6, 2015

New Publication! How Do Young Squirrels Deal With Rattlesnakes?

The second chapter of my Ph.D. dissertation was recently published in the September issue of Behavioral Ecology and Sociobiology. Here's a little post summarizing my study in case you are not a journal subscriber ;-)

Only one more chapter to publish for me to graduate!!!

Besides being ridiculously cute, baby animals are prime 
targets for hungry predators

Life’s tough for a newborn animal.

Imagine you are a young ground squirrel pup. You are small, weak, inexperienced, and not fully developed, and you have just encountered a hungry rattlesnake.

This sounds frightening, but you, as a prey animal, have multiple opportunities to thwart predators during an encounter. For example, prey can detect predators quickly (before the predator detects them), and use behaviors to deter an attack or to escape an attack. Additionally, if prey are vulnerable at one of these stages, they might make up for it by having super-awesome defenses at another stage. For example, an animal might not be very good at detecting predators, but superb at escaping attacks. Thus, we need to examine how prey respond to predators across all stages of a predator encounter to understand where their vulnerabilities lie. 

The five main stages of a predatory encounter during which prey can use different defenses to avoid death. An encounter occurs 
when both parties are at a distance where they are both able to detect each other. An interaction occurs when the prey positively 
detects the predator and exhibits a behavioral response. Prey may try to deter an attack via signaling or active predator 
harassment. If a predator attacks and captures its prey, the prey can still escape after capture (not shown on this diagram). 
Flow chart adapted from Lima and Dill (1990).

Predators are a big deal for young animals because predators typically prefer to attack them over adults. Young can be born with almost completely functional antipredator defenses (especially those without parental care). However, more often than not, newborns learn from their mothers and neighbors which predator species are dangerous and how to handle them. This is especially true for social animals like squirrels.

The big question is: why don’t animals evolve to be born with functional defenses?? Needing experience is theoretically good for an animal because different predators might be dangerous at different times or in different areas. Thus, experience allows animals to modify their responses to the specific threats in their local area, but young are exceptionally vulnerable during the learning period. However, as I mentioned above, defenses used at one stage of a predator encounter can compensate for deficiencies at another stage, which can reduce the overall risk of predation on young.

The fees are definitely high for animals trying to learn how to
outwit predators. One mistake could mean death!
Few studies had examined age differences in antipredator defenses across multiple stages of a predator encounter, and so I set out to test this in the California ground squirrel-rattlesnake system. I examined whether squirrel pups differed from adults in antisnake behaviors during the detection, interaction, and attack stages of rattlesnake encounters. I specifically looked at squirrels’ ability to detect wild rattlesnakes, snake-directed behaviors after discovery of a snake, and responses to simulated rattlesnake strikes. I predicted that if I did not find age differences in a behavior, then the behavior is ‘un-learned’ and squirrels are born with a functional defense. If I did find age differences in a behavior, squirrel pups could have an inappropriate defense that requires experience to become fully functional, or squirrel pups could have an appropriate defense that is different from the adult form because it protects pups against specific risks only they experience.

I recorded wild squirrel-snake encounters in the natural habitat during two summer field seasons. I found that squirrel pups were not very good at finding rattlesnakes in their habitat. Adult squirrels detected snakes during 52% of all encounters, while pups only detected snakes in 21% of encounters. I found that during an interaction with a discovered snake, adult squirrels spent more time harassing the snake (by tail flagging, a behavior that deters the snake from striking and forces it to leave the area). In addition, if squirrels discovered a rattlesnake that was hidden in a refuge, adults were more likely than pups to investigate the snake’s refuge.

Example video of squirrel pups that are completely oblivious to a rattlesnake that's in 
ambush right next to their burrow. One of them is attacked by the snake (about 1 min in), 
but it uses its ninja skills to avoid envenomation:

I simulated rattlesnake strikes on squirrels and used high-speed video cameras to record their responses to these attacks. Squirrels, like many small mammals that are preyed upon by snakes, are secret ninjas that use aerial leaps to propel themselves outside the strike trajectory. We call this response an evasive leap. When attacked, squirrels can choose to either run away (called a scramble) or use an evasive leap. I found that pups were equally as likely as adults to use an evasive leap to escape a rattlesnake strike. However, pups had much slower reaction times to strikes than adults.  

Example video of the strike-simulating device and the two distinct flee modalities squirrels 
use to escape attacks (scramble vs. evasive leap): 

This study showed that squirrels’ ability to detect snakes improves with age. Squirrel pups should not wander around much and avoid areas with dense vegetation where rattlesnake set up ambush. Although pups are slow to react to rattlesnake strikes, they seem to use appropriate behaviors when dealing with a discovered snake. Squirrel pups do not approach snakes as closely as adults, and minimize time spent in close proximity to them. Thus, the behaviors young squirrels use at the interaction stage of a rattlesnake encounter appear to compensate for their deficiencies at the attack stage. On the other hand, pups are just plain bad at detecting snakes, an inappropriate defense that leaves them exceptionally vulnerable. Snake detection must be refined through learning over time.  

In my study, squirrel pups spent less time harassing rattlesnakes (fewer tail-flagging bouts) (panel a), were less likely to investigate
a snake's refuge (panel b), and had slower reaction times to surprise attacks (panel c). Taken from Putman et al. (2015).

Check out my recently published article here! 

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