Experimental evidence for female choice and energetic costs of male tail elongation in red-collared widowbirds

A reformulation of Fisher's runaway identifies the heritability of mate choices as a key parameter and highlights limitations of the hypothesis

Fisher proposed that female preference for mates with extreme traits could evolve as an essentially arbitrary outcome of a self-reinforcing process. Although Fisher's runaway has been shown to be a theoretical possibility, it is not clear whether it occurs in real populations, in part because existing models express the necessary conditions in terms of parameters that would be nearly impossible to estimate in the wild. Here, I reformulate models of the runaway in terms of two estimable parameters, the heritability and phenotypic variance of realized mate choices. Higher values of both quantities make the runaway more likely. In the most realistic model considered, in which mate choices are based on a mixture of absolute and relative criteria, a runaway cannot occur unless mate choice increases the variance of the male trait, which seems incompatible with the strong directional mating preferences typically observed in polygynous species. Even in the most favourable case for the runaway, purely relative preference without direct selection on preference, a substantial heritability of realized mate choices would be required if there is moderately strong stabilizing selection on the male trait. These results cast some doubt on whether the runaway is a plausible outcome in natural populations.

Vertically inherited microbiota and environment-modifying behaviors indirectly shape the exaggeration of secondary sexual traits in the gazelle dung beetle

Many organisms actively manipulate the environment in ways that feed back on their own development, a process referred to as developmental niche construction. Yet, the role that constructed biotic and abiotic environments play in shaping phenotypic variation and its evolution is insufficiently understood. Here, we assess whether environmental modifications made by developing dung beetles impact the environment-sensitive expression of secondary sexual traits. Gazelle dung beetles both physically modify their ontogenetic environment and structure their biotic interactions through the vertical inheritance of microbial symbionts. By experimentally eliminating (i) physical environmental modifications and (ii) the vertical inheritance of microbes, we assess the degree to which (sym)biotic and physical environmental modifications shape the exaggeration of several traits varying in their degree and direction of sexual dimorphism. We expected the experimental reduction of a larva's ability to shape its environment to affect trait size and scaling, especially for traits that are sexually dimorphic and environmentally plastic. We find that compromised developmental niche construction indeed shapes sexual dimorphism in overall body size and the absolute sizes of male-limited exaggerated head horns, the strongly sexually dimorphic fore tibia length and width, as well as the weakly dimorphic elytron length and width. This suggests that environmental modifications affect sex-specific phenotypic variation in functional traits. However, most of these effects can be attributed to nutrition-dependent plasticity in size and non-isometric trait scaling rather than body-size-independent effects on the developmental regulation of trait size. Our findings suggest that the reciprocal relationship between developing organisms, their symbionts, and their environment can have considerable impacts on sexual dimorphism and functional morphology.

Which plumage patches provide information about condition and success in a female fairy-wren?

Recent evidence suggests that female ornaments can commonly act as signals. However, how signaling functions might be affected by the tendency for reduced ornament elaboration in relation to males is less well-understood. We address this in mutually ornamented purple-crowned fairy-wrens. We investigated putatively ornamental (tail, ear coverts, crown) and non-ornamental (throat, back) plumage patches in females and compared our findings to previous studies in males. Both sexes have brown backs, buff-white throats, and turquoise-blue tails (bluer in males), while ear coverts are rufous in females and black in males. Both sexes also have a seasonal crown (slate-gray in females, black-and-purple in males). Dominant (breeder) females expressed more complete and grayer (more ornamented) crowns, although variation in coloration should not be discriminable by individuals. Unexpectedly, subordinates showed more colorful (saturated) rufous ear coverts, which should be discriminable. Condition-dependence was only evident for crown completeness (% slate-gray cover). Females with more reddish-brown backs were more reproductively successful. Variation in plumage characteristics did not explain differential allocation by mates or chances of gaining dominance. Our outcomes were not entirely consistent with findings in males. The most notable disparity was for the crown, a signal used in male-male competition that in females seems to be expressed as an incomplete version of the male crown that is not associated with fitness benefits. Our study shows that in a species, multiple traits can vary in their information content and that female ornaments can sometimes be less informative than in males, even those that are produced seasonally.

