Elizabeth Allen, Sarah Bachar, Andrea Ferguson, Steven Rivera, Robert Thomas, Jimmy Nieto, ... The life span of Anolis sagrei is up to 3 years (Scott, 1984).
Behavioral Ecology Of The Brown Anole, Anolis sagrei: Aggressive Interactions And The Effects Of Gender And Size
Elizabeth Allen, Sarah Bachar, Andrea Ferguson, Steven Rivera, Robert Thomas, Jimmy Nieto, Caroline Dudley, Vishnu Manteuffel, Julian Lee
Abstract
There are many factors that influence aggression in lizards. This study investigated the effects of gender and size upon aggressive responses of adult male Anolis sagrei. Field and laboratory experiments were conducted by presenting adult males with either adult male, adult female, or juvenile intruders. We found that adult males respond with significantly higher levels of aggression toward other adult males than to either adult females or juveniles. Courtship behavior or advertisement displays were usually observed when males were presented with females. This is understandable because during the breeding season males want to mate with females, not chase them away. When presented with juveniles, males either ignored juveniles or responded with the lowest level of aggression. This suggests that juveniles may be tolerated within a males’ territory until the juvenile approaches a mature adult size.
Introduction
Behavioral ecologists investigate how organisms interact with their environment, seeking to explain aspects of animal behavior such as aggression, food preferences, habitat selection, and social structure. In nature, there is only a finite amount of space and energy available to individuals. When resources that an animal needs are limited in supply, individuals compete for them. Such resources may include food, mates, and living space. There are two types of competition: exploitative and interference. Exploitative competition is when one animal is better
equipped to obtain and use the resources. Interference competition occurs when two animals directly interact in trying to obtain resources.
We chose to examine questions involving interference competition. We wanted to investigate what factors influence aggressive, territorial displays in the brown anole, Anolis sagrei. We chose this lizard as our study organism for several reasons. This species is territorial and individuals vigorously compete for territorial space, especially during the breeding season. They are abundant and easy to observe in the field, with stereotypic behavioral displays. They are active during the day, tend to stay on or near the ground, and are easy to capture and handle.
There are many different factors that influence aggressive behavior in lizards. For example, population density may have a great effect on a lizard’s aggression (Manteuffel, unpubl. diss.). Manteuffel found that when the population density of the lizard Scelporus virgatus was high, aggression was higher than when the population density was low. Simon (1975) found that lowered food levels lead to increased aggression in the lizard Sceloporus jarrovi. During the breeding season, elevated testosterone levels have been shown to influence aggressive behavior in Sceloporus jarrovi (Moore 1987).
We looked at the effect of body size and gender upon aggressive responses of territorial male Anolis sagrei. We tested the null hypotheses that: 1) size does not affect the aggressive responses of adult males and 2) gender does not affect the aggressive response of adult males. The alternative hypotheses are: 1) size does affect the aggressive response of adult males, with larger intruders causing a higher aggressive response by adult males; 2) gender does affect the aggressive responses of adult males, with same gender intruders causing a higher aggressive response than opposite gender intruders.
Hypothesis1: Size does affect aggressive responses of males. Null hypothesis: Size does not affect the aggressive responses of males.
Hypothesis2: Gender does affect aggressive responses of males. Null hypothesis: Gender does not affect the aggressive responses of males.
Methods
In order to investigate the effect of sex and size upon aggressive displays of adult male brown anoles, we conducted field and lab experiments. Field studies allow observations of an animal in its natural environment, but the investigator can’t control all the variables that may be involved. Lab studies, on the other hand, allow the investigator to control variables, but the environment is artificial. By combining both kinds of study we hoped to get an accurate understanding of the effect of sex and size upon aggressive displays.
Our study was conducted between the dates of June 30-July 19, 2000. We looked at the lizards on the University of Miami campus where you can find the brown anole close to low-lying hedges. Anolis sagrei were introduced to Florida from Cuba 50 years ago. They are visual predators, alert to movement up to 15 feet away (Scott 1984). Since Anolis sagrei are poikilotherms, they are heliothermic (they regulate their metabolism by basking in the sun). At sexual maturity a males’ size (measured from the snout to the vent) is 44mm and a females’ size is 38mm. The breeding season is from May to July. Adult males have a territory of up to 6m. Their territory may overlap the territories of one or more females. Adults are sedentary and tend to stay in their territories. The life span of Anolis sagrei is up to 3 years (Scott, 1984).
The behavioral displays that the Anolis lizards perform have been well studied (Stamps and Barlow 1973; Jenssen 1977; Scott 1984). For our purposes, we categorized aggressive displays that are directed at an intruder into 4 levels (see Table 1). The lowest level of aggression is when the lizard has no response to the intruder. The next level is when the lizard extends its dewlap or performs push-ups or head-bobs. The third level of aggression is when the lizard extends its’
dorsal and/or nuchal crests. Finally, the highest level of aggression is when the lizard extends both crests and expands laterally by flattening its sides as it approaches the intruder (attacks involving biting may also occur at this level of aggression).
For the field experiments, we caught juveniles, adult females, and adult male lizards with a noose (a long, metal rod with a string that has a loop at the end). To catch the lizard we had to get the noose around the lizard’s neck. While still tethered, the lizard was placed in an adult male’s territory. We observed the resident male lizard’s aggressive response to the intruder for up to five minutes, unless level 3 occurred, at which point the experiment was terminated.
