Introduction

         Like humans, squirrels have evolved to better fit their environment in order to survive. Because squirrels are smaller mammals, they are a common prey among predators. Squirrels have evolved to instinctively protect themselves from its predators (Bertrand, 2016). The squirrel has many predators ranging from rattlesnakes all the way to hawks. The red-tailed hawk is a large predatory bird that hunts on small animals of both land and sky, but its prey of choice are small mammals, one of the most common being the squirrel (Henderson, 1934). The squirrel’s enemies, however, are not only in the sky. The coyote hunts on squirrels too (Mccleery et al, 2009). The small forager has to keep an eye out on both the land and sky (Stabile, 2010). Based on the background of the squirrel, we chose to look further into the topic of questioning squirrel’s ability to react differently to aerial predators (red-tailed hawks), ground predators (coyotes), and humans.

        Squirrels have attempted to adapt a mechanism for survival (Mccleery et al, 2009). While some animals, like the peacock or frog, change physical features to make them-selves more intimidating, or blend in with the environment around them, the squirrel produces alarm calls to warn fellow squirrels of impending danger while they them-selves escape. In squirrels, as in humans, the peripheral nervous system (PNS) triggers a response to danger, and interacts with the central nervous system to respond extremely quickly to danger stimuli (Cavendish et al, 2010). Squirrels developed dichromatic eyesight, which allows for equal sensitivity across the entire eye (Van Arsdel et al, 2004). This allows them to be able to notice not only food and ground predators easily, but also predators coming at them from the sky. Many behaviors, including defense mechanisms, developed by animals have drawbacks to them. In frogs, mating noises can be used by predators such as bats and possum to locate the frog. In the same way, the squirrel call can be eavesdropped on by predators in order to find and hunt the squirrel. For this reason, we believe they developed predator specific alarm calls, that make localization of the squirrel more difficult for each respective predator as it warns its comrades (Greene et al, 1998).

        Research has shown that squirrels have distinct fear-based reactions to an approaching predator based on what type of animal it is that is threatening them (Green et al, 1998). Green & Meagher (1998) performed an experiment to test whether or not red squirrels in Montana have different alarm calls for different predators. Their experiment involved using a trained dog, a human, and a model bird as predators. They performed multiple tests where they observed the vocal reactions of different squirrels to the individual predators. Their results showed that the squirrels do have different calls for aerial, terrestrial, and human predators. Our experiment will expand on theirs and decipher physical reactions to certain predators, as well as testing the same experiment on humans. Humans, like squirrels are often frightened by a variety of different things and they show different responses based on what the cause is. This fear, in humans and squirrels is an instantanuous reaction and is important in reacting to danger, as well as increasing survivability (BMJ, 1955). For example, we believe humans react with fear to a spider in the same way they do to any other predator (German et al, 2015). However, humans also have predator specific mechanisms- whether or not they react with aggression or anxiety (the fight or flight response) when faced with danger or stress. (Kunimatsu et al, 2012). This implies some similarity between humans and squirrels on a biological or genetic level.