Signal detection shapes ornament allometry in functionally convergent Caribbean Anolis and Southeast Asian Draco lizards

Visual ornaments have long been assumed to evolve hyper-allometry as an outcome of sexual selection. Yet growing evidence suggests many sexually selected morphologies can exhibit other scaling patterns with body size, including hypo-allometry. The large conspicuous throat fan, or dewlap, of arboreal Caribbean Anolis lizards was one ornament previously thought to conform to the classical expectation of hyper-allometry. We re-evaluated this classic example alongside a second arboreal group of lizards that has also independently evolved a functionally equivalent dewlap, the Southeast Asian Draco lizards. Across multiple closely related species in both genera, the Anolis and Draco dewlaps were either isometric or had hypo-allometric scaling patterns. In the case of the Anolis dewlap, variation in dewlap allometry was predicted by the distance of conspecifics and the light environment in which the dewlap was typically viewed. Signal efficacy, therefore, appears to have driven the evolution of hypo-allometry in what was originally thought to be a sexually selected ornament with hyper-allometry. Our findings suggest that other elaborate morphological structures used in social communication might similarly exhibit isometric or hypo-allometric scaling patterns because of environmental constraints on signal detection.

Meta-analysis reveals that animal sexual signalling behaviour is honest and resource based

Animals often need to signal to attract mates and behavioural signalling may impose substantial energetic and fitness costs to signallers. Consequently, individuals often strategically adjust signalling effort to maximize the fitness payoffs of signalling. An important determinant of these payoffs is individual state, which can influence the resources available to signallers, their likelihood of mating and their motivation to mate. However, empirical studies often find contradictory patterns of state-based signalling behaviour. For example, individuals in poor condition may signal less than those in good condition to conserve resources (ability-based signalling) or signal more to maximize short-term reproductive success (needs-based signalling). To clarify this relationship, I systematically searched for published studies examining animal sexual signalling behaviour in relation to six aspects of individual state: age, mated status, attractiveness, body size, condition and parasite load. Across 228 studies and 147 species, individuals (who were predominantly male) invested more into behavioural signalling when in good condition. Overall, this suggests that animal sexual signalling behaviour is generally honest and ability-based. However, the magnitude of state-dependent plasticity was small and there was a large amount of between-study heterogeneity that remains unexplained.

The peacock train does not handicap cursorial locomotor performance

Exaggerated traits, like the peacock train, are recognized as classic examples of sexual selection. The evolution of sexual traits is often considered paradoxical as, although they enhance reproductive success, they are widely presumed to hinder movement and survival. Many exaggerated traits represent an additional mechanical load that must be carried by the animal and therefore may influence the metabolic cost of locomotion and constrain locomotor performance. Here we conducted respirometry experiments on peacocks and demonstrate that the exaggerated sexually selected train does not compromise locomotor performance in terms of the metabolic cost of locomotion and its kinematics. Indeed, peacocks with trains had a lower absolute and mass specific metabolic cost of locomotion. Our findings suggest that adaptations that mitigate any costs associated with exaggerated morphology are central in the evolution of sexually selected traits.

Diurnal variation in corticosterone release among wild tropical forest birds

BACKGROUND

Glucocorticoids are adrenal steroid hormones essential to homeostatic maintenance. Their daily variation at low concentrations regulates physiology and behavior to sustain proper immunological and metabolic function. Glucocorticoids rise well above these baseline levels during stress to elicit emergency-state responses that increase short-term survival. Despite this essence in managing life processes under both regular and adverse conditions, relationships of glucocorticoid release to environmental and intrinsic factors that vary at daily and seasonal scales are rarely studied in the wild.

METHODS

This study on 41 passerine species of the Ecuadorian Chocó applied a standardized capture-and-restraint protocol to examine diurnal variation in baseline and stress-related release of corticosterone, the primary avian glucocorticoid. Tests for relationships to relative body mass, hemoglobin concentration, molt status and date complemented this evaluation of the time of day effect on corticosterone secretion in free-living tropical rainforest birds. Analyses were also partitioned by sex as well as performed separately on two common species, the wedge-billed woodcreeper and olive-striped flycatcher.

