Microhabitat Selection By Pacific Tree Frog, Hyla Regilla

Discuss about the Microhabitat selection by the Pacific tree frog, Hyla regilla.

Note: Note: This is a model take-home exam based on Kildaw (1995) to give students an idea of what sort of approach is expected. It may contain errors of fact or interpretation, and there is no need to imitate slavishly its style or organization.

Behavioural ecology is a field which includes the study of evolutionary basis of the behaviour of animals due to the ecological changes and pressures. The subject behavioural ecology stemmed from the ethology and it includes the study of the phylogeny of the behaviour of animals; survival value; ontogeny; proximate causes. If an organism exhibiting a certain trait is favoured by the environment then there is a big chance of an animal getting selective advantage within an environment (Davies, Krebs and West, 2012). This study is based on the analysis of an article which is based on a particular behaviour of an animal. This study also includes a critical overview along with an exclusive mention of the significance

The choice of the substrate colour provides a mean trough which predator can be avoided through a process called crypsis. The main question was what was the actual background colour of choice of the Pacific tree frog (Hyla regilla). This shows complexity in the colour polymorphism. In order to test this in a laboratory condition, the behaviour of the background choice of the of the non-changing brown and the non-changing green are considered for the test. The test was carried out to determine the whether these frogs have any kind of preference for a typical kind of background substrate. The hypothesis was based on the fact that the non-colour changing frogs (both the green and the brown nonchanging frogs) will prefer a background which was similar to their own skin colour. Another hypothesis was that the frogs will select a background which matches their own skin colour during the presence of a predator. The predictions were based on the behaviour of the non-colour changing frogs that whether there was any preference of the non-colour changing frogs with respect to their own skin colour (Wente and Phillips 2005). There were other tests like whether the color changes also exhibit the characteristics of matching the background substrates according to their color of the body. Hyla regilla species were collected from the Vanderberg Air Force Base which was located in Santa Barbara County of California. The sampling was done during the winter and the springtime. The substrates chosen are the soil and the bark substrates. The background substrate and the dorsal surface spectral reflectances were quantified with the help of a spectraradiometer. The change in colour of the frogs was measured through the colour change and it involved the alternate exposure to the light and the dark substrates for over a period of 3 to 4 weeks, at this time the dorsal skin surface reflectances was measured on a regular basis. Depending on the dorsal skin spectral reflectances, one frog was assigned to one of the three different types of morph groups (colour changer, non-changer brown, or non-changer green) (Wente and Phillips 2005).

Results

The test apparatus which was taken to be a large circular arena was divided into quadrats of equal measurements the opposite quadrates were made brown while the other two quadrates were made green. The test arena consisted of a vibration damped hood and it was light tight. In order to replicate the dusk and the dawn conditions, the frogs were tested based on the dimmed lighting conditions. The humidity and the temperature were recorded for each test and small bowls containing water were placed in each of the quadrates before the tests were carried out and the quadrates were misted before the conduct of the test. At the beginning of the test, a remotely controlled device was used to let a frog enter the arena. The frog was kept there for a night in dark and the scoring was done based on the matching of the background for every 5 minutes and this was carried out for over a period of 330 minutes. This resulted in the collection of the 66 different observations and the location of the frog was noted in each of the tests. It was found that the frogs that did not choose a background were around 44 times out of 66. It was found that the frogs were either clinched to the sides of the arena or sitting on the release device, such frogs were not taken into account for the analysis. If s frog found to be staying in a background color which was similar to its own color for more than 50 percent of the time, then such a behavior was taken into consideration for the data collection. Although only a single test was carried out with an individual, thus the test on the other color backgrounds was not performed. Therefore, frogs exhibiting the same morph was studied and tested whether their behavior was aligned to the random expectation (fifty percent on every background) or deviated from it (Wente and Phillips 2005).

Again, a small sample of the nonchanging frogs that included 10 nonchanging green frogs and 10 nonchanging brown frogs were tested for the background along with the presence of a predator. A common garter snake, called Thamnophis sitalis was selected and is known to be a predator of H. regilla (Schaub and Larsen Jr 1978). The procedure of this test was similar to the previous test procedure, however this time a solution of the fecal matter of T.sitalis was sprayed in the test area before the conduct of each test. The choice of the background was again analyzed via the binomial test. 

Critique

The principal findings of the tests reveal that the frogs had spent around 2/3^rd of the time on one of the 2 different colored substrates. The nonchanging green frogs showed a significant inclination towards the background having the green color and 78 percent of the frogs chose this color and they spent 50- percent of their time more than any other colored substrates. The mean of the observation where the nonchanging green frogs are found to be staying on the matching substrates was 67.7 percent. Whereas, the nonchanging brown frogs did not show any preference for the background colored brown. The mean of the observation where the brown frogs spent about 50 percent of the time on the brown background was found to be 51.2 percent. The rate of the background switching by the green, brown and the other types was analyzed and it was found that the nonchanging green switched around 3.64 times during a test on an average, and the nonchanging brown frogs switched around 3.45 times during a test on an average. The nonchanging frogs showed a preference for a background which matches their skin color during the presence of a predator. In the test, it was found that 7 frogs out of the 10 nonchanging brown frogs 5 out of the 6 nonchanging green frogs chose the background which matches their skin colour (figure 1).

