Troy A. Baird

Age and Gender-Related Differences in the Social Behavior and Mating Success of Free-Living Collared Lizards, Crotaphytus collaris

Field studies on collared lizards at Lake Arcadia Dam, Edmond, Oklahoma, revealed that the social and spatial behavior of individuals differed depending on gender and male age. Body size measurements of known lizards over a threeyear period revealed that females > 70 mm SVL at the beginning of the activity season in April become reproductively active in their first year, males > 100 mm in April are two years and older (adults), whereas smaller males (60-90 mm) are in their first full year (yearlings). Experimental introduction of female intruders onto occupied female home ranges elicited aggression in four of 17 trials. However, female home ranges overlapped one another greatly, and intrasexual female aggression occurred infrequently. These results suggest that female collared lizards at Arcadia Lake do not defend exclusive territories but that occasional aggression may play some other role in the social relationships among adjacent females. Areas occupied by adjacent adult males overlapped little and were defended against other adult males primarily by high rates of travel punctuated with frequent display. Yearling males occupied nondefended home ranges within adult male territories by adopting subordinate tactics characterized by low rates of display and travel and rapid retreat from adult males. The home ranges of approximately 60% of the females were overlapped by the territory of a single adult male. In cases of multiple adult male overlap, one of the males overlapped a larger percentage of the female home range and interacted more frequently with these females than did the other adult males. Most female home ranges were also overlapped partially by as many as four yearling males. Adult males initiated encounters with females much more frequently than did yearlings, and females were almost always receptive to adult males whereas they more often withdrew from yearling males. Estimates based on the relative frequencies of interactions of females with each overlapping male indicated that annual male mating success was correlated with age class, snout-vent length, hourly frequency of social acts initiated, and rate of travel, such that mating success of adult males far exceeded that of yearlings. Among adult males alone, one estimate of mating success was weakly correlated only with the number of females on territories. Our results show that, even though male mating success is low during their first year, yearlings remain on adult male territories by adopting subordinate behavior patterns, perhaps because this tactic enhances future opportunities to inherit territories and mates.

REPRODUCTIVE COLORATION IN FEMALE COLLARED LIZARDS, CROTOPHYTUS COLLARIS, STIMULATES COURTSHIP BY MALES

Female-specific reproductive coloration is widespread among lizards, prompting several hypotheses to explain the possible function(s) of such coloration in females. I tested four of these hypotheses using observational and experimental field studies of free-ranging collared lizard females, Crotaphytus collaris, which develop orange markings on the lateral torso in association with the reproductive cycle. Orange markings did not appear to function for advertisement to female competitors because females rarely displayed their coloration during interactions with consexual competitors, and markings developed well before the occurrence of peak aggression among females. By contrast, females frequently displayed their lateral torso when courted by males. The number of displays given to courting males did not differ in females that had and those that had not yet developed orange markings, suggesting that female coloration does not function for sexual recognition in collared lizards. Females developed orange pigmentation while they matured their first clutches, and markings were maintained between and throughout the production and oviposition of the second and third clutches. Male courtship encounters with females having naturally developed markings were longer and involved more male displays than those with females that had not yet developed their coloration, suggesting that development of coloration by females functions to stimulate male courtship when females are receptive. To test this hypothesis, I established nine female pairs of size- and age-matched females that had early vitellogenic eggs, but had not yet developed reproductive pigmentation. Female pairs were each residents on the territories of nine different males. One female in each pair was painted with orange spots and bars to mimic the natural orange coloration and the other with light brown to match the natural background coloration of female C. collaris and mask naturally developed pigments. Males began courting orange-painted females within 2 h, and, on average, orange females were courted over five times more frequently than brown females. The difference in courtship frequency did not appear to result from males avoiding brown females because the average distance between males and females in the two treatment groups was similar. These results support the hypothesis that reproductive coloration in C. collaris females stimulates courtship rather than signaling rejection of courtship to males.

Sexual Dichromatism and Color Conspicuousness in Three Populations of Collared Lizards (Crotaphytus collaris) from Oklahoma 1

Abstract The coloration of animal integuments evolves in response to numerous and often competing selective pressures. Although male-male competition and female mate choice characteristically select for increased color conspicuousness, visibility to predators and to prey often select for decreased conspicuousness. We examined three populations of Common Collared Lizards, Crotaphytus collaris, in Oklahoma (Arcadia Lake, Glass Mountains, Wichita Mountains) that have been argued to differ in the intensity of natural and sexual selection acting on their color patterns. Our study had two main objectives. First, reflectance spectra were obtained from the lizards to replicate and extend previous work on differences in sexual dichromatism among these populations. Second, spectra were gathered on components of visual backgrounds at our study sites to explore the possibility that each population may be relatively cryptic within its own habitat. Results showed that most body regions differed significantly in sexual dichromatism among populations, but in contrast to prior work, no one population was more sexually dichromatic than another for all body regions examined. Males exhibited less overlap in coloration with their visual backgrounds than did females (i.e., males were more conspicuous), and females overlapped more in coloration with rocks than with other visual backgrounds. The population estimated previously to experience the strongest predation pressure (Arcadia Lake) was shown in the present study to be the least conspicuous. Some support also was found for the proposition that even the most “colorful” population (Wichita Mountains) may not always be conspicuous when viewed against its typical visual background.