RESULTS

Interspecific analyses indicated maximum baseline corticosterone levels at the onset of the active phase and reductions thereafter. Stress-related levels did not correspond to time of day but accompanied baseline reductions during molt and elevations in birds sampled later during the September - November study period. Baseline corticosterone related negatively to hemoglobin in the wedge-billed woodcreeper and stress-related levels increased with body mass in the olive-striped flycatcher. There were no substantial sex-related differences.

CONCLUSIONS

The results of this study suggest a diurnal rhythmicity in baseline corticosterone release so robust as to emerge in pooled analyses across a highly variable dataset. While this detection in nature is singular, correspondent patterns have been demonstrated outside of the tropics in captive model species. Congruity in daily rhythms and links to physiological and life-history state across disparate taxa and environments may promote the yet unresolved utility of corticosterone release as a global metric for population health. However, certain results of this study also deviate from laboratory and field research at higher latitudes, cautioning generalization. Environmental distinctions such as high productivity and tempered seasonality may precipitate unique life-history strategies and underlying hormonal mechanisms in tropical rainforest birds.

Ecology and caudal skeletal morphology in birds: the convergent evolution of pygostyle shape in underwater foraging taxa

Birds exhibit a specialized tail that serves as an integral part of the flight apparatus, supplementing the role of the wings in facilitating high performance aerial locomotion. The evolution of this function for the tail contributed to the diversification of birds by allowing them to utilize a wider range of flight behaviors and thus exploit a greater range of ecological niches. The shape of the wings and the tail feathers influence the aerodynamic properties of a bird. Accordingly, taxa that habitually utilize different flight behaviors are characterized by different flight apparatus morphologies. This study explores whether differences in flight behavior are also associated with variation in caudal vertebra and pygostyle morphology. Details of the tail skeleton were characterized in 51 Aequornithes and Charadriiformes species. Free caudal vertebral morphology was measured using linear metrics. Variation in pygostyle morphology was characterized using Elliptical Fourier Analysis, a geometric morphometric method for the analysis of outline shapes. Each taxon was categorized based on flight style (flap, flap-glide, dynamic soar, etc.) and foraging style (aerial, terrestrial, plunge dive, etc.). Phylogenetic MANOVAs and Flexible Discriminant Analyses were used to test whether caudal skeletal morphology can be used to predict flight behavior. Foraging style groups differ significantly in pygostyle shape, and pygostyle shape predicts foraging style with less than 4% misclassification error. Four distinct lineages of underwater foraging birds exhibit an elongate, straight pygostyle, whereas aerial and terrestrial birds are characterized by a short, dorsally deflected pygostyle. Convergent evolution of a common pygostyle phenotype in diving birds suggests that this morphology is related to the mechanical demands of using the tail as a rudder during underwater foraging. Thus, distinct locomotor behaviors influence not only feather attributes but also the underlying caudal skeleton, reinforcing the importance of the entire caudal locomotor module in avian ecological diversification.

Signaling efficacy drives the evolution of larger sexual ornaments by sexual selection

Why are there so few small secondary sexual characters? Theoretical models predict that sexual selection should lead to reduction as often as exaggeration, and yet we mainly associate secondary sexual ornaments with exaggerated features such as the peacock's tail. We review the literature on mate choice experiments for evidence of reduced sexual traits. This shows that reduced ornamentation is effectively impossible in certain types of ornamental traits (behavioral, pheromonal, or color-based traits, and morphological ornaments for which the natural selection optimum is no trait), but that there are many examples of morphological traits that would permit reduction. Yet small sexual traits are very rarely seen. We analyze a simple mathematical model of Fisher's runaway process (the null model for sexual selection). Our analysis shows that the imbalance cannot be wholly explained by larger ornaments being less costly than smaller ornaments, nor by preferences for larger ornaments being less costly than preferences for smaller ornaments. Instead, we suggest that asymmetry in signaling efficacy limits runaway to trait exaggeration.