Adult Male Density Influences Juvenile Microhabitat Use in Brown Anoles

Photographs of the housing conditions used in the experiment. (a) One of the experimental enclosures (with an artificial tree) surrounded by blinds on all sides (note, the front blind was pulled back to reveal the tree and cage). (b) Close-up of the available horizontal perches. (c) Juvenile Anolis sagrei with its identification number on the lateral body surface for visual identification.

Fig 1. Photographs of the housing conditions used in the experiment. (a) One of the experimental enclosures (with an artificial tree) surrounded by blinds on all sides (note, the front blind was pulled back to reveal the tree and cage). (b) Close-up of the available horizontal perches. (c) Juvenile Anolis sagrei with its identification number on the lateral body surface for visual identification.

For many animals, optimal habitats vary across age classes, and individuals shift habitat use as they age. While many studies have documented such age-specific habitat use, most are observational and do not identify the causal factors. In addition, we know that competition between species has been an important driver of habitat use in Anolis lizards. However, less is known about the role of competition on habitat use within species of anoles, especially between age classes.

Dan Warner and I previously found that adults use higher and thicker perches than juveniles at our field site in northeastern Florida (Delaney and Warner 2016). We hypothesized that this variation was a result of adults forcing juveniles to suboptimal habitat. Thus, we altered the density of adult males in mesh enclosures (Fig. 1) in the lab and monitored changes in juvenile microhabitat choice.

Effects of adult male density on juvenile Anolis sagrei perch height during each period of the experiment. No adults were present during period 1. Period 2 represents observations over 8 d immediately following the introduction of adults to enclosures. Period 3 represents observations over 7 d approx. 1.5 mo after adults were introduced.

Fig 2. Effects of adult male density on juvenile Anolis sagrei perch height during each period of the experiment. No adults were present during period 1. Period 2 represents observations over 8 d immediately following the introduction of adults to enclosures. Period 3 represents observations over 7 d approx. 1.5 mo after adults were introduced.

 

We found that juveniles decreased perch height over time when no adults were present, but did so more rapidly when one adult male was present, and continue reducing height for a longer period when three adult males were present (Fig. 2). Juveniles had no preference for specific perch diameters in the absence of adults. However, juveniles used thicker perches when one adult male was present and had complex perch width use when three adult males were present (using two of the intermediate diameters most often, Fig. 3).

We also observed some unexpected results for substrate use. Leaf use increased over time when one adult male was present, but decreased over time in the presence of three adult males (Fig. 4). Adult males normally perched near the top of the tree trunk in each cage. When one adult was present, juveniles may have moved to the distal parts of the tree to distance themselves from the adult, thus placing them on leaves. However, in the high adult density treatment, adult males spread out on more of the tree, partitioning habitat amongst themselves. This would have left less room for juveniles, likely forcing them completely off the parts of the tree.

Percentage of observations on a given perch width for juvenile Anolis sagrei exposed to (a) control conditions (i.e., no adults), (b) low adult density (i.e., one adult male), and (c) high adult density (i.e., three adult males).

Fig 3. Percentage of observations on a given perch width for juvenile Anolis sagrei exposed to (a) control conditions (i.e., no adults), (b) low adult density (i.e., one adult male), and (c) high adult density (i.e., three adult males).

These nonlinear effects of adult density on perch diameter and leaf use suggest intraspecific interactions in this system are complex with competition likely occurring within and between age-classes. Overall, these results suggest that age-class competition does contribute to the microhabitat use of juvenile brown anoles.

For detailed information, including how snout-vent length and time-of-day affect juvenile microhabitat choice, the full paper is recently online: Delaney, D.M., and D.A. Warner. 2017. Adult male density influences juvenile microhabitat use in a territorial lizard. Ethology 123(2):157–167.

Other cited paper: Delaney, D.M., and D.A. Warner. 2016. Age- and sex-specific variations in microhabitat and macrohabitat use in a territorial lizard. Behavioral Ecology and Sociobiology 70(6):981-991.

Effects of treatment and period on leaf use by juvenile lizards. Black bars indicate the short-term change in substrate use (i.e., between periods 1 and 2, which represents behavioral changes immediately after adults were introduced) and gray bars indicate the long-term change (i.e., between periods 1 and 3, which represents behavioral changes approx. 1.5 mo after adult were introduced) in substrate use.

Fig 4. Effects of treatment and period on leaf use by juvenile lizards. Black bars indicate the short-term change in substrate use (i.e., between periods 1 and 2, which represents behavioral changes immediately after adults were introduced) and gray bars indicate the long-term change (i.e., between periods 1 and 3, which represents behavioral changes approx. 1.5 mo after adult were introduced) in substrate use.

 

 

 

 

 

 

 

About David Delaney

David is a Ph.D. student in Fred Janzen's lab at Iowa State University. He is interested in predator/prey relationships and microevolutionary processes.

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