Evolving nonapeptide mechanisms of gregariousness and social diversity in birds

Abstract

Of the major vertebrate taxa, Class Aves is the most extensively studied in relation to the evolution of social systems and behavior, largely because birds exhibit an incomparable balance of tractability, diversity, and cognitive complexity. In addition, like humans, most bird species are socially monogamous, exhibit biparental care, and conduct most of their social interactions through auditory and visual modalities. These qualities make birds attractive as research subjects, and also make them valuable for comparative studies of neuroendocrine mechanisms. This value has become increasingly apparent as more and more evidence shows that social behavior circuits of the basal forebrain and midbrain are deeply conserved (from an evolutionary perspective), and particularly similar in birds and mammals. Among the strongest similarities are the basic structures and functions of avian and mammalian nonapeptide systems, which include mesotocin (MT) and arginine vasotocin (VT) systems in birds, and the homologous oxytocin (OT) and vasopressin (VP) systems, respectively, in mammals. We here summarize these basic properties, and then describe a research program that has leveraged the social diversity of estrildid finches to gain insights into the nonapeptide mechanisms of grouping, a behavioral dimension that is not experimentally tractable in most other taxa. These studies have used five monogamous, biparental finch species that exhibit group sizes ranging from territorial male-female pairs to large flocks containing hundreds or thousands of birds. The results provide novel insights into the history of nonapeptide functions in amniote vertebrates, and yield remarkable clarity on the nonapeptide biology of dinosaurs and ancient mammals. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Fig. 1

Distribution of VT-ir cell groups in estrildid finches. (A) The rostral-most cell groups in a female Angolan blue waxbill, showing VT-ir neurons in the periventricular POA (Pe), SON, suprachiasmatic nucleus (SCh), and lateral SCh (LSCh). This photo was taken rostral to the main body of cells in the SCh, thus just a few neurons are visible. Scale bar = 200 μm. (B) The PVN of a male zebra finch, showing the separation of VT and MT populations. Note that low levels of MT mRNA extend in the surrounding hypothalamus and are not restricted to the PVN. Scale bar = 50 μm. (C) VT-ir cells and fibers at the level of the anterior commissure (AC) in a male zebra finch, showing cell groups of the PVN and BSTm, and apparent overlapping projections from these cell groups in the BSTm and ventral LS. Large-caliber, heavily beaded axons (small arrows) are observed coursing from the PVN through the BSTm and directly into the ventrolateral zone of the caudal LS (LSc.vl). Relatively heavier projections are observed to the lateral BST (BSTl), but terminate immediately adjacent to the BSTm and LSc.vl. Within the BSTm (box), fine-caliber, beaded axons of local origin (large arrowheads) mix with the heavier axons of apparent PVN origin. Scale bars = 200 μm (left) and 20 μm (right). Other abbreviations: AC, anterior commissure; AH, anterior hypothalamus; LSc.d, dorsal zone of the LSc; LSc.v, ventral zone of the LSc; MS, medial septum; nPC, nucleus of the pallial commissure; OM, occipital-mesencephalic tract; POM, medial preoptic nucleus; SH, septohippocampal septum. Panel C modified from Goodson and Kabelik (2009).

Fig. 2

Valence sensitivity of vasotocin (VT) neurons in the bed nucleus of the stria terminalis (BSTm), as demonstrated by socially-induced changes in the immunocytochemical colocalization of VT and the proxy activity marker Fos. (A) Representative colocalization of VT (green) and Fos (red) in the BSTm of a male zebra finch following a courtship interaction. Note that most VT neurons express Fos. Scale bar = 20 μm. (B) In the zebra finch, which is a highly gregarious species, isolation in a quiet room followed by exposure to a same-sex conspecific through a wire barrier produces a robust increase in VT neuronal activity in the BSTm of both males and females. Total n = 10. (C) This same manipulation produces a significant decrease in VT-Fos colocalization in the territorial violet-eared waxbill, a species that does not naturally exhibit same-sex affiliation, but exposure to the subject’s pair bond partner (a presumably positive stimulus), produces a robust increase in neuronal activity. Sexes are shown pooled. Total n = 16. (D) VT-Fos colocalization increases in zebra finches following competition with a same-sex individual for courtship access to an opposite-sex bird, but not if the subject is paired with a highly aggressive partner and intensely subjugated. Subjugated animals were aggressively displaced or attacked 71–210 times during a 10-min interaction, demonstrating that social arousal alone does not increase VT-Fos colocalization in the BSTm. Sexes are shown pooled. Total n = 15. Panel A is modified from Goodson et al. (Goodson et al., 2009c); panels B–D are modified from Goodson and Wang (2006).