The conclusion of the article reveals that there are three different color morphs of Hyla regilla that has the ability to change color in accordance with the background color, capable to change body color and at the same time differ in body color (Wente and Phillips 2003).

The strengths of the study can be attributed to the two different sets of the hypothesis and the study results exactly reveal data which are in accordance with the hypothesis. The three different types of the frogs are found to behave properly as expected them to be. Another major strength is the presence of the predator element. Instead of harming the frogs, the usage of the fecal matter of the snake Thamnophis sitalis as a sign of the presence of the predator eliminate the chances of human harm and error in the conduct of the study. Another strength of this study is that the microhabitat selection in the laboratory setting was well executed and the test is replicable. The study also included the measurement of the visual mechanism that plays a major role in the detection of the matching of the background color by the frogs. The major weakness of the study is contributed by the small sample size considered for the study of the effect of a predator on the behavior of the frogs. The data collected from the predator is very small and was unable to detect any major difference. The study failed to measure any significant differences in the movement of the nonchanging green and brown frogs in the presence of a predator odor. The methodological approach considering the study is fine, however, a greater sample for measuring the presence of the predator would have been beneficial for overall applicability of the study. The conclusions although very short but has a logical flow with respect to the findings of the study and also suggests the scopes of the further study for better understanding. The writing is lucid, however highlighting the prime findings that were in accordance with the hypothesis is expected. The article also lacks the mentions of the two hypothesis upon which the whole experiment is based. The one area which needs to be improved is the sample size is very small with respect to the predator experiment and thus the article loses its applicability to a great extent due to this.

The topic of the paper is based on the cryptic behavior of the frogs and this topic can be found in the cost-benefit approach of the cryptic behavior (chapter 5). This chapter deals with the Darwinian puzzle of conspicuous behavior. According to Alcock (2005), the mimicry and the cryptic behavior are the antipredator strategies to stay away from the predator. These strategies involve the usage of the biological deceitful methods and this helps the prey to portray itself as different from the reality. The several strategies of cryptosis involve camouflage, mimicry, and hiding. Background matching is one of the strategies employed by the frogs. The paper considered here does not need any major change in the perspective. The data collected via the predator experiment might give a feel that the article selected is a poor reflection of a previous work. The major area of the study is the prey behavior with respect to the predator and this very area is not experimented by the authors. The sample size selected is so small that it lacks a wider representation. The article can be cited with respect to the background study and the spectral measurements, however, the article does not require any change in the conclusion as stated by Alcock.

Further studies can be conducted in the fitness consequences with respect to the variation in color and the other related behavior among the Hyla regilla species; the ecological correlation, a sensory basis of the variation of color requires further study. This will help in increasing the understanding of the different interactions, several processes that help to maintain the variations phenotypically within the natural population. The question will be based upon how the frogs sense the presence of the predator and how long a frog will try to mimic the background or change its body color. The presence of the predator will force the prey to stay in the mimicked position without making any other physical movement can be stated as a hypothesis. The major assumption here can be considered as the total lack of any other external influences. The possible prediction is that frogs will try to mimic the background as long as the presence of the predator sends a signal of danger to a frog.

Thus, from the study, it can be concluded that behavioral ecology is a field of study which encompasses the behavior of the animals, insects, and birds. The behaviors generally emphasized here is the presence of the predator and its impact on the prey. The prey-predator relationship is the main goal of the study and for that purpose, Hyla regilla is taken as a prey and its behavior is measured is with respect to the presence of the Thamnophis sitalis as a predator.

References

Alcock, J. 2005. Animal Behavior: An Evolutionary Approach. 8th edition. Sinner Associates, Inc., Suckerland, Massachusettes. 115 pp

Davies, N.B., Krebs, J.R. and West, S.A., 2012. An introduction to behavioural ecology. John Wiley & Sons. 1 pp.

Schaub, D.L. and Larsen Jr, J.H., 1978. The reproductive ecology of the Pacific treefrog (Hyla regilla). Herpetologica, pp.409-416.

Wente, W.H. and Phillips, J.B., 2003. Fixed green and brown color morphs and a novel color-changing morph of the Pacific tree frog Hyla regilla. The american naturalist, 162(4), pp.461-473.

Wente, W.H. and Phillips, J.B., 2005. Microhabitat selection by the Pacific treefrog, Hyla regilla. Animal Behaviour, 70(2), pp.279-287.