Henry B. John-Alder

A COMPARATIVE TEST OF ADAPTIVE HYPOTHESES FOR SEXUAL SIZE DIMORPHISM IN LIZARDS

Abstract.— It is commonly argued that sexual size dimorphism (SSD) in lizards has evolved in response to two primary, nonexclusive processes: (1) sexual selection for large male size, which confers an advantage in intrasexual mate competition (intrasexual selection hypothesis), and (2) natural selection for large female size, which confers a fecundity advantage (fecundity advantage hypothesis). However, outside of several well‐studied lizard genera, the empirical support for these hypotheses has not been examined with appropriate phylogenetic control. We conducted a comparative phylogenetic analysis to test these hypotheses using literature data from 497 lizard populations representing 302 species and 18 families. As predicted by the intrasexual selection hypothesis, male aggression and territoriality are correlated with SSD, but evolutionary shifts in these categorical variables each explain less than 2% of the inferred evolutionary change in SSD. We found stronger correlations between SSD and continuous estimates of intrasexual selection such as male to female home range ratio and female home range size. These results are consistent with the criticism that categorical variables may obscure much of the actual variation in intrasexual selection intensity needed to explain patterns in SSD. In accordance with the fecundity advantage hypothesis, SSD is correlated with clutch size, reproductive frequency, and reproductive mode (but not fecundity slope, reduced major axis estimator of fecundity slope, length of reproductive season, or latitude). However, evolutionary shifts in clutch size explain less than 8% of the associated change in SSD, which also varies significantly in the absence of evolutionary shifts in reproductive frequency and mode. A multiple regression model retained territoriality and clutch size as significant predictors of SSD, but only 16% of the variation in SSD is explained using these variables. Intrasexual selection for large male size and fecundity selection for large female size have undoubtedly helped to shape patterns of SSD across lizards, but the comparative data at present provide only weak support for these hypotheses as general explanations for SSD in this group. Future work would benefit from the consideration of alternatives to these traditional evolutionary hypotheses, and the elucidation of proximate mechanisms influencing growth and SSD within populations.

Locomotor capacity and foraging behaviour of kalahari lacertid lizards

Abstract Closely related lacertid lizards (Eremias, Nucras) in the Kalahari desert differ in patterns of foraging behaviour. Some species are relatively sedentary (‘sit-and-wait’) whereas others are more active (‘widely-foraging’) predators. We determined whether whole-animal locomotor capacities (cruising endurance on a treadmill, initial speed and maximum burst speed in a racetrack, and sprint endurance in a torus-shaped track) correlated with interspecific differences in foraging behaviour. Two of three widely-foraging species had greater cruising endurance, graater sprint endurance, but lower burst speed than did a sit-and-wait species. However, the two species that sprinted quickly also had limited endurance, and vice versa. Pre-feeding negatively influenced endurance but not sprint capacity. Theoretical models of foraging behaviour should recognize that ectotherms have limited endurance, that there can be a trade-off between speed and endurance, and that pre-feeding can reduce some aspects of locomotor capacity.

Plasma corticosterone and testosterone levels during the annual reproductive cycle of male brown anoles (Anolis sagrei)

The objective of the present study was to determine the plasma levels of testosterone and corticosterone in male Anolis sagrei during the annual reproductive cycle and to examine the relationships between seasonal change in the levels of these hormones, male reproductive activity, and body condition. Both testosterone and corticosterone levels in adult males captured in the Miami, Florida, area varied significantly with month and with season (i.e., breeding vs. nonbreeding period), although they were not significantly correlated with each other or with body mass. Mean monthly testosterone levels were higher during the breeding season (March–August) than during the non‐breeding season, with the one exception that the highest mean testosterone level occurred in February immediately before the beginning of the breeding season. Testosterone levels in the overall sample of 144 males were significantly correlated with testes mass but not with any of the other measured variables. Corticosterone levels were highest during the nonbreeding season and lowest during the breeding season. Corticosterone levels on a monthly basis were negatively correlated with monthly changes in testes mass and positively correlated with monthly changes in abdominal fat‐body mass.

