Female aggression towards same-sex rivals depends on context in a tropical songbird.

Agonistic conflict is ubiquitous throughout taxa, although the intensity of aggression observed is often highly variable across contexts. For socially monogamous species, a coordinated effort by both pair members can improve the chances of successfully warding off challengers and reinforce pair bonds. However, the intensity of aggression exerted by any one pair member may vary with respect to contextual factors, including the intensity of their mate's aggression. Thus, experimentally exploring how individuals respond to potential rivals via multiple assays with varying social contexts can advance our basic understanding of how aggression varies in socially monogamous systems. We used simulated territorial intrusion and mirror image simulation assays to explore this issue in white-shouldered fairywrens (Malurus alboscapulatus moretoni) of Papua New Guinea. While males tended to be more responsive than females during simulated territorial intrusions, females were more aggressive towards their mirrored reflection than males. Further, individual females that were most aggressive in mirror image simulations were the least aggressive during simulated territorial intrusions, whereas males were inconsistent. These results suggest that female behavioral phenotypes appear to be flexible, relative to context. We discuss how multiple commonly used measurements of aggression might in fact measure different types of responses.

Male White-shouldered Fairywrens (Malurus alboscapulatus) elevate androgens greater when courting females than during territorial challenges.

Androgens like testosterone mediate suites of physical and behavioral traits across vertebrates, and circulation varies considerably across and within taxa. However, an understanding of the causal factors of variation in circulating testosterone has proven difficult despite decades of research. According to the challenge hypothesis, agonistic interactions between males immediately prior to the breeding season produce the highest levels of testosterone measured during this period. While many studies have provided support for this hypothesis, most species do not respond to male-male competition by elevating testosterone. As a result, a recent revision of the hypothesis ('challenge hypothesis 2.0') places male-female interactions as the primary cause of rapid elevations in testosterone circulation in male vertebrates. Here, we offer a test of both iterations of the challenge hypothesis in a tropical bird species. We first illustrate that male White-shouldered Fairywrens (Malurus alboscapulatus) differ by subspecies in plasma androgen concentrations. Then we use a social network approach to find that males of the subspecies with higher androgens are characterized by greater social interaction scores, including more time aggregating to perform sexual displays. Next, we use a controlled experiment to test whether males respond to simulated territorial intrusion and/or courtship competition contexts by elevating androgens. We found that males elevated androgens during territorial intrusions relative to flushed controls, however, males sampled during courtship competitions had greater plasma androgens both relative to controls and males sampled while defending territories. Ultimately, our results are consistent with challenge hypothesis 2.0, as sexual interactions with extra-pair females were associated with greater elevation of androgens than territorial disputes.

Correlated evolution of distinct signals associated with increased social selection in female white-shouldered fairywrens.

Conspicuous female signals have recently received substantial scientific attention, but it remains unclear if their evolution is the result of selection acting on females independently of males or if mutual selection facilitates female change. Species that express female, but not male, phenotypic variation among populations represents a useful opportunity to address this knowledge gap. White-shouldered fairywrens (Malurus alboscapulatus) are tropical songbirds with a well-resolved phylogeny where female, but not male, coloration varies allopatrically across subspecies. We explored how four distinct signaling modalities, each putatively associated with increased social selection, are expressed in two populations that vary in competitive pressure on females. Females in a derived subspecies (M. a. moretoni) have evolved more ornamented plumage and have shorter tails (a signal of social dominance) relative to an ancestral subspecies (M. a. lorentzi) with drab females. In response to simulated territorial intrusions broadcasting female song, both sexes of M. a. moretoni are more aggressive and more coordinated with their mates in both movement and vocalizations. Finally, M. a. moretoni songs are more complex than M. a. lorentzi, but song complexity does not vary between sexes in either population. These results suggest that correlated phenotypic shifts in coloration and tail morphology in females as well as song complexity and aggression in both sexes may have occurred in response to changes in the intensity of social selection pressures. This highlights increased competitive pressures in both sexes can facilitate the evolution of complex multimodal signals.

Sex role similarity and sexual selection predict male and female song elaboration and dimorphism in fairy-wrens.

Historically, bird song complexity was thought to evolve primarily through sexual selection on males; yet, in many species, both sexes sing and selection pressure on both sexes may be broader. Previous research suggests competition for mates and resources during short, synchronous breeding seasons leads to more elaborate male songs at high, temperate latitudes. Furthermore, we expect male-female song structure and elaboration to be more similar at lower, tropical latitudes, where longer breeding seasons and year-round territoriality yield similar social selection pressures in both sexes. However, studies seldom take both types of selective pressures and sexes into account. We examined song in both sexes in 15 populations of nine-fairy-wren species (Maluridae), a Southern Hemisphere clade with female song. We compared song elaboration (in both sexes) and sexual song dimorphism to latitude and life-history variables tied to sexual and social selection pressures and sex roles. Our results suggest that song elaboration evolved in part due to sexual competition in males: male songs were longer than female songs in populations with low male survival and less male provisioning. Also, female songs evolved independently of male songs: female songs were slower paced than male songs, although only in less synchronously breeding populations. We also found male and female songs were more similar when parental care was more equal and when male survival was high, which provides strong evidence that sex role similarity correlates with male-female song similarity. Contrary to Northern Hemisphere latitudinal patterns, male and female songs were more similar at higher, temperate latitudes. These results suggest that selection on song can be sex specific, with male song elaboration favored in contexts with stronger sexual selection. At the same time, selection pressures associated with sex role similarity appear to favor sex role similarity in song structure.

Sub-lethal exposure to lead is associated with heightened aggression in an urban songbird.

Many urban areas have elevated soil lead concentrations due to prior large-scale use of lead in products such as paint and automobile gasoline. This presents a potential problem for the growing numbers of wildlife living in urbanized areas as lead exposure is known to affect multiple physiological systems, including the nervous system, in vertebrate species. In humans and laboratory animals, low-level lead exposure is associated with neurological impairment, but less is known about how lead may affect the behavior of urban wildlife. We focused on the Northern Mockingbird Mimus polyglottos, a common, omnivorous North American songbird, to gain insights into how lead may affect the physiology and behavior of urban wildlife. We predicted that birds living in neighborhoods with high soil lead concentrations would (a) exhibit elevated lead concentrations in their blood and feathers, (b) exhibit lower body condition, (c) exhibit less diverse and consistent vocal repertoires, and (d) behave more aggressively during simulated conspecific territorial intrusions compared to birds living in neighborhoods with lower soil lead concentrations. Controlling for other habitat differences, we found that birds from areas of high soil lead had elevated lead concentrations in blood and feathers, but found no differences in body condition or vocal repertoires. However, birds from high lead areas responded more aggressively during simulated intrusions. These findings indicate that sub-lethal lead exposure may be common among wildlife living in urban areas, and that this exposure is associated with increased aggression. Better understanding of the extent of the relationship between lead exposure and aggression and the consequences this could have for survival and reproduction of wild animals are clear priorities for future work in this and other urban ecosystems.