Evolutionary costs of aggression revealed by testosterone manipulations in free-living male lizards

Summary We examined the hypothesis that increased aggression results in decreased survivorship. We tested this hypothesis by increasing aggression of free-living male lizards Sceloporus jarrovi with testosterone implants and evaluating the effects on survivorship. A previous study showed that testosterone-implanted males were more aggressive than controls, suggesting a greater degree of success in male-male competition. Results of the present study show that the same testosterone-implanted lizards experienced greater mortality. Testosterone-treated males were also seen more frequently and more conspicuous ones were less likely to survive. Testosterone-treated males lost more weight over the summer. In controls, survivorship was negatively correlated with the body weight index. These data suggest that conspicuousness and energetic demands interact in their influence on survivorship. Thus, the natural plasma level of testosterone may be at an optimal level balancing any potential selection pressures for a higher level of testosterone through sexual selection with selection pressures for a lower level through a decrease in survivorship.     

Thermoregulatory cost affects territorial behavior in hummingbirds: a model and its application

A common assumption in behavioral ecology is that the valuation of a resource by consumers depends on the energetic value of the resource itself. Nevertheless, the value of a resource may be relative to the condition of the organism, which is in turn related to the abiotic conditions such as ambient temperature. We developed a theoretical model—incorporating these untested assumptions—to predict a functional relationship between territorial aggression and ambient temperature for individuals sensitive to daily variations in energy availability. We evaluated our theoretical predictions against a field experiment carried out with the hummingbird Sephanoides sephaniodes. The model predicted a quadratic relation between aggression intensity and ambient temperature. Field data were better explained by a quadratic equation than a linear function, suggesting the existence of lower and upper thresholds of temperature which determine the intensity of territorial defense. Ambient temperature affects energy expenditure for thermoregulation, and therefore, it fixes the benefit level that must be produced by the territory to pay the costs of its defense. Our findings strongly suggest that abiotic conditions can change an animal evaluation of the yield of a resource and in turn influence the behavioral strategy which it adopts.     

Social,environmental and genetic factors in the ontogeny of phenotypic differentiation in a lizard with alternative male reproductive strategies

Summary Adult male tree lizards, Urosaurus ornatus, practise alternative (territorial or sneaker/satellite) reproductive strategies that are correlated with differences in throat color and body size. In this study we raised tree lizards from hatching in the laboratory to examine the question of whether the phenotypic expression of secondary sex coloration and body size can be facultatively influenced by social or abiotic environmental factors. We compared males reared in the laboratory under different social and environmental conditions to males in the field and found no effect of different conditions on phenotypic differentiation (Figs. 2–4). Thus, phenotypic differences between morphs probably result largely from nonfacultative expression of different genotypes. This suggests that alternative male morphs practise a mixed evolutionary stable strategy (ESS) rather than one morph making the best of a bad situation. However, in the context of ESS theory it is difficult to explain our further result that the nonterritorial morph in this species grows faster and reaches a larger adult body size than the territorial morph (Fig. 5).     

The economics of harem maintenance in the sac-winged bat, Saccopteryx bilineata (Emballonuridae)

Saccopteryx bilineata has a polygynous mating system in which males defend females in a harem territory. Harem defense and courtship include energetically costly flight maneuvers and hovering displays. We tested if (1) harem males have a greater field metabolic rate than non-harem males or females and if (2) the field metabolic rate of harem males is correlated with the number of females in a harem territory. We measured the energy budget in 32 S. bilineata with the doubly labeled water method and compared these estimates with behavioral observations in the daytime roost. Among adult bats, field metabolic rate varied with body mass by an exponent of approximately two. We found no significant difference in field metabolic rate or mass-specific field metabolic rate between harem and non-harem males. The mass-specific field metabolic rate of harem-males increased with harem size. The latter finding supports the hypothesis that the energy costs of courtship display and territorial defense influence the energy budget of harem males. Overall, field metabolic rates of S. bilineata were lower than those of similarly sized bats of the temperate zone and only 2.3 times above the basal metabolic rate recorded for this species. We suggest that male S. bilineata did not take advantage of their metabolic capacity because a prudent allocation of energy to activities of harem maintenance is an adaptive strategy for males in this mating system.     

