Alverno College, 2009
Studies have shown that the frequency of the chirping sound made when a snowy tree cricket (Oecanthus fultoni) rubs its wings together is directly related to the rise in temperature (Connally, et al., 2007). This led us to question whether the acoustic intensity of a cricket’s chirp also rises in warmer temperatures and, in turn, design an experiment to measure that. Two groups of twelve crickets were tested in separate environmental temperatures to determine whether temperature had an effect on the intensity of cricket chirps. It appears that Gryllus assimilis did not chirp louder within the warmer temperature, but actually chirped louder within the ambient temperature environment. This finding led us to reject our hypothesis despite a significant difference between the two groups (p < 0.05).
Keywords: chirp, loudness, acoustic, crickets, temperatures, Gryllus assimilis
Acoustic signals are imperative to natural selection among crickets. In a study done by Ulagaraj & Walker (1973), in which they studied the orientation of mole crickets (Scapteriscus acletus and Scaperiscus vicinus) movement towards a source of sound. They compared natural and synthetic male calling sounds being played on speakers placed in a field of mole crickets with speakers that produced no sound (controls). The results showed mole crickets from as far as 100 meters away flying on or near the speakers where the calling songs were broadcast being from. The controls “caught” next to no crickets, further emphasizing the acoustic importance in cricket communication.
The chirping sound produced by a variety of common crickets is created when the cricket elevates and rubs his wings together (Walker, 1969) creating many small vibrations that occur with each beat of the wings (Bennet-Clark & Ewing, 1968). These chirps are often synchronized among the species for mate selection as well as to ward off predators (Velez & Brockmann, 2006).
The vibrational movement of a cricket’s wings is a result of muscle contractions (Ritchie, et al. 2001). These muscle contractions in the wings are what determine the rate and loudness of a cricket’s calling sounds. Since the temperature can affect muscle movement, it could, in turn, have an impact on a cricket’s ability to vary their acoustic signals (Ritchie, et al. 2001).
Ritchie, et al., (2001) studied the female preference for male calling signals in the Drosophila montana. Their study showed the mean carrier frequencies having a direct correlation with the temperature. They found that the carrier frequency of chirps was more commonly associated with temperature variations in the males but not in females. Since the quality of the male calling signal is what determines the reliability of sexual selection (Andersson, 1994), crickets with higher frequencies of chirp sounds would be more likely to be favored for mate selection.
These findings have led us to conduct an experiment that tests the impact of temperature in relation to chirp loudness in G. assimilis. We hypothesized that G. assimilis will chirp louder within a warmer environment than within an ambient temperature environment.
The twenty-four crickets that were used consisted of a mixture of adult male and female Gryllus assimilis. They were randomly selected from a pre-existing environment within Pet World Warehouse Outlet pet store. Twelve crickets were placed in each of two separate containers. The subjects were tested between the hours of 12:00pm and 12:30pm Monday through Friday for one week. The crickets were already habituated to human vocalization and activity.
Two 21.59cm x 12.7cm x 13.97cm critter cages were used to house the crickets. Cardboard egg cartons were collected and placed into each cage for shelter purposes and to prevent the crickets from killing one another. Fluker’s Calcium Fortified Cricket Quencher was the food source used daily to nourish the crickets during experimentation. A Traceable to Nist Calibrator Decibel Meter (part number: 981001448) was used to read the decibels in which the crickets were chirping at a given time. Two Biotronette Mark III environmental chambers were used to house the cricket cages in order to maintain stable, predetermined temperatures for each cricket population. Two Taylor digital thermometers (part number: 1455) were used, one in each environmental chamber, to determine and maintain correct temperatures in each chamber.
The decibel meter was obtained and calibrated according to the instructions within its original box. It was calibrated to 94.0dB. Before any recordings were taken, the decibel meter was set to the following settings: Range = low (35-100dB); Response = slow; and C- Function.
After the crickets were purchased, cardboard egg cartons were placed into each cage. Twelve crickets were housed in the cage placed into the heated environmental chamber. The temperature was set to 6 on the temperature dial. Twelve were housed in the cage placed into the ambient temperature environmental chamber. No lights were turned on within either chamber and no timers were set. The digital thermometers were each set to the “out” setting and placed near the cage within each chamber. The crickets were given 4 cubes per cage of Fluker’s Calcium Fortified Cricket Quench and a few sprinkles of water. They were then given 24 hours to habituate to their new environments and to allow the temperature to rise within the heated environment. The temperature at test time within the heated environment was 25 degrees Celsius and the temperature at test time within the ambient temperature environment was 23 degrees Celsius.
The recordings began the afternoon of Monday, March 23, 2009 and ended Friday, March 27, 2009. The decibel readings were recorded between the times of 12:00pm and 12:30pm each day. Prior to any recordings the decibel meter was placed 5cm away from the cage within the heated chamber first (set to the appropriate settings) for three minutes. This three minute time period allowed the crickets to regain their normal behavior after being disrupted by the noise from opening and closing the chamber door and placing the meter next to the cage. After this three minute process, the recordings were taken for one minute only when crickets chirped. The same process was repeated for the ambient temperature cage in the other environmental chamber. After the recordings were finished each cricket cage was given 4 cubes of Fluker’s cricket quench and a few sprinkles of water to prevent dehydration and starvation of the animals. After data was gathered an independent T-test was used to calculate the p-value to determine significance. This experiment was one tailed because past research has lead to believe that crickets chirp more in warmer temperatures than colder temperatures.
Our findings concluded that our hypothesis was not supported. However, there was significance in the chirping loudness of crickets between heated and ambient temperature environments (Fig. 1, p = 0.0008). The heated environment ranked lower for cricket loudness (dB) (M= 73.7, SD = 1.48) than the loudness within the ambient temperature environment (M=78.7, SD = 1.84). The independent T-test and the variance suggest that there was a significant difference between the loudness of cricket chirping between the heated environment and the ambient temperature environment. There was louder chirping from crickets within the ambient temperature environment than within the heated environment.
Figure 1. Loudness of chirping by Gryllus assimilis in relation to the type of environment (room temperature M=78.7dB, SD= 1.84; heated M=73.7dB, SD=1.48); Error bars reflect standard deviation.
The hypothesis was not supported in that the loudness of cricket chirps was not affected by warmer temperatures but rather by ambient temperatures. Several challenges arose during the limited duration of this experiment. Such challenges included maintaining consistent environmental temperatures, maintaining a consistent quantity of crickets, and maintaining a constant balance between male and female crickets within each environment. Each of these factors may have influenced the results of the experiment. It would have been beneficial to have a prior working knowledge of the intended environmental chambers as well as familiarity with cricket behavior if we were to repeat this experiment.
Because the loudness of chirps has been found to be more characteristic of male crickets than female crickets (Ritchie, et al., 2001), an option for similar research in the future may be to use only male crickets for a more accurate measure of chirp loudness. Since our results indicated a significant difference in the level of loudness in ambient (cooler) temperatures as opposed to in the warmer environment, this might be an option for future research, as well.
It is possible that temperature may not have an impact at all on how loud the crickets chirp, but rather the noise of the outside environment may cause cricket chirping to be more noticeable, especially at night.
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