Soft Song in Song Sparrows: Acoustic Structure and Implications for Signal Function

In many species of songbirds, males sometimes produce songs at distinctly lower amplitude than in normal singing. Depending on the species, these 'soft songs' may be sung in the context of female courtship, male–male aggression, or both. In song sparrows, males produce soft songs during aggressive interactions with other males, and the amount of soft song produced is the only singing behavior that can be used to reliably predict a subsequent attack by the singer. Although soft song is clearly an important signal in this species, little is known about the acoustic structure of soft song or about how that structure compares to the structure of normal 'broadcast song'. We recorded a large sample of soft songs and broadcast songs from 10 male song sparrows, and measured song amplitudes in the field while controlling the subject's distance to a calibrated microphone. We show that song sparrow males produce songs over a wide range of amplitudes, with soft songs in the range of 55–77 dB sound pressure level and broadcast songs in the range of 78–85 dB. We present evidence for two types of soft song: 'crystallized' soft songs that are broadcast repertoire song types sung at low amplitude, and 'warbled' soft songs that are not found in the broadcast repertoire. Although highly variable, warbled soft songs produced by individual birds could be grouped into song types based on spectrographic similarity. To our knowledge, a distinct repertoire of soft song types has not been previously reported for any songbird.     

Aggressive and Foraging Behavioral Interactions Among Ruffe

The ruffe, Gymnocephalus cernuus, is a nonindigenous percid in the Great Lakes. Ruffe are aggressive benthivores and forage over soft substrates. Laboratory studies in pools (100cm diameter, 15cm water depth) were conducted to determine whether fish density (low=2, medium=4, high=6 ruffe per pool) changed foraging and aggressive behaviors with a limited food supply of chironomid larvae. All fish densities demonstrated a hierarchy based on aggressive interactions, but ruffe were most aggressive at low and high fish densities. Time spent in foraging was lowest at the low fish density. The best forager at the low fish density was the most aggressive individual, but the second most aggressive fish at the medium and high fish density was the best forager and also the one chased most frequently. A medium fish density offered the best energetic benefits to ruffe by providing the lowest ratio of time spent in aggression to that spent foraging. Based on our results, ruffe should grow best at an intermediate density. With high ruffe densities, we would also expect disparity in size as the more aggressive fish are able to garner a disproportionate amount of the resources. Alternatively, as the Great Lakes are a fairly open system, ruffe could migrate out of one area to colonize another as populations exceed optimal densities.     

Thermal Behaviour of Honeybees During Aggressive Interactions

We report here on the interrelationship of aggressive behaviour and thermoregulation in honeybees. Body temperature measurements were carried out without behavioural disturbance by infrared thermography. Guard bees, foragers, drones, and queens involved in aggressive interactions were always endothermic, i.e. had their flight muscles activated. Guards made differential use of their endothermic capacity. Mean thorax temperature was 34.2–35.1°C during examination of bees but higher during fights with wasps (37°C) or attack of humans (38.6°C). They usually cooled down when examining bees whereas examinees often heated up during prolonged interceptions (maximum >47°C). Guards neither adjusted their thorax temperature (and thus flight muscle function and agility) to that of examined workers, nor to that of drones, which were 2–7°C warmer. Guards examined cool bees (<33°C) longer than warmer ones, supporting the hypothesis that heating of examinees facilitates odour identification by guards, probably because of vapour pressure increase of semiochemicals with temperature. Guards in the core of aggressive balls clinged to the attacked insects to fix them and kill them by heat (maximum 46.5°C). Bees in the outer cluster layers resembled normal guards behaviourally and thermally. They served as active core insulators by heating up to 43.9°C. While balled wasps were cooler (maximum 42.5°C) than clinging guards balled bees behaved like examinees with maximum temperatures of 46.6°C, which further supports the hypothesis that the examinees heat up to facilitate odour identification.     