Individually variable energy management strategies in relation to energetic costs of egg production

Marked interindividual variation in metabolic rate suggests considerable complexity in energy management strategies, but attempts to further our understanding of the relationship between resting metabolic rate (RMR), daily energy expenditure (DEE), and reproductive effort have been hampered by the complexity of studying this system in the field. Here, we describe energy management strategies in a captive-breeding system, using Zebra Finches (Taeniopygia guttata), to demonstrate the high level of complexity and interindividual variability in energy expenditure, food intake, locomotor activity, and reproductive effort. In particular, we investigated whether the increase in RMR associated with egg production is additive, resulting in higher DEE and a need for elevated food intake, or whether this cost is compensated by reduced expenditure in nonreproductive components of the energy budget. We found high levels of intra-individual variation in energy expenditure associated with egg production in female Zebra Finches, e.g., comparing nonbreeding stage with the one-egg stage, change in RMR varied from 4.0% and 41.3%, and change in DEE varied from -33.3% to +46.4%. This variation was systematically related to aspects of locomotor activity and reproductive effort. Females with the largest increase in RMR during egg production decreased locomotor activity the most but still had increased DEE at the one-egg stage, and females with high DEE at the one-egg stage produced larger clutches. Our study suggests that females minimize increases in DEE during egg production through behavioral energy reallocation (reduced locomotor activity) but that individuals differ in their use of this strategy, which, in turn, is related to the absolute level of reproductive investment. This suggests a very complex, individually variable system of energy management to meet the demands of egg production.     

Sympatric sexual signal divergence among North American Calopteryx damselflies is correlated with increased intra- and interspecific male–male aggression

Divergence of sexual signals in sympatry can arise as a consequence of (1) interspecific competition for resources, (2) selection against maladaptive hybridization, or (3) as a result of selection to reduce the cost of interspecific aggression; termed agonistic character displacement (ACD). Calopterygid damselflies have emerged as a model system for studying the evolution of divergent sexual signals due to the repeated evolution of sympatric species pairs with fully and partially melanized wings. Damselfly wing patterns function during both courtship and territory defense. However, the relative contributions of natural and sexual selection to phenotypic divergence and enhanced isolation in sympatry remain unclear in many cases. Here, we investigated the hypothesis that interference competition, in the form of increased interspecific male–male aggression, drives the evolution of character displacement in sympatry between two species of North American damselflies, Calopteryx aequabilis and Calopteryx maculata, that show no evidence of ecological divergence or ongoing hybridization. In paired behavioral trials, we found that interspecific male aggression related to territory defense varied between site, species, and as a function of the relative abundance of con- vs. hetero-specific males. Specifically, we found that large-spotted C. aequabilis males received increased intra- and interspecific aggression but that aggression against large-spotted males declined during the middle of the flight season when both species were equally abundant. Based on these results, we suggest that ACD leads to enhanced species recognition, and may be a common outcome of the antagonism between interspecific male–male competition and the countervailing force of intraspecific sexual selection favoring increased wing melanization among territorial damselfly species.     

Ectoparasite loads in free-ranging northern fence lizards, Sceloporus undulatus hyacinthinus: effects of testosterone and sex

More knowledge of the proximate factors that influence parasite loads would help us understand the selective pressures faced by hosts and host-parasite evolution. Testosterone has been associated with increased parasite loads in vertebrates. Here we asked whether experimentally elevated testosterone affected ectoparasite loads in free-ranging northern fence lizards (Sceloporus undulatus hyacinthinus). Males were captured, given testosterone or sham implants, and released. In 2 consecutive years, testosterone-implanted males had significantly more ectoparasites at recapture than did controls. Additionally, ectoparasite loads were positively correlated with testosterone concentrations in unmanipulated males, and males had significantly more ectoparasites than did females. The results are consistent with an effect of testosterone on parasite loads. However, rather than elevated testosterone increasing mite loads in experimental males, it appeared that high testosterone inhibited a natural seasonal decline in mite loads. Testosterone-implanted males also lost body mass whereas controls gained mass. Among controls, those retaining the most ectoparasites over the course of the experiment experienced the smallest gains in body mass, suggesting that the mites are costly.     