Fig. 3

Antisense knockdown of VT production in the BSTm and intraseptal infusions of a VP V_1a antagonist reduce gregariousness and increase anxiety-like behavior. (A) Choice apparatus design. A 1 m wide testing cage was subdivided into zones by seven perches (thin lines). Subjects were considered to be within close proximity when they were within 6 cm of a stimulus cage (i.e., on the perches closest to the sides of the testing cage). The stimulus cages contained either two or ten same-sex conspecifics. The percent of test time spent in close proximity to conspecifics yields a measure of “contact,” and the percent of contact time that is spent next to the larger group yields a measure of “gregariousness.” (B) Relative to scrambled oligonucleotide control subjects, male zebra finches infused with VT antisense oligonucleotides exhibit a median reduction in gregariousness of approximately 80% (C) Gregariousness is likewise reduced by V_1a antagonist infusions into the LS, relative to vehicle. (D–E) Latency to feed in the presence of a novel object is strongly increased by both VT antisense infusions into the BSTm (D) and V_1a antagonist infusions into the LS (E). Box plots show the median (red line), 75^th and 25^th percentile (box) and 95% confidence interval (whiskers). Modified from Kelly et al. (2011).

Fig. 4

Antagonism of oxytocic receptors reduces preferences for larger groups in zebra finches. Relative to vehicle treatments, subcutaneous (s.c.) or intracerebroventricular (i.c.v.) administrations of the oxytocin antagonist desGly–NH₂, d(CH₂)₅[Tyr(Me)², Thr⁴]OVT (OTA), reduce the amount of time that zebra finches spend in close proximity to the large group (B–C) and increase time in close proximity to the small group (D–E). *P < 0.05, ***P < 0.001, main effect of Treatment; #P < 0.5 Sex*Treatment; n = 12 m, 12 f. Letters above the error bars denote significant within-sex effects. Modified from Goodson et al. (2009d).

Fig. 5

Species-specific distributions of oxytocin-like binding sites reflect evolutionary convergence and divergence in flocking and territoriality. (A–C) Representative autoradiograms of ¹²⁵I-OT antagonist binding sites in the caudal LS (LSc) in two sympatric, congeneric finches – the territorial violet-eared waxbill (A) and the gregarious Angolan blue waxbill (B), plus the highly gregarious zebra finch (C). (D) Densities of binding sites in the dorsal (pallial) LSc of two territorial species (Melba finch, MF, and violet-eared waxbill, VEW), a moderately gregarious species (Angolan blue waxbill, ABW), and two highly gregarious species (spice finch, SF, and zebra finch, ZF). No sex differences are observed and sexes were pooled. Total n = 23. Different letters above the boxes denote significant species differences (Mann-Whitney P < 0.05) following significant Kruskal-Wallis. (E) Binding densities tend to reverse in the subpallial LSc (P = 0.06), suggesting that species differences in sociality are most closely associated with the relative densities of binding sites along a dorso-ventral gradient, as confirmed in the bottom panel (F) using a dorsal:ventral ratio. Abbreviations: Hp, hippocampus; LSc.d, dorsal zone of the LSc; LSc.v,vl, ventral and ventrolateral zones of the LSc; N, nidopallium; PLH, posterolateral hypothalamus; TeO, optic tectum. Modified from Goodson et al. (2009d).

Fig. 6

Neuromodulation of aggression varies across contexts and phenotypes. For panels A–C, subjects were tested in a within-subjects design following injections of saline control or JNJ-17308616, a novel V_1a antagonist that crossed the blood-brain barrier. Tests were 7 min. Total n = 9 for all *p = 0.015. (A) Total numbers of aggressive behaviors (means ± SEM) exhibited by aggressive, dominant male violet-eared waxbills in the context of territorial defense (resident-intruder tests). (B) Aggressive behaviors exhibited in resident-intruder tests (as in panel A) by male violet-eared waxbills that were typically subordinate. Total n = 6; *p = 0.043. (C) Aggressive behaviors exhibited by aggressive, dominant male violet-eared waxbills in the context of mate competition. Panels A–C modified from Goodson et al. (2009b). (D) Aggressive behavior per minute (when not in a nest; means ± SEM) exhibited by male zebra finches in colony cages that contained 4 males and 5 females. Subjects were administered intraventricular infusions of a V₁-V_1a antagonist cocktail twice daily. Focal 10-min observations were conducted in the morning and afternoon for three days (corresponding to sessions 1–6). Data are shown separately for session 1, when aggression is focused on competition for females, and during sessions 2–6 when most aggression is focused on the defense of nest cups. Data are displayed separately for unpaired and pair-bonded individuals. In session 1, paired males exhibited more aggression than unpaired males (P = 0.0002), and VP antagonist treatment resulted in a decrease in aggression relative to treatment with saline (P = 0.006). Different letters above the error bars denote significant group differences (P < 0.05). In sessions 2 to 6, the antagonist resulted in an increase in aggression levels relative to saline treatment (*P = 0.04). Data for all males are shown for session 1; analyses for sessions 2–6 are restricted to males for which unpaired and paired data are available. Modified from Kabelik et al. (2009).