Oxytocin mechanisms of stress response and aggression in a territorial finch.

All jawed vertebrates produce a form of oxytocin (OT), and in birds, mammals and fish, OT is strongly associated with affiliation. However, remarkably few data are available on the roles of OT and OT receptors (OTRs) in aggression. Because OT and OTRs exert anxiolytic effects in mammals (although context-specific) and modulate stress coping, we hypothesized that OTR activation is at least permissive for territorial aggression. Indeed, we find that peripheral injections of an OTR antagonist significantly reduce male-male and female-female aggression in a highly territorial finch. This finding suggests the hypothesis that aggression is accompanied by an increase in transcriptional (Fos) activity of OT neurons, but contrary to this hypothesis, we find that dominant male residents do not elevate OT-Fos colocalization following an aggressive encounter and that OT-Fos colocalization in the preoptic area and hypothalamus correlates negatively with aggression. Furthermore, OT-Fos colocalization increases dramatically in males that were aggressively subjugated or pursued by a human hand, likely reflecting OT modulation of stress response. Because OT inhibits the hypothalamo-pituitary-adrenal axis, the antagonist effects may reflect the fact that aggressive birds and mammals tend to be hyporesponsive to stress. If this is correct, then 1) the observed effects of OTR antagonism may reflect alterations in corticosterone feedback to the brain rather than centrally mediated OTR effects, and 2) the negative correlation between OT-Fos colocalization and aggression may reflect the fact that more aggressive, stress hyporesponsive males require less inhibition of the hypothalamo-pituitary-adrenal axis than do less aggressive males, despite the requirement of that inhibition for the normal display of aggression.

To flock or fight: neurochemical signatures of divergent life histories in sparrows.

Many bird species exhibit dramatic seasonal switches between territoriality and flocking, but whereas neuroendocrine mechanisms of territorial aggression have been extensively studied, those of seasonal flocking are unknown. We collected brains in spring and winter from male field sparrows (Spizella pusilla), which seasonally flock, and male song sparrows (Melospiza melodia), which are territorial year-round in much of their range. Spring collections were preceded by field-based assessments of aggression. Tissue series were immunofluorescently multilabeled for vasotocin, mesotocin (MT), corticotropin-releasing hormone (CRH), vasoactive intestinal polypeptide, tyrosine hydroxylase, and aromatase, and labeling densities were measured in many socially relevant brain areas. Extensive seasonal differences are shared by both species. Many measures correlate significantly with both individual and species differences in aggression, likely reflecting evolved mechanisms that differentiate the less aggressive field sparrow from the more aggressive song sparrow. Winter-specific species differences include a substantial increase of MT and CRH immunoreactivity in the dorsal lateral septum (LS) and medial amygdala of field sparrows but not song sparrows. These species differences likely relate to flocking rather than the suppression of winter aggression in field sparrows, because similar winter differences were found for two other emberizids that are not territorial in winter--dark-eyed juncos (Junco hyemalis), which seasonally flock, and eastern towhees (Pipilo erythropthalmus), which do not flock. MT signaling in the dorsal LS is also associated with year-round species differences in grouping in estrildid finches, suggesting that common mechanisms are targeted during the evolution of different life histories.

Mammal-like organization of the avian midbrain central gray and a reappraisal of the intercollicular nucleus.

In mammals, rostrocaudal columns of the midbrain periaqueductal gray (PAG) regulate diverse behavioral and physiological functions, including sexual and fight-or-flight behavior, but homologous columns have not been identified in non-mammalian species. In contrast to mammals, in which the PAG lies ventral to the superior colliculus and surrounds the cerebral aqueduct, birds exhibit a hypertrophied tectum that is displaced laterally, and thus the midbrain central gray (CG) extends mediolaterally rather than dorsoventrally as in mammals. We therefore hypothesized that the avian CG is organized much like a folded open PAG. To address this hypothesis, we conducted immunohistochemical comparisons of the midbrains of mice and finches, as well as Fos studies of aggressive dominance, subordinance, non-social defense and sexual behavior in territorial and gregarious finch species. We obtained excellent support for our predictions based on the folded open model of the PAG and further showed that birds possess functional and anatomical zones that form longitudinal columns similar to those in mammals. However, distinguishing characteristics of the dorsal/dorsolateral PAG, such as a dense peptidergic innervation, a longitudinal column of neuronal nitric oxide synthase neurons, and aggression-induced Fos responses, do not lie within the classical avian CG, but in the laterally adjacent intercollicular nucleus (ICo), suggesting that much of the ICo is homologous to the dorsal PAG.