        Based on our research, we theorize that squirrels are more desensitized to human playbacks in comparison to aerial and terrestrial playbacks. In the presence of humans, squirrels produce less danger signals, like vocalizations and body language, because of the learned factor that humans are not a common predator (Mccleery et al, 2009). We also noted that squirrels have different vocalizations for different predators. For aerial predators, squirrels use an acoustic, low amplitude, high frequency call so it is not easily perceived by raptors. For terrestrial predators, squirrels make a loud bark (Greene, Meagher et al, 1998). The comparison between human and squirrel danger signals is that they are processed through the central and peripheral nervous system. Like humans, the PNS and CNS trigger the “fight or flight” response when there is a sense of danger (Cavendish et al, 2010).
        In the presence of humans, squirrels produce less danger signals, like vocalizations and body language, because of the learned factor that humans are not a common predator (Mccleery et al, 2009). We also noted that squirrels have different vocalizations for different predators. For aerial predators, squirrels use acoustic, low amplitude, high frequency call so it is not easily perceived by raptors. For terrestrial predators, squirrels make a loud bark (Greene et al, 1998). The comparison between human and squirrel danger signals is that they are processed through the central and peripheral nervous system. Like humans, the PNS and CNS trigger the “fight or flight” response when there is a sense of danger (Cavendish et al, 2010). There is a gene in both human and squirrels that causes aggression/ anxiety, it is called monoamine oxidase A and B. Monoamine oxidase A and B is a gene in mammals located on the X-chromosome (Shih et al, 1995). A study performed on mice, showed that when the gene for MAO B was removed, they presented aggression similar to human males. When MAO A was removed, the mice did not exhibit aggression, only fear when threatened (Shih et al, 1995). The MAO A and B gene triggers the response that we name “fight or flight.” Overall, we hypothesized that if we presented playback calls of ariel and terrestrial predators to the squirrel, the squirrel would respond by performing a predator specific alarm call, and running away. This protects both itself and the squirrels around it. For humans, because their responses of "fight" or "flight" vary, we expect some "fight" responses as well. For the larger predator presentation to humans (the bear playback), we expect more "flight" typr responses and in some cases a vocal response. For the spider, we expect more vocal responses as well as aggressive behavior towards the threat.

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        All our data collection was conducted on campus behind Holmes Hall. First, we located a squirrel and moved to a reasonably close distance to observe it, but not so close that it ran away from us (approximately 10-25ft). We then began filming the squirrel before we started any playback to ensure we accurately captured the squirrel’s reaction. For our first control, we did not play any sound for 10 seconds. This was repeated five times, with our team documenting any observations made regarding the squirrel's behavior. Our second control was a noise that squirrels consider to be non-threatening, (birds chirping) so that the squirrel did not identify this sound as a predator. We played the sound of a common Robin chirping, through a UE Boom speaker at 100% of max volume, or 88dBA, for 10 seconds. Our non-threatening noise came from a clip off of YouTube. Members of our team observed all noticeable change and recorded it in data charts as well as filming it to ensure no change in behavior was missed. This test was conducted on five different squirrels in order to account for error.

        Next our team waited at least one day so that the squirrels behind Holmes hall were no longer influenced by the non-threating noise. Then we found an accurate Red-Tailed Hawk call on YouTube. We used this sound and played it at 100% of max volume (88dBA) on the UE Boom Speaker and repeated the same method as used with the non-threatening Robin call. This was also tested on five different squirrels and for each one we recorded all the squirrel’s behaviors in the data sheet and filmed each encounter.

        The next step of the experiment was performed at least a day after the Red-Tailed Hawk playback was used so that the squirrels would no longer be cautious or alarmed. We picked an accurate coyote howl playback noise from YouTube. We then located a squirrel behind Holmes Hall and waited at least sixty seconds to allow the squirrel to adjust to our presence so that we did not affect the animal’s behavioral response. Then we played the coyote howl on a UE Boom speaker at 100% volume (88dBA). Members of our team recorded any notable behavioral reactions and filmed the encounter. This procedure was then repeated four more times for a total of five trials for the coyote howl playback sound. .

        The next step in this experiment was to find a common human conversation on YouTube. At least one day after the previous step, involving the coyote howl, we performed the same procedure as with the bird chirping, the hawk call, and the coyote howl. The only difference is that this time we used a human conversation playback sound. This was done with five different squirrel specimens; and the data from each encounter was recorded in a data sheet as well as recorded on video.

        The second part of this experiment tested the same behavior in humans. Because humans have similar responses to danger, we are going to compare human to squirrel responses. To test the human’s reactions we purchased a fake arachnid, which is thought to normally elicit a fear response in humans. We approached a human; put our hands out with the fake spider in it and requested that they hold it for us. We then recorded their responses in a predetermined data sheet as well as videoing each encounter. We then repeated these steps four more times for a total of five trials. Then we found a Black Bear recording on YouTube and played it near a human walking alone behind Holmes Hall. We filmed the encounter and recorded all observations of a data sheet. This was then repeated four more times for a total of five trials. The human responses were compared to those of the squirrels as natural reactions to fear stimuli. After completion of all the trials we blended the consistent reactions that appear for both squirrels and humans, for example, squirrel calls (of all kinds) and human screaming. We lastly created a chart comparing the frequency of overall reactions between the two species to understand a trend of overall reactions for analysis.