Female House Wrens may Use a Low‐Amplitude Call as an Aggressive Signal

The majority of work on aggressive signaling has focused on male–male contests. Although females in many species compete over important resources, female signals are understudied. In house wrens (Troglodytes aedon), females compete with other females to protect nesting cavities and eggs. We suggest that a high‐pitched, low‐amplitude call, hereafter abbreviated as ‘HI’, may be an aggressive signal used by female house wrens. Using playback of simulated female intruders, we found that females used HI calls significantly more often during playback of female house wrens compared to playback of heterospecifics. Additionally, HI calls were given significantly more often in the minute preceding physical attack. In comparison, song rates did not predict future attacks. Finally, we present pilot data that suggest listening females may respond more aggressively to female playback containing HI calls compared to playback containing only songs. This suggests the reliability of HI calls could be maintained by a receiver retaliation rule. HI calls bear a striking resemblance to the low‐amplitude songs of many male songbirds, in terms of both acoustic structure and social context. This is one of the few reports of a putative low‐amplitude aggressive signal in a female songbird. However, the nature of female competition may make low‐amplitude signaling an underappreciated signaling form in female animals.     

Aggressive Interactions and Risk of Fish Predation for Larval Damselflies

Larval damselflies frequently engage in aggressive interactions that may increase their risk of fish predation. To test this we analyzed the behavior of larval Ischnura verticalis exposed to both conspecifics and fish predators. Larvae in the presence of conspecifics oriented, struck, and swam more but crawled less compared to solitary larvae; the presence of fish reduced, or tended to reduce, all behaviors. Fish struck more at interacting larvae compared to noninteracting larvae. Increased attack rate by fish likely reflects the increase in the very active swimming behavior by larvae and suggests a conflict between antipredator behaviors. Swimming is an appropriate response to avoid predation by odonate larvae which normally ambush prey but is clearly dangerous when fast-swimming fish that cue in on movement are nearby.     

Radiation Interception, Partitioning and Use in Grass -Clover Mixtures

Mixed swards of perennial ryegrass /white clover were grownin competition under controlled environmental conditions, attwo temperatures and with different inorganic nitrogen supplies.The swards were studied after canopy closure, from 800 to 1200°C d cumulative temperatures. Clover contents did not varysignificantly during the period. A simulation model of lightinterception was used to calculate light partitioning coefficientsand radiation use efficiencies for both components of the mixturein this controlled environment experiment. Additionally, thissame radiative transfer model was applied to the field datafrom Woledge (1988) (Annals of Applied Biology112: 175 –186)and from Woledge, Davidson and Dennis (1992) (Grass and ForageScience47: 230 –238). The measured and simulated valuesof light transmission, at different depths in the mixed canopy,were highly correlated (P<0.001) with more than 80% of thetotal variance explained. The daily average of photosyntheticallyactive radiation (PAR) interception in a natural environmentwas estimated from simulations, for the field and controlledenvironment data. Under these conditions, white clover capturedsignificantly more light per unit leaf area than perennial ryegrassat low, but not at high, nitrogen supply. In the controlled environment experiment, the radiation useefficiency of the legume was lower than that of its companiongrass. For both species, radiation use efficiency was negativelycorrelated with the mean irradiance of the leaf. The role ofa compensation between light interception and light use forstabilizing the botanical composition of dense grass –cloverswards is discussed. Light interception; radiation transfer model; growth analysis; radiation use efficiency; white clover; perennial ryegrass; Trifolium repensL.; Lolium perenneL.; grassland     

Hiding Behaviour of Long-tailed Macaques (Macaca fascicularis): II. Use of Hiding Places during Aggressive Interactions

In an observational study on captive long-tailed macaques ( Macaca fascicularis ), the ability to hide was investigated in the social context of aggression. The use of five different types of hiding place during highly aggressive interactions was recorded and the choices of six different rank-sex classes were compared using an index of attractiveness of the different types of hiding place during highly aggressive interactions. There was no significant difference between the rank-sex classes in selection of the hiding places that could be connected with visibility: no specific location or type was preferred, either by the group as a whole or by any specific rank-sex class. Thus long-tailed macaques do not seem to hide shortly after aggressive interactions.     

