1. 2. 3. 4. Figure 1 . Spatial dispersal pattern of eastern chipmunks. Due to male- biased dispersal pattern, an adult female tends to have most of her relatives near her burrow, whereas juvenile males rarely have any relatives either near or far from their burrows. .
Figure 1. Spatial dispersal pattern of eastern chipmunks. Due to male-
biased dispersal pattern, an adult female tends to have most of her relatives
near her burrow, whereas juvenile males rarely have any relatives either
near or far from their burrows.
n=10 n=24 n=4 n=20 n=4 n=11 n=4 n=10
Figure 2. Percentages of trilling in each age-gender group when released at
two distances from burrow. Results adapted from Burk Da Silvaet al., 2002.
The effect of population density on the trilling call behavior of eastern chipmunks
By Phebe Quan
Eastern chipmunks make three types of context specific alarm calls (Burk Da Silva et al., 2002). Chipping is loud and repetitive and is mostly given by stationary chipmunks when a terrestrial predator is detected. Chunking sounds like an axe hitting wood and is given in the presence of aerial predators. Trilling is short and subtle and is only given immediately after evading a pursuing predator. Chipmunks exposed to these conspecific alarm calls exhibit immediate alert responses by fleeing the area or assuming a motionless, alert posture (Weary and Kramer, 1995). Given that the callers are more vulnerable to predation, sending an alarm signal in only the most appropriate situation becomes crucial to their survival.
A previous study by Burk Da Silva et al. (2002) in Quebec categorized chipmunks into four age-gender groups based on the abundance of kin living in the vicinity of each individual (Figure 1). The study indicated that the frequencies of trill vary among the four groups as well as between two experimental conditions: those released within their home range and those released outside their home range (Figure 2). Chipmunks with more relatives in their proximity trill more often than those with fewer local relatives. Therefore it is concluded that chipmunks’ trilling call behavior is subject to kin selection.
If more nearby relatives means that chipmunks trill more often, then the frequency of trilling should not only differ among age-gender groups, but also within age-gender groups in areas with different population densities. I hypothesize that when being chased, the chipmunks living in a highly populated area (more kin) are more likely to trill than the ones living in a less populated area (fewer kin).
This study was conducted on the Marietta College campus in Ohio, from September 2006 to April 2007. The study area consisted of 1.5-ha section with dogwood, maple, sycamore and chestnut trees, and lower bushes. The burrows of resident chipmunks were located and mapped1. Live traps were set everyday from September to November and in April using walnuts as bait2. They were checked once or twice a day depending on the weather (chipmunks usually don’t came out when it rains).
Captured chipmunks were transferred to a plastic tube and marked with a unique pattern of fur clipping3, then categorized into different age-gender groups based on weight (juvenile < 65g, adults > 90g) and anal-genital distance. Then they were released where they were captured4. Immediately after release, individuals were chased by the experimenter until they dove into a burrow or climbed up a tree. Whether or not each chipmunk made a trilling call was recorded.
The density of chipmunks in this area was calculated and the frequencies of trill in each age-gender group were compared with the counterparts in Burk Da Silva’s study using SPSS.
Figure 3. Comparison of chipmunk population densities
The results from this study support the hypothesis that there is a relationship between population density and the frequency of trilling by adult female chipmunks. However the results from adult males and
both male and female juveniles are equivocal due to small sample sizes.
The first reason for the lower chipmunk density in Marietta College campus is the severe fragmentation of habitat in an urban environment (Reunanen and Grubb, 2005). Second, since chipmunk populations greatly depend on the availability of food, reduced food production leads to more competition over limited resources (Wishner, 1982). In addition, the lower coverage of bushes and the absence of woody debris, which serve as their refuge and travel path, make this area less preferablefor chipmunks.
I observed less frequent trills in adult female chipmunks in my study area. Lower population density, which means a lower density of relatives, reduced the tendency of calling. Also, since trilling is a soft vocalization, buildings and background noise may reduce the carrying distance of the trilling call. As a result fewer kin could benefit, so the call is comparably less beneficial to the caller. Moreover, low predation rates, due to a lower abundance of prey as well as human settlement, limited the exposure of chipmunks to danger and in turn probably affected the acquisition of the trilling call behavior.
In order to confirm a correlation between population density and the frequency of trilling calls, more experiments in different study areas with various population densities will be required. Plus, it will be important to compare the frequency of trill by juvenile males before and after dispersal to further demonstrate kin selection.
Burke Da Silva K, Mahan C, and Jack Da Silva. 2002. “The trill of the chase: eastern chipmunks call to
warn kin.” Journal of Mammalogy 83(2): 546-552.
Reunanen P. 2005. Density of eastern chipmunks (Tamias striatus) in farmland woodlots decline with
increasing area and isolation. Am. Midl. Nat. 154:433-441.
Weary DM and Kramer DL. 1995. “Response of eastern chipmunks to conspecific alarm calls.” Animal
Behavior 49: 81-93.
Wishner L. 1982. Eastern chipmunks: secrets of their solitary lives. Smithsonian Institution Press:
Thanks Chou-chou, Chi-chi & local chipmunks for giving me this wonderful idea.
Thanks Dr. McShaffrey, Dr. Hogan, Dr. Spilatro & Dr. McCabe for guidance & advice.
Thanks Jeremy, Yukun, Niuniu, Jaclyn and Andrea for also helping.
Thirty individuals were identified in the area of this study. As a result, the population density in this research area was 20/ha, compared to 30/ha in the study by Burk Da Silva et al. in Quebec (Figure 3). The frequencies of trilling by each age-gender group from the two study areas were compared (Figure 4). Results from a Chi-Square test indicated that adult females in Quebec trilled significantly more often than their counterparts in Marietta (X2(1, n=34) =6.17,
p= 0.013). Adult males in Marietta trilled less as well, however no significant difference was detected due to the small sample size. The results for juvenile male and female age-gender classes were not significant either.
Before juvenile dispersal After juvenile dispersal
Figure 4. Percentages of trilling in each age-gender group in the two study areas