Effects of habitat complexity on male socio-spatial behavior and mating system dynamics in collared lizards.

Abstract

Territory defense typically involves costly behavioral tactics that may detract from mating opportunities and other fitness enhancing activities. Selection on males therefore is expected to result in their establishing territories and adopting behavior patterns that maximize mating opportunities while simultaneously minimizing costs of spatial defense, which may vary among microhabitat patches that differ in physical parameters such as size, shape, and structural complexity. Using field behavioral studies, I first tested the influence of microhabitat patch structure (simple versus complex) on social and spatial behavior in territorial and non-territorial male eastern collared lizards (Crotaphytus collaris). As a result of markedly different structural conditions in the two microhabitat types, I proposed two alternative sexual selection models to explain factors that may govern male behavior and fitness. Despite the relatively small size and narrow dimensions of simple microhabitats, lizards colonized both simple patches, resulting in high local densities. Nevertheless, some males still defended territories on simple patches, and they did so without initiating contests with rivals or giving broadcast displays more frequently than territorial individuals on complex patches. By contrast, territorial males on simple patches moved throughout their territories more, and also courted a greater number of females more frequently than territorial males on complex patches. Non-territorial males in the two microhabitat types did not differ in any of the social variables measured. Increased visibility owing to the relatively flat and unobstructed surface topography of simple patches appears to promote increased courtship opportunities, while at the same time allowing males to deter same-sex competitors without significantly increasing costly defensive behaviors. Moreover, prioritization of courtship in highly competitive neighborhoods suggests that male behavior is shaped more by opportunities to interact with females than by competition for intrasexual dominant social status, perhaps because proximity to females coupled with simple habitat structure promotes monopolization of female mates. Although sexual selection theory predicts that socially dominant males will sire more offspring than males adopting subordinate social tactics, increased structural complexity of microhabitats may compromise the ability of territory owners to detect non-territorial rivals and prevent them from mating with female residents. To test the hypothesis that the ability of males to monopolize matings with females is negatively related to the structural complexity of microhabitats, I used molecular genetic techniques to quantify reproductive success for territorial and non-territorial males in each microhabitat type. Consistent with this prediction, males defending territories on simple patches sired a greater proportion of the offspring produced by individual mates compared to territorial males on complex patches. Contrary to the expectation that increased mate monopolization by territorial males would decrease mating opportunities for non-territorial males, neither the total number of offspring sired, nor the number of female mates differed as a function of male social status on simple patches, most likely as a result of high local female densities. By contrast, territorial males on complex patches sired more offspring total than their non-territorial rivals, and also tended to mate with more females (but not statistically so). The observation that territorial males on simple microhabitats sired a larger proportion of the offspring produced by their female mates compared to territorial males on complex patches suggests that mate monopolization may be more feasible in microhabitats that are less structurally complex because they afford territory owners high visibility while also limiting undetected movement by non-territorial males. Higher levels of mate monopolization without increased defense costs suggests that territory defense may be more economical in structurally simple microhabitats. Because the simple human-constructed microhabitats at this study site mimic some features of the natural rock outcrops and washes on which the socio-spatial behavior of collared lizards evolved, these results are more similar to what might be expected in populations in natural habitats.