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         Behind Holmes hall near the river trail and behind Wells hall are the two sites where we will be colleting data (Figure 1A). When we document the reactions of squirrels we will also document where on campus we found them. This allows the creation of a Pie Chart showing where we performed the most squirrel observations (Figure 1B). This graph indicates squirrel populations based on our ability to find and test squirrels for observation. We found the most squirrels behind Holmes Hall. Near the river trail there are not as many people around, and so we found more squirrels along the trail. Behind Wells Hall there is traffic around the middle of the day, and especially at the start and end times of classes. We found that more people came through the river trail behind Wells than Holmes because it is in central campus which connects north, south, east and west sides of campus. We had a majority of our observations taken behind Holmes Hall due to more abundance of trees and less people disturbing the squirrels (Figure 1B).

        In preliminary research we found that squirrels will produce a high “seet” noise when threatened by an aerial predator because this noise is low-amplitude and high-frequency, meaning that the aerial predator will have a more difficult time hearing it (Green et al, 1998). When we played the playback of a hawk call, a common aerial threat, the squirrels immediately ran up the nearest tree they could find (Figure 2). The squirrel made no vocal noises like we had thought it would. Prior researchers found that when threatened by a terrestrial predator squirrels would sometimes use a lower barking sound to signal others of this threat (Green et al, 1998). When we played the coyote playback the squirrel, once again, did not make any vocal noises but ran up a tree for safety. Squirrels have become so comfortable with humans on Michigan State campus. Unlike many of animals, the squirrels here will actually approach the human if it thinks you may have food to offer. When we approached the squirrels to see if the squirrel would run to safety all it would do is move away a couple feet if it felt uneasy. It never ran up a tree to seek shelter and safety. A common bird chirping playback sound is going to be used as a control because the bird sound will be non-threatening. This will allow us to test whether or not it is our human presence or the presence of the speaker potentially affecting the squirrel. When the bird call was played it had no affect on the squirrel (Figure 2). The no sound playback call is to completely ensure that the speaker does not affect the squirrel’s response to the other tests. All of the squirrels we tested had no response to the no call sound because they barely recognize our presence in the area (Figure 2).

        Since humans have limited natural predators, we thought the best way to initiate the “fight or flight” response would be through something that we are commonly afraid of, which is spiders. When we presented the spider to humans, several were unaffected by it. Only two people screamed and hit at the spider when we showed it to them and one ran from the spider (Figure 3). We thought it’d be best to have a small predator and large predator so we also used a playback of a bear. When played in front of the human we had three people run and two people who were unaffected by the noise (Figure 3).

        We found that squirrels will react more often than humans will when presented with a threat (Figure 4). Humans aren’t known to have many predators so when encountering a threat that we had presented, it didn’t elicit a response. A squirrel on the other hand, has many predators, hence a higher reaction percentage occurred for them. The gene Monoamine Oxidase A and B occurs both in humans and squirrels. Even though predators for the two are different and humans aren’t necessarily in as much danger, it still elicits a fight or flight response in both. Therefore, higher reactions in squirrels were recorded because they are wary of potential danger. While humans still have the gene that codes for the "fight or flight" response, they only need to react to danger when it arises randomly. They do not necessarily have any predators they need to be constantly aware of. This is why humans showed a decent percentage of responses, but not quite as high as squirrel reactions.

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Figure 3- Human Fight or Fight or Flight Responses to Bear Playback and Fake Spider Fear Stimuli. This data is to compare human and squirrel “fight or flight” responses. Since humans and squirrels share an aggression gene, MAO A and B, we wanted to see if squirrels and humans had similar behaviors to predator stimuli (Shih et al, 1995). These human responses can be compared to squirrel responses as natural reactions to fear stimuli. We performed five experiments involving the fake spider and five experiments involving the bear playback. More responses for “fight or flight” were shown with the fake spider. This may be because there is more of a negative stigma to spiders in comparison to bears.