Single Aggressive Interactions Increase Urinary Glucocorticoid Levels in Wild Male Chimpanzees

A basic premise in behavioural ecology is the cost-benefit arithmetic, which determines both behavioural decisions and evolutionary processes. Aggressive interactions can be costly on an energetic level, demanding increased energy or causing injuries, and on a psychological level, in the form of increased anxiety and damaged relationships between opponents. Here we used urinary glucocorticoid (uGC) levels to assess the costs of aggression in wild chimpanzees of Budongo Forest, Uganda. We collected 169 urine samples from nine adult male chimpanzees following 14 aggressive interactions (test condition) and 10 resting events (control condition). Subjects showed significantly higher uGC levels after single aggressive interactions compared to control conditions, likely for aggressors as well as victims. Higher ranking males had greater increases of uGC levels after aggression than lower ranking males. In contrast, uGC levels showed no significant change in relation to aggression length or intensity, indicating that psychological factors might have played a larger role than mere energetic expenditure. We concluded that aggressive behaviour is costly for both aggressors and victims and that costs seem poorly explained by energetic demands of the interaction. Our findings are relevant for studies of post-conflict interactions, since we provide evidence that both aggressors and victims experience a stress response to conflict.     

Previous Experiences Alter the Outcome of Aggressive Interactions Between Males in the Crayfish, Procambarus Clarkii

Various measures of size have been associated with increased likelihood of winning such agonistic encounters between decapod crustaceans. Recent social experience, in the form of 'winner' or 'loser' effects, has also been shown to alter the outcome of encounters between size-matched individuals. This study further explores the importance of social experience in crustacean agonistic encounters. Crayfish, Procambarus clarkii , of different size classes were given a series of winning or losing experiences to determine whether this would affect their likelihood of success in subsequent fights. Recent wins or losses in a series of fights did alter the likelihood of an individual winning a subsequent encounter, despite an apparent disparity in size between opponents that would otherwise serve as an accurate predictor of the outcome.     

Soft Interactions with Model Crowders and Non-canonical Interactions with Cellular Proteins Stabilize RNA Folding

Living cells contain diverse biopolymers, creating a heterogeneous crowding environment, the impact of which on RNA folding is poorly understood. Here, we have used single-molecule fluorescence resonance energy transfer to monitor tertiary structure formation of the hairpin ribozyme as a model to probe the effects of polyethylene glycol and yeast cell extract as crowding agents. As expected, polyethylene glycol stabilizes the docked, catalytically active state of the ribozyme, in part through excluded volume effects; unexpectedly, we found evidence that it additionally displays soft, non-specific interactions with the ribozyme. Yeast extract has a profound effect on folding at protein concentrations 1000-fold lower than found intracellularly, suggesting the dominance of specific interactions over volume exclusion. Gel shift assays and affinity pull-down followed by mass spectrometry identified numerous non-canonical RNA-binding proteins that stabilize ribozyme folding; the apparent chaperoning activity of these ubiquitous proteins significantly compensates for the low-counterion environment of the cell.     

Possible Role of Mother-Daughter Vocal Interactions on the Development of Species-Specific Song in Gibbons

Mother-infant vocal interactions play a crucial role in the development of human language. However, comparatively little is known about the maternal role during vocal development in nonhuman primates. Here, we report the first evidence of mother-daughter vocal interactions contributing to vocal development in gibbons, a singing and monogamous ape species. Gibbons are well known for their species-specific duets sung between mates, yet little is known about the role of intergenerational duets in gibbon song development. We observed singing interactions between free-ranging mothers and their sub-adult daughters prior to emigration. Daughters sang simultaneously with their mothers at different rates. First, we observed significant acoustic variation between daughters. Co-singing rates between mother and daughter were negatively correlated with the temporal precision of the song’s synchronization. In addition, songs of daughters who co-sang less with their mothers were acoustically more similar to the maternal song than any other adult female’s song. All variables have been reported to be influenced by social relationships of pairs. Therefore those correlations would be mediated by mother-daughter social relationship, which would be modifiable in daughter’s development. Here we hypothesized that daughters who co-sing less often, well-synchronize, and converge acoustically with the maternal acoustic pattern would be at a more advanced stage of social independence in sub-adult females prior to emigration. Second, we observed acoustic matching between mothers and daughters when co-singing, suggesting short-term vocal flexibility. Third, we found that mothers adjusted songs to a more stereotyped pattern when co-singing than when singing alone. This vocal adjustment was stronger for mothers with daughters who co-sang less. These results indicate the presence of socially mediated vocal flexibility in gibbon sub-adults and adults, and that mother-daughter co-singing interactions may enhance vocal development. More comparative work, notably longitudinal and experimental, is now needed to clarify maternal roles during song development.     

