Male White-shouldered Fairywrens (Malurus alboscapulatus) elevate testosterone when courting females but not during territorial challenges

Testosterone mediates 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 testosterone concentrations. Then we use a social network approach to find that males of the subspecies with higher testosterone 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 or courtship interaction contexts by elevating testosterone. Males sampled during courtship had greater plasma testosterone both relative to flushed controls and males sampled during simulated intrusion. Ultimately, our results are consistent with challenge hypothesis 2.0, as males rapidly elevated testosterone following interactions with females, but not during territorial challenges.

The communicative value of complex singing in passerine birds

Birdsong is one of the most complex signals in the animal world, as it may consist of many different sounds grouped according to certain rules. Singing acts as a distant signal, indicating, e.g., the species and gender identity of the singer. However, territorial songbirds also use singing as an interactive social signal during territorial disputes, as well while interacting with female. In these contexts, males vary the type and timing of their songs to convey graded information about their motivational state, and those variations can play a role in communication. In this review, we considered how male songbirds vary their singing in territorial context. To study such variations, researchers usually simulated territorial intrusion by broadcasting conspecific singing in territories, including singing modified in a manner necessary for the researcher. For comparison, we considered briefly how singing vary in intersexual context. The author of the paper focuses on the role of singing complexity in communication. Therefore, not all known context-dependent changes in singing are considered, but only those related to “complexity”: the diversity of song/sound types and the transitional patterns of different song/sound types in the course of singing. Our review has shown that males change their singing when they detect environmental changes such as the appearance of a female or a competitor as follows: 1) song rate increases, 2) syllable rate increases, 3) song-type switching rate increases, 4) song-type diversity increases (i.e., the observed repertoire size), and 5) longer and more complex songs are predominantly used. In some species, the song bout organization may also change, but the data is still scarce. Typically, one or more, but not all the aforementioned acoustic behaviors have been found in a given song-bird species. All these behaviors (tactics) come down to a single strategy, namely: maximizing the acoustic diversity over a short period of time (e.g., several minutes), that is, increasing the number of different song and/or note types. The proximate causes of how the increased acoustic diversity work in the territorial competition context might lie in a sensory, or perceptual bias of the receiver. Namely, habituation should occur to repeated presentation of the same song type faster than to presentation of different song types. Therefore, by vocalizing more diversely, males more effectively influence the signal recipient’s behavior.

Territorial Responses of Nuthatches Sitta europaea—Evaluation of a Robot Model in a Simulated Territorial Intrusion

In birds, aggressive behavior can be elicited in the field with a simulated territory intrusion (STI). Here, we compared four different STI methods in nuthatches in the non-breeding season: playback only, playback combined with an inactive wooden model mounted on a robot device, playback and an active model mounted on a robot device, and playback with the robot device only. In the control treatment, birds were not exposed to STI. Experiments were carried out in 12 territories. Behavioral observations included latency to approach, latency to start calling, time spent in the same tree, number of flyovers, minimum distance, number of individuals, number of flights into the tree, and number of calls. There was no significant influence of stimulus presentation. Nuthatches responded equally to all four experimental manipulations, but order of presentation had a strong influence.

Defending as a unit: sex- and context-specific territorial defence in a duetting bird

Behaviours such as territorial defence represent functionally integrated traits that underlie multiple behavioural variables such as physical and acoustic responses. Characterizing the multivariate structure of such traits is fundamental to understand their evolution. In bird species that form stable pair bonds and are territorial year-round, both sexes are expected to defend their territory; however, the role that each sex plays in defending their shared territory remains largely unknown. Evidence for the sex-roles during territorial defence is mixed and sex- and context-specific characterizations of territorial defence embracing the multivariate nature of the trait are currently lacking. Here we investigated sex- and context-specific variation in a hypothesised latent variable called “territorial defence” and tested whether duets were part of territorial defence in a wild population of rufous hornero (Furnarius rufus). To do so, we combined a simulated territorial intrusion approach during nest building and provisioning contexts with a structural equation modelling approach. Our results showed that, in males and females, the six measured behavioural variables were linked by a single latent trait, territorial defence, in both contexts. Flights over the decoy and duet songs were equally good proxies of territorial defence. Although males were defending more the territory than females, pair members showed a positive correlation in their behaviour. The structural equation modelling framework enabled us to capture a complex correlation pattern among behavioural variables, expanding upon a classic body of research on territorial defence. Thus, the combination of classical behavioural approaches with sophisticated statistical analyses brings new exciting possibilities to the field of behavioural ecology.Significance statementTerritorial defence is a key behaviour in territorial species as it plays a major role in an individual’s reproductive success and survival. Additionally, territorial defence has been proposed as one possible evolutionary driver of duetting behaviour, one of the most fascinating vocal behaviours in birds. As behaviours are evolutionary characters, they must be studied in a multivariate framework. In this study we focused on characterizing territorial defence during a simulated territorial intrusion in an integrative manner using a classical territorial intrusion framework. We did so in male and female rufous horneros (Aves: Furnaridae) across two breeding contexts, while simultaneously testing theoretical predictions about the role of duetting behaviour as key part of territorial defence. Overall, our study provides for the first time a sex- and context-comparison of the multivariate, latent variable “territorial defence” in duetting birds, while highlighting the potential of combining field behavioural approaches with structural equation modelling.

