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.
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 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:
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). |
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