Effect of water constraint on growth rate, activity and body temperature of yearling common lizard (Lacerta vivipara)

Abstract We investigated the effect of water constraints on yearling Lacerta vivipara, a widespread species of lizard inhabiting European peat bogs and heath land. We conducted a laboratory experiment to investigate plasticity of growth rate, activity level and preferred body temperature. We subjected individuals of two source habitats (dry vs humid) to two laboratory conditions of water supply resulting in different air relative humidity and water availability (high vs low). We observed that a low water supply induced a lower growth rate and lower activity level, suggesting that growth limitation is correlated with adaptive responses to avoid dehydration. However, individuals from the two habitats selected different body temperatures when restricted in water and showed different ratios between growth and activity. This suggests that there is population variability in phenotypic plasticity with respect to water availability in the habitat. Field observations conducted in six natural populations, classified into two groups (dry vs humid habitat) also suggest that growth rate in nature is constrained by water availability.

THERMAL PHYSIOLOGY, PHENOLOGY, AND DISTRIBUTION OF TREE FROGS

Seasonal and latitudinal gradients in the thermal sensitivity of jumping performance have been examined in 10 species of hylids representing five different seasonal and distributional categories. Among temperate species, low-temperature performance is best in early-breeding northern species, intermediate in later-breeding northern species, and worst in southern species. Subtropical and tropical species have progressively higher critical thermal minima, but their thermal performance curves are not much different from those of southern temperate species in other respects. Critical upper thermal limits on jumping performance are much less variable than are lower limits, and the critical thermal minimum (CTmin) is not correlated with the critical thermal maximum (CTmax) in our sample. Effectively, CTmax is relatively inflexible, and consequently an improvement in low-temperature performance (reduction in L80, the lowest temperature at which frogs can attain 80% of maximal performance) results in a widening of the thermal breadth of performance at or above some arbitrarily selected standard (e.g., B80, 80% of maximal performance). The strong negative correlation between L80 and B80 in our sample of hylids is not found in lizards. Differences in thermal sensitivity of jumping performance are strongly correlated with naturally experienced minimum body temperatures. The thermal sensitivity of locomotion may itself be (or may be correlated with) a mechanism underlying distributional differences and temporal partitioning of habitat among frogs. We suggest that some aspects of the community organization of hylids, such as breeding phenology and the depauperate faunas of high latitudes, are constrained at least in part by thermal physiology. Ecological consequences of this constraint are currently being investigated.

Home-Range Analysis in Sceloporus undulatus (Eastern Fence Lizard). I. Spacing Patterns and the Context of Territorial Behavior

Abstract Understanding the spacing patterns of individuals in a population of animals is important for establishing the specific functions of territorial behavior. Here, we couple a comprehensive analysis of home range with demographic information and focal behavioral observations of Sceloporus undulatus (eastern fence lizard) to investigate (1) spacing patterns, (2) the determinants of home range, and (3) the functional significance of territorial behavior and traits related to territorial behavior (body size, color). Male home-range area is an order of magnitude larger in the New Jersey population of S. undulatus described here than in other populations, whereas female home ranges are comparable in area to other populations of this species. Home-range area is positively correlated with body size in adult males, although this relationship is not found within age classes, and males share about 50% of their home range with other males. After removal of the effect of body size, residual home-range area is smaller in two-year olds (first breeding season) than in older males. Home-range area increases as a function of the number of overlapped females, even after the effect of body size is statistically removed, and the sightings of males within their home ranges are tightly clustered in and near the home ranges of females. Males courted females in 60% of focal behavioral observations and contested other males in 14%, always in close proximity to a female. Relative body size, color, and behavior were displayed by contest “winners” and “losers” in the same manner as previously characterized in captive lizards. Of the three status traits examined, only body size explained variation in home-range area, apparently reflecting the ability of males to court increasing numbers of females as they get older and larger. The winners of contests gained access to the nearby female, but contests had no lasting influence on the locations where males subsequently were sighted. Thus, the size and shape of male home ranges are determined largely by the number and locations of females with whom males interact. Females themselves and not specific sites or home-range area are the contested resources. Plasma corticosterone was correlated with the area of a males home range overlapped by female home ranges. Because this measure of overlap may reflect the frequency of interactions between lizards and assuming that elevated corticosterone reflects a stress response, social interactions including contests between males and courtship of females appear to be stressful in free-living male S. undulatus.