Estrogenic regulation of dopaminergic neurons in the opportunistically breeding zebra finch.

Steroid-induced changes in dopaminergic activity underlie many correlations between gonadal hormones and social behaviors. However, the effects of steroid hormones on the various behaviorally relevant dopamine cell groups remain unclear, and ecologically relevant species differences remain virtually unexplored. We examined the effects of estradiol (E2) manipulations on dopamine (DA) neurons of male and female zebra finches (Taeniopygia guttata), focusing on numbers of tyrosine hydroxylase-immunoreactive (TH-ir) cells in the A8-A15 cell groups, and on TH colocalization with Fos, conducted in the early A.M., in order to quantify basal transcriptional activity. TH is the rate-limiting enzyme for catecholamine synthesis, and specifically DA in the A8-A15 cell groups. In contrast to other examined birds and mammals, reducing E2 levels with the aromatase-inhibitor Letrozole failed to alter TH-ir neuron numbers within the ventral tegmental area (VTA; A10), while increasing neuron numbers in the central gray (CG; A11) and caudal midbrain A8 populations. Consistent with findings in other birds, but not mammals, we also found no effects of E2 manipulations (Letrozole or Letrozole plus E2 replacement) on TH-Fos colocalization in any location. In accordance with previous observations in both mammals and birds, E2 treatment decreased the number of TH-ir neurons in the A12 population of the tuberal hypothalamus, a cell group that inhibits the release of prolactin. In general, males and females exhibited similar TH-ir neuron numbers, although males exhibited significantly more TH-ir neurons in the A11 CG population than did females. These results suggest partial variability in E2 regulation of DA across species.

Vasotocin neurons and septal V1a-like receptors potently modulate songbird flocking and responses to novelty.

Previous comparisons of territorial and gregarious finches (family Estrildidae) suggest the hypothesis that arginine vasotocin (VT) neurons in the medial bed nucleus of the stria terminalis (BSTm) and V(1a)-like receptors in the lateral septum (LS) promote flocking behavior. Consistent with this hypothesis, we now show that intraseptal infusions of a V(1a) antagonist in male zebra finches (Taeniopygia guttata) reduce gregariousness (preference for a group of 10 versus 2 conspecific males), but have no effect on the amount of time that subjects spend in close proximity to other birds ("contact time"). The antagonist also produces a profound increase in anxiety-like behavior, as exhibited by an increased latency to feed in a novelty-suppressed feeding test. Bilateral knockdown of VT production in the BSTm using LNA-modified antisense oligonucleotides likewise produces increases in anxiety-like behavior and a potent reduction in gregariousness, relative to subjects receiving scrambled oligonucleotides. The antisense oligonucleotides also produced a modest increase in contact time, irrespective of group size. Together, these combined experiments provide clear evidence that endogenous VT promotes preferences for larger flock sizes, and does so in a manner that is coupled to general anxiolysis. Given that homologous peptide circuitry of the BSTm-LS is found across all tetrapod vertebrate classes, these findings may be predictive for other highly gregarious species.

Natural selection on testosterone production in a wild songbird population.

