Snake Scent Application LaboratoryOmar Moreno

Introduction

Snakes are squirrels' primary predators, and evading them is key for the squirrels' survival. It has been noted that squirrels chew on shedded snake skin and proceed to lick certain parts of their body, thus spreading the scent of the snake. This skill may have stemmed from an evolutionary advantage by those who possess it; squirrels that know how to protect themselves live longer and reproduce, teaching their offspring. The squirrels' use of the snake skin can serve as a defense mechanism. The purpose of this lab was to see why squirrels apply snake scent, from the skin of the snake to their bodies, by analyzing where on their bodies do squirrels apply the snake scent. Through statistical analysis, the places where the snake scent is applied can be determined to be random or not. According to a 2006 study, squirrels apply snake scent to their bodies to deter other snakes from hunting them (Clucas). The snake scent overrides that of the squirrels, thus hoodwinking the snakes into believing that there are other snakes around. Our hypothesis is that the squirrels apply the snake scent in a deliberate, systematic, and non-random manner. Additionally, the part of the squirrels' bodies to which the most amount of snake scent will be applied will probably be the tail, as it makes up a substantial amount of the squirrels' surface area, and it waves in the air, wafting the scent to potential predators. In order for this hypothesis to be supported, the null hypothesis, which states that the application of the snake scent is completely random, must be rejected unconditionally. (Demonstration Manual)

Methods

In order to determine whether the application of the snake scent was random or not, we watched four videos, each with a different individual, of two distinct species of squirrels, S. variegatus (rock squirrel) and S. beecheyi (ground squirrel), applying the snake scent to their bodies. S. variegatus are primarily found in California, specifically a city called Winters, which is where they were recorded, while S. beecheyi is found in New Mexico, and were filmed in Caballo. (Clucas) Snake skin was placed on the ground and left alone for the squirrels to find after following a trail of sunflower seeds. The field researcher then remained about 30m away and video taped the squirrels behavior when they reached the snake skin. The squirrels' behaviors in the videos were analyzed by categorizing the areas where the scent was applied; Tail, Head, Flank, Hind Limbs, and Front Limbs. (Demonstration Manual)

Results

Using statistical analysis, we can see that the snake scent application is not random. If it were random then each category would have about an equal chance of having the snake scent applied upon. These are the expected values (e), found by dividing the number of instances by the number of categories. The actual observations (o) were gathered using the previously-mentioned methods. Looking at Tables 1 and 2, it can be seen that each squirrel prefers to apply the snake scent to the tail, followed by the flank. The head and the front limbs received very little to no application of the snake scent.

TailHeadFlankHind LimbFront LimbTotals

Expected (e)23.823.823.823.823.8119

Observed (o)67028231119

43.2-23.84.2-0.8-22.8

1866.24566.4417.64.64519.84

78.41323.80.7410.02721.842124.823

Table 1 S. variegatus chi-square test calculations

TailHeadFlankHind LimbFront LimbTotals

Expected (e)11.211.211.211.211.256

Observed (o)340175056

formula22.8-11.25.8-6.2-11.2

formula519.84125.4433.6438.44125.44

formula46.41411.23.0033.43211.275.249

Table 2 S. beecheyi chi-square test calculations

The values highlighted in red are the results of the chi-square test. These are compared to the given values which depend on the degrees of freedom and the levels of confidence; if the chi-square test is less than these values, then the application of snake scent is random, and vice versa. In this case, there are five variables, therefore, the degrees of freedom are four, seeing as there are four independent variables and one variable which depends on these. Additionally, we are going to look at two levels of confidence, 0.05 and 0.01. At four degrees of freedom and 0.05 level of confidence, the given value is 9.488, while the chi-square tests for S. variegatus and S. beecheyi are greater than 9.488 at 124.823 and 75.249, respectively (Demonstration Manual). At four degrees of freedom and confidence levels of 0.01, the given value is 13.277, while the chi-square tests for S. variegatus and S. beecheyi are greater than 13.277 at 124.823 and 75.249, respectively. This means that we can be 99% certain that the snake scent application is not random. These statements reject the null hypothesis, which stipulated the application of snake scent as random, while at the same time supporting the hypothesis that the snake scent application is not random.

Discussion

The results of the statistical analysis support our hypothesis that the snake scent application is not random. Additionally, squirrels applied most of the snake scent to their tails. The reason for this merits further research, though the movement and behavior of the tail are possible reasons, as they allow for the scent to spread throughout the environment. It should be researched whether there are other reasons for the snake scent application, apart from the defense mechanism.References

Clucas, B., Rowe, M. P., Owings, D. H., Arrowood, P. C. (2006). Snake scent application in ground squirrels, Spermophilus spp.: a novel form of antipredator behaviour?. Animal Behaviour, 75, 299-307.

Life Science Core, UCLA. 2008-14. Life Science 1 Evolution Ecology and BiodiversityDemonstration Manual. Edition 10.1