Dominance and competition in an herbivorous lizard

Summary A study of social organization in an herbivorous lizard Ctenosaura hemilopha investigated the role of dominance in group member behavior. Attention focused on a very populous (16 ind.) colony to examine causes and effects of crowding with respect to competitive factors. Lizards were colonial, with a top-rank male, one or more adult females, and various subadults and juveniles. Top-rank males defended harems, with colonies female-biased (1:4 in focal colony). Dominance hierarchy was observed, with females more aggressive than males, except for the top-rank male. Group awareness facilitated adaptive responses to threatening dominants and predators. Strong correlations existed among individual size, rank, and aggression. Top-rank male aggressiveness was partly explained by harem defense. Food resource competition, which causes aggression in female insectivorous lizards, did not explain female C. hemilopha behavior. Response to predators, predator fecal pellet analysis, and tail break frequencies implicate crevice escape sites for predator avoidance as a prime controller of social and population structure in these lizards.     

Territorial defense in the bannertail kangaroo rat (Dipodomys spectabilis): footdrumming and visual threats

Summary Footdrumming and visual displays associated with territorial defense in the bannertail kangaroo rat (Dipodomys spectabilis) were studied in the field in Arizona. We predicted that foot-drumming functioned like bird song in territorial advertisement. Rats were tested in their territories for their responses to the presentation of four different stimuli: a playback of airborne footdrumming patterns (audio); a stuffed and mounted kangaroo rat in an aggressive posture (visual); a combination of both (audio-visual); and a playback of cricket stridulation as a control. Kangaroo rats investigated all experimental stimuli equally by becoming alert, approaching, and retreating (Fig. 2). Rats, therefore, seemed to assess the amount of threat and footdrummed at higher rates and sooner in response to the more threatening visual stumulus of a stuffed rat than to the apparently less threatening ones of a airborne footdrumming playback or cricket stridulation (Fig. 1). We conclude that rats footdrum at long ranges to repel a potential intruder and also at close ranges when territories are threatened by a persistent intruder. Chase and attack, as in territorial birds, are the final lines of defense.     

Significance of spectral and temporal call parameters in the auditory communication of male grass frogs

Summary During the spawning period, male grass frogs (Rana temporaria L.) frequently produce short and long territorial calls in addition to mating calls. The calls differ in mean pulse number, duration, and pattern of amplitude modulation. Experiments in which recorded natural calls are played back reveal that male grass frogs are capable of discriminating the different conspecific calls. A male frog stimulated by mating calls always responds by producing mating calls in greater numbers (Fig. 3). Territorial calls presented at low intensity also cause an increase in the mating-call rate (Fig. 4), but at high intensity they clicit territorial calls and turning toward the loudspeaker. A combination of short and long territorial calls was especially effective in eliciting the phonotaxis response. As play-back experiments with simulated calls show, the carrier frequency and the pulse repetition rate are particularly important cues for recognition of conspecific calls (Fig. 5). A simulated call with a 400-Hz carrier frequency (the dominant frquency of the mating call) is just as effective as the natural call with the complete frequency spectrum (Fig. 3), whereas a 1100-Hz simulated call is ineffective (Fig. 5). The chief factors in discrimination among the conspecific calls are the call repetition rate and probably the amplitude and frequency modulation. Changes in the duration of the calls had little effect (Fig. 6). The available evidence suggests that the mating call has a reciprocally stimulating action on males in a chorus, whereas the territorial calls experess aggressiveness and give warning to other males.