Because of their role in mediating life-history trade-offs, hormones are expected to be strongly associated with components of fitness; however, few studies have examined how natural selection acts on hormonal variation in the wild. In a songbird, the dark-eyed junco (Junco hyemalis), field experiments have shown that exogenous testosterone alters individuals' resolution of the survival-reproduction trade-off, enhancing reproduction at the expense of survival. Here we used standardized injections of gonadotropin-releasing hormone (GnRH) to assay variation in the testosterone production of males. Using measurements of annual survival and reproduction, we found evidence of strong natural selection acting on GnRH-induced increases in testosterone. Opposite to what would be predicted from the survival-reproduction trade-off, patterns of selection via survival and reproduction were remarkably similar. Males with GnRH-induced testosterone production levels that were slightly above the population mean were more likely to survive and also produced more offspring, leading to strong stabilizing selection. Partitioning reproduction into separate components revealed positive directional selection via within-pair siring success and stabilizing selection via extrapair mating success. Our data represent the most complete demonstration of natural selection on hormones via multiple fitness components, and they complement previous experiments to illuminate testosterone's role in the evolution of life-history trade-offs.

Mesotocin and nonapeptide receptors promote estrildid flocking behavior.

Proximate neural mechanisms that influence preferences for groups of a given size are almost wholly unknown. In the highly gregarious zebra finch (Estrildidae: Taeniopygia guttata), blockade of nonapeptide receptors by an oxytocin (OT) antagonist significantly reduced time spent with large groups and familiar social partners independent of time spent in social contact. Opposing effects were produced by central infusions of mesotocin (MT, avian homolog of OT). Most drug effects appeared to be female-specific. Across five estrildid finch species, species-typical group size correlates with nonapeptide receptor distributions in the lateral septum, and sociality in female zebra finches was reduced by OT antagonist infusions into the septum but not a control area. We propose that titration of sociality by MT represents a phylogenetically deep framework for the evolution of OT's female-specific roles in pair bonding and maternal functions.

Dynamic neuromodulation of aggression by vasotocin: influence of social context and social phenotype in territorial songbirds.

The homologous neuropeptides vasotocin (VT) and vasopressin (VP) influence agonistic behaviours across many taxa, but peptide-behaviour relationships are complex and purportedly species-specific. Putative species-specific effects in songbirds are confounded with context, however, such that territorial species have been tested only in resident-intruder paradigms and gregarious species have been tested only in a mate competition paradigm. Using the territorial violet-eared waxbill (Estrildidae: Uraeginthus granatina), we now show that a V(1a) receptor antagonist reduces male aggression during mate competition (as in gregarious finches), but does not affect resident-intruder aggression in dominant males. However, the V(1a) antagonist disinhibits aggression in less aggressive (typically subordinate) males. These results are consistent with recent data on the activation of different VT cell groups during positive and negative social interactions. Thus, VT influences aggression similarly across territorial and gregarious species, but in context- and phenotype-specific ways that probably reflect the differential activation of discrete VT cell groups.

Seasonal variation in volatile compound profiles of preen gland secretions of the dark-eyed junco (Junco hyemalis).

Quantitative stir bar sorptive extraction methodology, followed by gas chromatography-mass spectrometry (GC-MS) and element-specific atomic emission detection (AED) were utilized to analyze seasonal changes in volatile components of preen oil secretions in Junco hyemalis. Juncos were held in long days to simulate breeding conditions, or short days to simulate nonbreeding conditions. Linear alcohols (C(10)-C(18)) were the major volatile compounds found in preen oil, and in both sexes their levels were higher when birds were housed on long as opposed to short days. Methylketones were found at lower levels, but were enhanced in both sexes during long days. Levels of 2-tridecanone, 2-tetradecanone, and 2-pentadecanone were also greater on long days, but only in males. Among carboxylic acids (C(12), C(14), and C(16)), linear but not branched acids showed some differences between the breeding and nonbreeding conditions, although the individual variation for acidic compounds was large. Qualitatively, more sulfur-containing compounds were found in males than females during the breeding season. Functionally, the large increase in linear alcohols in male and female preen oil during the breeding season may be an indication of altered lipid biosynthesis, which might signal reproductive readiness. Linear alcohols might also facilitate junco odor blending with plant volatiles in the habitat to distract mammalian predators. Some of the volatile compounds from preen oil, including linear alcohols, were also found on the wing feather surface, along with additional compounds that could have been of either metabolic or environmental origin.