Great tits in noisy territories and avoiding overlapping respond stronger to territorial intruders

Animals often communicate with each other in noisy environments where interference from the ambient noise and other signallers may reduce the effectiveness of signals. Signalling behaviours may also evolve to interfere with signals of their opponents, e.g. by temporally overlapping them with their own, such as the song overlapping behaviour that is seen in some songbirds during aggressive interactions. Song overlapping has been proposed to be a signal of aggressive intent, but few studies directly examined the association between song overlapping and aggressive behaviours of the overlapping bird (the predictive criterion). In the present paper we examined the question of whether song overlapping is correlated with aggressive behaviours displayed during a simulated territorial intrusion in a population of great tits (Parus major) living in an urban-rural gradient. We also examined whether aggressive behaviours are correlated with the ambient noise levels. We found that overlapping was associated negatively with aggressive behaviours males displayed against a simulated intruder. These results fail to support the predictive criterion for song overlapping, raising the question whether overlapping is in fact a signal of aggressive intent. Ambient noise levels were associated positively with aggressive behaviours but did not correlate with song rate, song duration or song overlapping. Great tits in noisy urban habitats may display higher levels of aggressive behaviours due to either interference of noise in aggressive communication or another indirect effect of noise.

Not one hormone or another: Aggression differentially affects progesterone and testosterone in a South American ovenbird

Behaviors such as territorial interactions among individuals can modulate vertebrate physiology and vice versa. Testosterone has been pointed out as a key hormone that can be rapidly affected by aggressive interactions. However, experimental evidence for such a link is mixed. In addition, behaviors can elicit changes in multiple hormones, which in turn have the potential to synergistically feedback to behavior. For example testosterone and progesterone can act interdependently in modulating male behavior. However, if aggression can affect progesterone levels in males remain unknown and – to the best of our knowledge – no one has yet tackled if and how aggressive behavior simultaneously affects testosterone and progesterone in free-living animals. We addressed these questions by performing simulated territorial intrusion experiments measuring both hormones and their ratio in male rufous horneros (Aves, Furnarius rufus) during the mating and parental care periods. Aggression affected testosterone and progesterone differentially depending on the period of testing: challenged birds had higher levels of progesterone during the mating period and lower levels of testosterone during parental care compared to controls. Challenged individuals had similar progesterone to testosterone ratios during both periods and these ratios were higher than those of control birds. In summary, territorial aggression triggered hormonal pathways differentially depending on the stage of the breeding cycle, but equally altered their ratio independent of it. Our results indicate that multiple related hormones could be playing a role rather than each hormone alone in response to social interactions.

Ignoring the challenge? Male black redstarts ( Phoenicurus ochruros ) do not increase testosterone levels during territorial conflicts but they do so in response to gonadotropin-releasing hormone

Competition elevates plasma testosterone in a wide variety of vertebrates, including humans. The ‘challenge hypothesis’ proposes that seasonal peaks in testosterone during breeding are caused by social challenges from other males. However, during experimentally induced male–male conflicts, testosterone increases only in a minority of songbird species tested so far. Why is this so? Comparative evidence suggests that species with a short breeding season may not elevate testosterone levels during territory defence. These species may even be limited in their physiological capability to increase testosterone levels, which can be tested by injecting birds with gonadotropin-releasing hormone (GnRH). We studied two populations of black redstarts that differ in breeding altitude, morphology and the length of their breeding season. Unexpectedly, males of neither population increased testosterone in response to a simulated territorial intrusion, but injections with GnRH resulted in a major elevation of testosterone. Thus, black redstarts would have been capable of mounting a testosterone response during the male–male challenge. Our data show, for the first time, that the absence of an androgen response to male–male challenges is not owing to physiological limitations to increase testosterone. Furthermore, in contrast to comparative evidence between species, populations of black redstarts with a long breeding season do not show the expected elevation in testosterone during male–male challenges.

Free-Living Male Mountain White-Crowned Sparrows Exhibit Territorial Aggression Without Modulating Total or Free Plasma Testosterone

Abstract In some species, expression of territorial aggression is accompanied by a rise in testosterone secretion, but in others aggressive behavior is expressed while testosterone levels remain unchanged. Corticosteroid binding globulin (CBG) binds both corticosterone and testosterone in avian plasma. Thus, increasing corticosterone may result in fluctuations in unbound (“free”) testosterone; this could result in greater biological activity of testosterone without an increase in testosterone secretion. We investigated whether such plasma interactions of testosterone, corticosterone, and CBG might result in alterations of free testosterone in male Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha). We conducted simulated territorial intrusions during incubation and compared total and free testosterone of males captured immediately following a simulated territorial intrusion with that of males captured passively. All experimental males showed aggressive behavior, but apparently did not modulate total or free testosterone relative to controls.