Testosterone Regulates Sexually Dimorphic Coloration in the Eastern Fence Lizard, Sceloporus undulatus

Abstract Adult male Eastern Fence Lizards (Sceloporus undulatus) possess blue and black ventral patches that function in sex recognition and intrasexual social signaling, but this ventral coloration is absent or greatly reduced in females and juvenile males. Adult males also exhibit a relatively uniform, reddish brown dorsal coloration, while females and juvenile males are cryptically colored, with two rows of dark brown or black chevrons set against a background of gray and brown. In the present report, we show that sexual divergence in ventral coloration is temporally correlated with sexual divergence in plasma testosterone in free-living juvenile males and females, supporting the hypothesis that sexual dichromatism is regulated at least in part by testosterone. We experimentally tested this hypothesis by (1) removing the primary source of circulating testosterone in juvenile males via surgical castration, (2) restoring testosterone in castrated males with tonic-release implants, and (3) implanting intact juvenile females with exogenous testosterone. As predicted by our hypothesis, the development of blue and black ventral coloration in S. undulatus was (1) inhibited by castration in juvenile males, (2) restored by exogenous testosterone following castration in juvenile males, and (3) promoted by exogenous testosterone in juvenile females. The expression of male-specific dorsal coloration was also (1) inhibited by castration and (2) restored by exogenous testosterone following castration in juvenile males. Our results are consistent with established literature supporting the critical role of androgens in the mediation of sexually dimorphic coloration among phrynosomatid lizards.

Thermal relations of some Australian skinks (Sauria: Scincidae)

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Physiological correlates of natural activity and locomotor capacity in two species of lacertid lizards

Summary1.Physiological factors relating to activity metabolism were measured in two species of African lacertid lizards that differ greatly in natural foraging patterns:Eremias lineoocellata, a sit-and-wait predator, andE. lugubris, a widely foraging animal.2.Maximal oxygen consumption at 37°C is greater inE. lugubris [3.22 ml O2/(gxh)] than inE. lineoocellata [2.49 ml O2/(gxh)].3.Anaerobic scope and capacity at 37°C are greater inE. lineoocellata [2.56 mg lactate/(gx min) and 1.81 mg lactate/g] than inE. lugubris [2.12 mg lactate/(gxmin) and 1.40 mg lactate/g].4.Relative heart mass and hematocrit are greater inE. lugubris than inE. lineoocellata (0.28% and 0.24% body mass; 30.1 and 24.4 respectively).5.No significant interspecific differences occur in hind limb muscle mass as a percentage of body mass or in myoglobin concentration, citrate synthase or myofibrillar ATPase activity of hind limb skeletal muscle.6.No significant interspecific differences occur in isometric contractile properties (twitch and tetanic tension, twitch rise time and half-relaxation time, and fatigue response to tetanic stimulation) of the iliofibularis muscle.7.Organismal metabolic patterns of aerobic and anaerobic capacity reflect differences in locomotor capacity and natural foraging patterns of these species.

Testosterone Inhibits Growth in Juvenile Male Eastern Fence Lizards (Sceloporus undulatus): Implications for Energy Allocation and Sexual Size Dimorphism

In the eastern fence lizard, Sceloporus undulatus, female‐larger sexual size dimorphism develops because yearling females grow faster than males before first reproduction. This sexual growth divergence coincides with maturational increases in male aggression, movement, and ventral coloration, all of which are influenced by the sex steroid testosterone (T). These observations suggest that male growth may be constrained by energetic costs of activity and implicate T as a physiological regulator of this potential trade‐off. To test this hypothesis, we used surgical castration and subsequent administration of exogenous T to alter the physiological and behavioral phenotypes of field‐active males during the period of sexual growth divergence. As predicted, T inhibited male growth, while castration promoted long‐term growth. Males treated with T also exhibited increased daily activity period, movement, and home range area. Food consumption did not differ among male treatments or sexes, suggesting that the inhibitory effects of T on growth are mediated by patterns of energy allocation rather than acquisition. On the basis of estimates derived from published data, we conclude that the energetic cost of increased daily activity period following T manipulation is sufficient to explain most (79%) of the associated reduction in growth. Further, growth may have been constrained by additional energetic costs of increased ectoparasite load following T manipulation. Similar studies of the proximate behavioral, ecological, and physiological mechanisms involved in growth regulation should greatly improve our understanding of sexual size dimorphism.