Estimating Lizard Home Range: The Rose Model Revisited

Abstract In 1982, B. Rose pointed out that home-range area could easily be underestimated unless sufficient numbers of sightings were obtained for each individual, and recommended a population-specific test to determine the minimum sample size at which home-range area can be accurately measured. This recommendation has been generally ignored. We explore how sample size affects interpretation of space use data in male lava lizards, Microlophus albemarlensis, and male eastern collared lizards Crotaphytus collaris. Using the criteria established by Rose, the minimum sample sizes for collared lizards and lava lizards were 45 and 84 sightings, respectively. These are much larger samples than we anticipated and reinforce the need for empirical determination of minimum sample size. Our data suggest that lizard movement rate is a better predictor of minimum sample size than home-range area. Collared lizard home ranges were over 10 times larger than lava lizard home ranges, yet the higher movement rates of collared lizards resulted in a minimum sample size that was about one-half that of lava lizards. Restricting data collection to any one-quarter of the total study period would have markedly underestimated home-range area and home-range overlap in both species. Every lizard of both species had a core area within their home range, suggesting the absence of major temporal shifts during the study. On average, lava lizard core areas occupied 21.7 ± 2.8% of total home-range area, yet core areas included 78.8 ± 3.1% of all lizard sightings. Collared lizard core areas averaged 25.4 ± 2.4%, with a lower percentage of collared lizard sightings occurring within core areas (66.3 ± 3.0%). There was only one incidence of core overlap in either species. Lava lizards may be defending core areas but in an unusual manner, what amounts to “sit-and-wait territoriality.” This unusual mode of space use would not be detectable from a smaller number of sightings per individual.

Geographic Variation in Sexual Dichromatism in the Collared Lizard, Crotaphytus collaris (Sauria: Crotaphytidae)

We recorded the color of male and female collared lizards (Crotaphytus collaris collaris) from three populations using reflectance spectrophotometry to examine interpopulation differences in sexual dichromatism. The degree of sexual dichromatism within this species varied significantly among local populations. We also examined brightness of coloration and sexual dichromatism in these populations using appraisal by naive observers. Subjective appraisal also showed significant differences among these populations. However, the population appraised as most brightly colored was not appraised as most dichromatic. This study demonstrates that sexual dichromatism may vary significantly among populations of a single species. Although subjective appraisal of sexual dichromatism may yield reliable results when carried out under carefully controlled conditions, assessment of a single pair of individuals or assessment of only male coloration is not sufficient for a study of sexual dichromatism.

Hormone levels in territorial and non-territorial male collared lizards

For species displaying plastic alternative reproductive tactics, the relative plasticity hypothesis (RPH) combined with the positive relationship between androgens and aggression predicts higher androgen levels in more aggressive socially dominant males relative to less aggressive subordinate males (directional RPH). We tested this prediction of the directional RPH by comparing plasma levels of testosterone, dihydrotestosterone, and corticosterone in 2y+-collared lizard males that defended territories with those of first-year males that were mature, but did not defend territories. As expected, 2y+-males exhibited higher rates of advertisement, aggression, and courtship than first-year males. Contrary to expectations of the directional RPH, levels of testosterone, dihydrotestosterone and corticosterone were similar in the males displaying these alternative tactics. Furthermore, whereas display by non-territorial males increased after a neighboring territorial male died, levels of testosterone and corticosterone decreased in these males, counter to prediction that the territorial tactic is activated by increased androgens. This result also suggests that high rates of aggression do not alter plasma steroids, although behavioral manipulations are needed to more fully test this hypothesis. Secretion of testosterone in non-territorial males may promote their high growth rates, and/or may prime them for the rapid behavioral changes that occur when opportunities for territory acquisition arise as a consequence of predation on territorial males. Relationships among hormones differed between these types of males: corticosterone was negatively correlated with both testosterone and dihydrotestosterone in territorial males, but androgens and corticosterone did not significantly covary in non-territorial males.