Housing Shortages in Urban Regions: Aggressive Interactions at Tree Hollows in Forest Remnants

Urbanisation typically results in a reduction of hollow-bearing trees and an increase in the density of particularly species, potentially resulting in an increased level of competition as cavity-nesting species compete for a limited resource. To improve understanding of hollow usage between urban cavity-nesting species in Australia, particularly parrots, we investigated how the hollow-using assemblage, visitation rate, diversity and number of interactions varied between hollows within urban remnant forest and continuous forest. Motion-activated video cameras were installed, via roped access to the canopy, and hollow usage was monitored at 61 hollows over a two-year period. Tree hollows within urban remnants had a significantly different assemblage of visitors to those in continuous forest as well as a higher rate of visitation than hollows within continuous forest, with the rainbow lorikeet making significantly more visitations than any other taxa. Hollows within urban remnants were characterised by significantly higher usage rates and significantly more aggressive interactions than hollows within continuous forest, with parrots responsible for almost all interactions. Within urban remnants, high rates of hollow visitation and both interspecific and intraspecific interactions observed at tree hollows suggest the number of available optimal hollows may be limiting. Understanding the usage of urban remnant hollows by wildlife, as well as the role of parrots as a potential flagship for the conservation of tree-hollows, is vital to prevent a decrease in the diversity of urban fauna, particularly as other less competitive species risk being outcompeted by abundant native species.     

Structure and Use of Song in the Yellow-headed Blackbird (Xanthocephalus xanthocephalus)

Analysis of yellow-headed blackbird songs indicated that accenting and buzzing songs differed in note structure, energy distribution, and loudness. Buzzing songs showed a shift in energy distribution as a result of propagation through marsh habitat whereas accenting songs did not. Results suggest that accenting songs should propagate further in marsh habitat than buzzing songs. Two experiments, done to test whether song types differed in functional distance, showed that the presence of a male song of either type, or a male or female model on their territories increased use of buzzing songs by males. In the absence of auditory or visual stimulation, males used mainly accenting songs. Results supported the hypothesis that buzzing songs are used primarily for short-range interactions whereas accenting songs are used primarily for long-range general advertising.     

Aggressive Signal Design in the Jacky Dragon (Amphibolurus muricatus): Display Duration Affects Efficiency

Design characteristics of signals, such as their duration, may have evolved to maximize signal efficiency. It is commonly assumed that constraints on signal design have usually shaped the most optimal display characteristics to improve signal transmission and information transfer of the signaller, and detection by intended receivers. In this study, we tested whether the characteristics (duration, speed and frequency) of an aggressive display, the push‐up body rock, exhibited by the Jacky dragon (Amphibolurus muricatus) have likely evolved for optimal signal efficiency, as it is able to draw attention to the signaller. We performed two video playback experiments using high‐resolution 3D animations testing the effect of variation in push‐up body rock structure. In experiment 1, we manipulated push‐up body rock display structure. We gradually increased the number of push‐ups exhibited by a digitally animated Jacky dragon increasing the overall display duration. In experiment 2, we developed four stimuli based on population‐typical push‐up body rock display for duration (short and long), and frequency of push‐ups (1 or 5 consecutive push‐ups) by manipulating push‐ups’ speed. In both experiments, we measured the probability of an orienting response and response latency of focal lizards when being exposed to the different stimuli. Our results showed that display duration is critically important for signal efficiency in the aggressive push‐up body rock display. If we are to understand the design characteristics of signals used in animal communication, then it appears important to consider the possible trade‐off between signal efficiency and costs.     