For the lab experiments, we put one adult male lizard each into 12 aquaria (10 gallon). Sand and dirt were placed in the aquarium to provide a natural environment. The aquaria were stored in a shade house to ensure ambient environmental conditions. We allowed the lizards to establish themselves within the aquaria for two days. After the two days, we placed another lizard (either an adult male, an adult female, or a juvenile) in the aquarium to observe the response of the resident. We waited up to 5 minutes, unless level 3 occurred, at which point the experiment was terminated. In order to prevent the lizards from responding to the researchers, we observed from behind a blind (black net screen). We used the chi-square test to analyze our data.
RESULTS
An initial analysis comparing field and laboratory experiments revealed non-significant differences. Therefore we combined the field and lab results for further analysis. Size effects We found that size has an effect upon the aggressive response levels of adult males (X2 = 48.0, p < .001). Aggressive responses by adult males were higher to other adult males than to juveniles (see figure 1). Gender effects
We found that gender has an effect upon the aggressive response levels of adult males (X2 = 44.2, p < .001). Aggressive responses by adult males were higher to other adult males than to adult females (see figure 1).
DISCUSSION
In this study, we found that adult males respond with the highest level of aggression to other adult males. Alternatively, when presented with juveniles, adult males reacted at the two lowest levels of aggression. This indicates that size does have an effect on adult male’s aggressive behaviors. A possible function of this aggressive response to size, in terms of interference competition for territorial space, are that adult males exclude other similar sized males, but tolerate juveniles in their territories. A possible reason why adult males respond more aggressively to other adult males is because they probably want the same resources, such as food, shelter, and mates. Adult males may respond with such a high level of aggression toward intruders in order to obtain exclusive rights to a territory. In support of this conclusion, Stamps and Krishnan (1998) found that individuals of the lizard Anolis aeneus who obtain high social status and exclusive home ranges are more aggressive to intruders. Additionally, Fox and Myers (1991) found that dominance was positively related to obtaining superior home ranges in the lizard Uta stansburiana.
We found that adult males respond with little or no aggression toward juvenile lizards. A possible explanation for this is that the juveniles don’t want the same resources. The juveniles are not ready to mate, they may not want the same sizes of food items, and juveniles may choose different shelter sites. This could be tested by designing experiments to investigate resource use by adults and by juveniles to see if any overlap occurs between resources used.
Alternatively, juveniles and adults may use the same resources (except mates), but adults tolerate the juveniles. They may tolerate them because the juveniles may use very little of the
resources and minimally affect the adults resource use. Quantifying the impact that juveniles have on resources used by adults could test this.
We found that adult males respond with little or no aggression toward the females. The reason why we believe that the males do not show high aggression levels toward the females is because the males want to reproduce. It doesn’t make sense that a male would chase away females during the breeding season.
In conclusion, gender and size do affect aggressive responses by adult male brown anoles.
REFERENCES Grier, J.W. 1984. Biology of animal behavior. Times Mirror/Mosby College Publishing. Jenssen, T.A. 1977. Evolution of anoline lizard display behavior. Amer. Zool., 17: 203-215. Licht, P., and G.C. Gorman. 1970. Reproductive and fat cycles in Caribbean Anolis lizards. Univ. Calif. Publs. Zool., 95: 1-52. Manteuffel, V.M.S. 2000. Density-dependent processes within the early stages of the life-cycle of the striped plateau lizard, Sceloporus virgatus. Unpublished dissertation. Moore, M.C. 1987. Circulating steroid hormones during rapid aggressive response of territorial male mountain spiny lizards, Sceloporus jarrovi. Horm. Behav., 21: 511-521. Scott, M.P. 1984. Agonistic and courtship displays of male Anolis sagrei. Breviora, 479: 1-22. Simon, C.A. 1975. The influence of food abundance on territory size in the iguanid lizard Sceloporus jarrovi. Ecology 56: 993-998. Stamps, J.A., and G.W. Barlow. 1973. Variation and stereotypy in the displays of Anolis aeneus. Behaviour, 47: 67-94. Stamps, J.A., and S. Tanaka. 1981. The relationship between food and behavior in juvenile lizard (Anolis aeneus). Copeia, 2: 422-434. Stamps, J.A., V.V. Krishnan. 1998. Territory acquisition in lizards. IV. How settlers attained high status and exclusive home ranges. Animal Behaviour, 55:461-472.
Williams, E.E. 1976. West Indian Anoles: a taxonomic and evolutionary summary. I. Introduction and species list. Breviora Mus. Comp. Zool. No. 440: 1-21.
Table 1. Scale Used To Score Maximum Behavioral Response Of Lizards
Level 0 1 2 3
Associated Displays No response Advertisement Display (dewlap, head-bobs, pushups) Aggressive Display w/ crests Aggressive Display w/ crests and sagittal expansion [Note: Courtship displays (jiggle bobs) placed in level 1]
Figure 1. Aggressive responses of adult male brown anoles toward juvenile, adult male, and adult female intruders
Number Of Individuals
25 20
juvenile male
15
female 10 5 0 0
1
2
Aggressive Response Categories
3
