Roger A. Anderson

Sexual Selection Versus Alternative Causes of Sexual Dimorphism in Teiid Lizards

SummaryThe presence and extent of sexual dimorphisms in body form (size and shape) of adult macroteiid lizards were investigated. Males were significantly larger than females in the temperate species, Cnemidophorus tigris, and in the tropical species, Ameiva ameiva and C. ocellifer. Young adult C. tigris males grew faster than young adult females within and between reproductive seasons. Adult males of all species had larger heads than adult females of the same body size; this difference increased with body size. Moreover, male C. tigris were heavier than females of the same snout-vent length. The causes and consequences of the sexual dimorphisms were also examined. The possible causes of body size are especially numerous, and distinguishing the relative influences of the various causal selection factors on body size is problematical. Nevertheless, observational field data were used to tentatively conclude that intrasexual selection was the cause of larger body size of C. tigris males relative to females because (1) larger males won in male aggressive interactions, (2) the winning males gained access to more females by repelling competitors and by female acceptance, (3) larger males consequently had higher reproductive success, and (4) other hypothetical causes of larger male size were unsupported.

Mesocosm Experiments on Habitat Choice by an Endemic Lizard: Implications for Timber Management

We investigated the impacts of various logging practices on habitat choice by the endemic Florida scrub lizard (Sceloporus woodi) in the Florida scrub of Ocala National Forest (ONF). We used large outdoor mesocosms as a novel means to evaluate lizard preferences for habitats with different structural features produced by standard forestry practices. Captive lizards were offered a choice between two adjoin- ing habitats (= sides of a mesocosm) created using one of two substratum treatments (SAND = 75% open sand; WOOD = 75% coarse woody debris (CWD)) coupled with one of two insolation treatments (LIGHT = ambient sunlight; DARK = 45% ambient sunlight). The mesocosms proved to be an effective technique for evaluating lizard habitat preferences. Lizards were easily observed and remained active and healthy throughout the experiment Sighting frequencies differed significantly among the four mesocosm sides, yielding an overall preference ranking of DARK SAND > LIGHT SAND > DARK WOOD - LIGHT WOOD. Analysis of sighting frequencies by treatment factors (substratum and insolation) and of dissimilarity ma- trices both indicated that habitat choice was based primarily on substratum composition and only weakly determined by insolation level. In addition, size- and gender-specific preferences suggest that social inter- actions may help shape patterns of habitat used in conjunction with individual preferences. We conclude that the least favored mesocosm side (LIGHT WOOD) represents a habitat type that could potentially serve as a population sink for scrub lizards and recommend several methods to reduce the accumulation of CWD or to ameliorate its potential thermal stress on lizards. In addition, the most favored mesocosm side (DARK SAND) represents a shaded sandy habitat type not currently found in ONF timber stands. We present several alternative harvesting and site preparation methods that could produce such habitats and recommend further research on their potential value for enhancing populations of scrub lizards and other open-habitat scrub

Sexual Size Dimorphisms and Bite Force in the Northern Alligator Lizard, Elgaria coerulea

Abstract Bite force capacity is a directly related performance correlate of head shape and is an integrative measure of performance in capturing and handling prey, fighting, and mating, especially for males. We investigated head shape and bite force dimorphisms in a small semifossorial lizard (Elgaria coerulea) that exhibits females-larger sexual size dimorphism (FL-SSD) in some populations but not in others. Specifically, we explored how body size, head shape, and bite force compare between the sexes relative to any dimorphisms in body or head size in a coastal population of E. coerulea. Female larger-SSD was confirmed for this population, but it contrasted with males-larger sexual dimorphism in head size (ML-SDHS). Males also had greater bite force than females of similar size. The secretive habit of E. coerulea hampers observations of behavioral interactions among conspecifics. However, it is expected that lizards with FL-SSD will have ML-SDHS if males with greater bite force win fights with other males over access to mates and/or if greater bite force increases copulation success.

Rock-dwelling Lizards Exhibit Less Sensitivity of Sprint Speed to Increases in Substrate Rugosity

Effectively moving across variable substrates is important to all terrestrial animals. The effects of substrates on lizard performance have ecological ramifications including the partitioning of habitat according to sprinting ability on different surfaces. This phenomenon is known as sprint sensitivity, or the decrease in sprint speed due to change in substrate. However, sprint sensitivity has been characterized only in arboreal Anolis lizards. Our study measured sensitivity to substrate rugosity among six lizard species that occupy rocky, sandy, and/or arboreal habitats. Lizards that use rocky habitats are less sensitive to changes in substrate rugosity, followed by arboreal lizards, and then by lizards that use sandy habitats. We infer from comparative phylogenetic analysis that forelimb, chest, and tail dimensions are important external morphological features related to sensitivity to changes in substrate rugosity.

ADJUSTING PREY HANDLING TIMES AND METHODS AFFECTS PROFITABILITY IN THE BROAD-HEADED SKINK (EUMECES LATICEPS)

Time spent pursuing, capturing, and subduing prey, and time spent preparing and swallowing it all affect profitability of a prey item (net energy gain/handling time). In the diet selection model of classical optimal foraging theory, handling time is constant for each prey type, but several factors affect it in some predators. We studied factors that might cause time spent to capture and handle prey by broad-headed skinks (Eumeces laticeps) to vary between and within prey types. Pursuit time did not vary with prey (cricket) size in experimental conditions. In three experiments using crickets of the same size, handling times for the first prey eaten were shorter when more prey were simultaneously present. Reduced handling time likely maximizes number of prey captured before the remaining prey escape. Preparation time (capture to initiation of swallowing) did not vary with prey number, but swallowing time decreased with number of prey present. This result suggests that lizards must prepare crickets thoroughly for digestion by bites puncturing the exoskeleton, which constrains them from decreasing preparation time. However, lizards can reduce handling time by swallowing more quickly, increasing the likelihood of capturing additional prey. Handling times were shortest for hungry skinks and longest just before satiation. Two factors may contribute to the increase in handling time with degree of satiation: (1) Motivation to capture prey declines as a predator becomes satiated, removing the reason for shortening handling times when groups of prey are present; (2) as the stomach and the esophagus are filled, mechanical constraints may slow swallowing. Preparation time, number of bites, swallowing time, and handling time increased with cricket size. Consequently, although it appears that the largest prey were the most profitable for sizes of crickets studied, this may not be true for other ranges of prey size. Preparation time was shorter for prey bitten on the head than on the side or back, but prey orientation during swallowing did not affect swallowing time. In summary, (1) when prey are present simultaneously, total energetic profit may be increased by reducing handling time, and (2) handling for specific prey types vary with satiation and handling method.

Sequential analyses of foraging behavior and attack speed in ambush and widely foraging lizards

Food acquisition mode in lizards (i.e., ambush vs. widely searching) has been intensely scrutinized for the past decade to identify correlations between food acquisition mode, diet, sprint speed, and other aspects of phenotypic diversity. To begin to understand these correlations, we studied foraging mode variation in natural foraging behavior and attack speed in three ambush predators and two widely foraging species in the field. Sequential analyses revealed considerable variation in the temporal structure of behavioral repertoires associated with acquiring food. Ambush and wide-foraging species use unique combinations of behaviors prior to prey attack with differences among and between food acquisition modes. Attack speeds were well below maximum sprint speed for these species. Thus, the widely demonstrated correlation between food acquisition mode and sprint speed is not related to prey capture per se. The striking variation in prey capture repertoires in these model ambush and wide foragers shows that we have a long way to go before we will understand the ecological relevance of many performance and phenotypic traits that are related to foraging mode in lizards.