Zippers Make Signals: NCAM-mediated Molecular Interactions and Signal Transduction

The neural cell adhesion molecule, NCAM, is involved in multiple cis- and trans-homophilic interactions (NCAM binding to NCAM) thereby facilitating cell–cell adhesion through the formation of zipper-like NCAM-complexes. NCAM is also involved in heterophilic interactions with a number of proteins and extracellular matrix molecules. Some of these heterophilic interactions are mutually exclusive, and some interfere with or are dependent on homophilic NCAM interactions. Furthermore, both homo- and heterophilic interactions are modulated by posttranslational modifications of NCAM. Heterophilic NCAM-interactions initiate several intracellular signal transduction pathways ultimately leading to biological responses involving cellular differentiation, proliferation, migration and survival. Both homo- and heterophilic NCAM-interactions can be mimicked by synthetic peptides, which can induce NCAM-like signalling, and in vitroand in vivo studies suggest that such NCAM mimetics may be used for the treatment of neurodegenerative disorders.Special issue dedicated to Lawrence F. Eng.     

Multiple Cues in Status Signalling: The Role of Wingbars in Aggressive Interactions of Male House Sparrows

During aggressive interactions, animals may signal their competitive ability by various ornaments referred to as badges of status. The use of a single badge predicting dominance rank occurs in many vertebrate species. However, animals often display multiple ornaments that may convey information about either different or the same aspects of the signaller's quality, or alternatively, may serve as signal amplifiers. We observed the fighting behaviour of male house sparrows in two captive flocks to investigate whether they may use multiple cues in status signalling during aggressive interactions. Beside the status‐signalling bib, male sparrows possess a conspicuous white wingbar that they often display upon aggressive encounters. We tested whether bib size and the wingbar's conspicuousness (i.e. its achromatic contrast with the neighbouring dark feathers) or its area predicted success in various aspects of fighting. We found that bib size strongly predicted overall fighting success (i.e. proportion of fights won) and defence success (i.e. proportion of successful defences out of all attacks received). Wingbar conspicuousness was positively related to defence success after controlling for the effect of bib size in multivariate analyses. Furthermore, displaying the wings also tended to improve the birds’ success in defence but not in attack. Wingbar area was unrelated to any measured aspect of fighting ability. We suggest that bib size and wingbar conspicuousness may convey multiple messages on fighting abilities, specifically on overall aggressiveness and defending potential, respectively. Alternatively, wingbars may serve as amplifiers for the wing displays of aggressive motivation. Thus, male sparrows may use multiple cues in assessing the competitive ability of opponents during social interactions.     

Interactions between Biliverdin,Oxidative Damage,and Spleen Morphology after Simulated Aggressive Encounters in Veiled Chameleons

Stressors frequently increase oxidative damage–unless organisms simultaneously mount effective antioxidant responses. One putative mitigative mechanism is the use of biliverdin, an antioxidant produced in the spleen during erythrocyte degradation. We hypothesized that both wild and captive-bred male veiled chameleons (Chamaeleo calyptratus), which are highly aggressive to conspecifics, would respond to agonistic displays with increased levels of oxidative damage, but that increased levels of biliverdin would limit this increase. We found that even just visual exposure to a potential combatant resulted in decreased body mass during the subsequent 48-hour period, but that hematocrit, biliverdin concentration in the bile, relative spleen size, and oxidative damage in plasma, liver, and spleen were unaffected. Contrary to our predictions, we found that individuals with smaller spleens exhibited greater decreases in hematocrit and higher bile biliverdin concentrations, suggesting a revision to the idea of spleen-dependent erythrocyte processing. Interestingly, individuals with larger spleens had reduced oxidative damage in both the liver and spleen, demonstrating the spleen’s importance in modulating oxidative damage. We also uncovered differences in spleen size and oxidative damage between wild and captive-bred chameleons, highlighting environmentally dependent differences in oxidative physiology. Lastly, we found no relationship between oxidative damage and biliverdin concentration, calling into question biliverdin’s antioxidant role in this species.