PURSUIT DETERRENT SIGNALLING BY THE BONAIRE WHIPTAIL LIZARD CNEMIDOPHORUS MURINUS

Pursuit-deterrent signalling may account for several behaviors, often subtle, that have no current satisfactory explanation. We conducted field observations and experiments to determine whether arm-waving by Bonaire whiptail lizards, Cnemidophorus murinus , is consistent with pursuit deterrent signalling. The hypotheses that lizards wave their arms to remove feet from hot substrates and that arm-waving is solely a social signal were falsified. Lizards preferentially waved the arm closer to an investigator simulating a predator, typically while looking directly at the predator. This strongly suggests signalling and contradicts the hypothesis that arm-waving is merely a flight intention movement. Most individuals retreat, providing a margin of safety, and then wave. As expected for a pursuit-deterrent signal, arm-waving is related to risk. During direct approaches lizards waved when the predator drew within 3-6 m if approached slowly, but fled without signalling if approached rapidly. Lizards approached tangentially signalled frequently when the minimum bypass distance was 2-4 m, but rarely when it was 10 m. They arm-waved when predators approached, but rarely while they retreated. Arm-waving was rare where lizards were habituated to human presence, but frequent elsewhere. Collectively, the data strongly suggest that arm-waving has a pursuit-deterrent function.

Aggressive transition between alternative male social tactics in a long-lived Australian dragon (Physignathus lesueurii) living at high density.

Theory predicts the evolution of alternative male social tactics when intense competition coupled with the superior competitive ability of some individuals limits access to reproductive opportunities by others. How selection has shaped alternative social tactics may be especially interesting in long-lived species where size among sexually mature males varies markedly. We conducted experimental studies on long-lived eastern Australian water dragons living where competition was intense to test the hypotheses that mature males adopt alternative social tactics that are plastic, and that large size and body condition determine resource-holding potential. Approximately one-half of mature males (N = 14) defended territories using high rates of patrol and advertisement display, whereas 16 smaller mature males having lower body condition indices utilized non-territorial social tactics. Although territorial males were larger in absolute size and head dimensions, their heads were not allometrically larger. Territorial males advertised very frequently using displays involving stereotypical movements of the head and dewlap. More aggressive displays were given infrequently during baseline social conditions, but increased during periods of social instability. Female home ranges overlapped those of several territorial and non-territorial males, but females interacted more frequently with territorial males. The extreme plasticity of social tactics in this species that are dependent on body size was confirmed by two instances when relatively large non-territorial males spontaneously evicted territory owners, and by marked shifts in tactics by non-territorial males in response to temporary experimental removals of territory owners, followed (usually) by their expulsion when original owners were reinstated. The high level of social plasticity in this population where same-sex competitors are densely concentrated in preferred habitat suggests that chronic high energetic costs of defense may select for males to cycle between territorial and non-territorial social tactics depending upon their changing energetic status and their current capacity for competition with rivals.

A new heterospecific foraging association between the puddingwife wrasse,Halichoeres radiatus, and the bar jack,Caranx Tuber: evaluation of the foraging consequences

SynopsisThe bar jack,Caranx Tuber, was commonly observed to follow individual puddingwife wrasses,Halichoeres radiatus, that were foraging on the substrate. Individuals of both species actively pursued the other to maintain these heterospecific foraging ‘teams’, were sometimes attracted to feeding acts initiated by team partners, and the foraging rates of teamed jacks and wrasses were positively correlated. Pilfering of food items was rare, suggesting little, if any, competition cost of this foraging association. The ratio of bites to search in teamed jacks was over three times that when solitary, and jacks were sometimes aggressive to conspecifics attempting to join their team, suggesting that the association is beneficial to the jacks. Both bite and search rates were higher in puddingwifes when teamed with a jack, indicating that the association also benefits the wrasses. Benefits to puddingwifes may be derived directly from attendants because wrasses were sometimes attracted to jack foraging acts. However, increased foraging in wrasses may also be a consequence of heightened motivation to feed owing to heterospecific social facilitation.

Unexpected high fitness payoff of subordinate social tactics in male collared lizards

Sexual selection theory often predicts that dominant males will sire more offspring than males displaying subordinate social tactics. We combined the records of space use by collared lizard, Crotaphytus collaris, females and records of social and spatial behaviour of males displaying two markedly different social tactics (territorial and nonterritorial) with genetic determination of parentage to test how variation in male social tactics influences the distribution of reproductive success. In marked contrast with predictions based on their social and spatial behaviour, territorial males did not monopolize paternity of offspring with the females that they defended, and on average, nonterritorial males obtained reproductive success equal to that of territorial males, both within and among successive clutches. When all males were analysed together, none of the traits that are often hypothesized to promote fitness in other lizards were under strong sexual selection in collared lizard males. Among territory owners alone, however, there was positive directional selection on body size. Both the unexpected success of nonterritorial males and exceptionally high levels of multiple paternity appear linked in part to features of the habitat at our study site that diminish the ecological potential for territorial males to monopolize mating opportunities with females that reside in their defended areas. If female collared lizards derive any fitness benefits by mating with multiple males, it is likely that these are genetic rather than material.