vertebrate animals affiliate in multiple contexts , but sexual behavior is likely the only context of affiliation that is nearly ubiquitous across vertebrate species .
thus , the various forms of social contact that we observe across species are subject to independent evolution in different taxa , although some forms are conserved across very large clades .
the provision of maternal care to offspring post - hatching or post - parturition is a good example .
this kind of behavior typifies most mammals and most birds , but mammals and birds have evolved extended maternal care independently ( alcock , 2009 ; goodenough et al . , 2010 ) .
studies of many vertebrate taxa demonstrate that the neural mechanisms of sexual behavior are strongly conserved ( goodson , 2005 ; martinez - garcia et al .
, 2008 ) , and limited evidence suggests that mechanisms of parental care are at least grossly similar in mammals and birds , as well ( buntin et al . , 2006 ) .
however , almost no generalizations can be made regarding the mechanisms of evolutionarily labile aspects of social behavior , such as cooperative breeding , mating systems , and grouping .
these variables represent major defining features of species - specific social structures and have therefore garnered an extraordinary amount of attention from evolutionary biologists and behavioral ecologists .
however , evolutionarily labile behaviors pose fairly extreme challenges for comparative neurobiology , largely for reasons of feasibility .
for instance , after two decades of intensive research , the neural mechanisms of selective partner preference ( a hallmark of monogamy ) , paternal care , and alloparental care are reasonably well understood for only a single species , the monogamous prairie vole ( microtus ochrogaster ) , and we know a limited amount about the critical variables that differentiate prairie voles from other non - monogamous vole species ( carter et al . , 2008 ;
donaldson and young , 2008 ; aragona and wang , 2009 ; ross and young , 2009 ; mcgraw and young , 2010 ) .
what we do not yet know is whether neural mechanisms have evolved convergently in other monogamous or alloparental species , although this is a matter of substantial current interest ( fink et al . , 2006 ;
this example points out a fundamental difference in the approaches to social organization that are employed by evolutionary biologists and neuroethologists .
evolutionary biologists can often bring extensive information to bear on a given question , perhaps using data from dozens or hundreds of species , and analyze it in a phylogenetically corrected way that allows the extrapolation of general patterns . to some extent
, neurobiologists can employ similar techniques if they are based on anatomical characteristics or genes ( northcutt , 2008 ; pollen and hofmann , 2008 ) .
nonetheless , we can not make any strong claims about evolutionary patterns of regulation if the experimental approaches of behavioral neuroscience are not employed , and it simply not feasible to replicate experiments in dozens or hundreds of species .
for this reason , neuroethological approaches to questions about evolutionarily labile behaviors will likely continue to be somewhat different from evolutionary biological approaches , even when both are employed by the same investigators
. however , if neuroethologists want to establish predictive validity for other species , comparative experimentation focused on divergent and convergent evolution is essential . in the sections that follow ,
we describe a quasi - experimental approach that we have taken to study convergent and divergent evolution in grouping behavior within the estrildid finch family .
although only five species have been used ( table 1 ) , they have been carefully selected and our findings should therefore have good predictive validity for other estrildid finches . whether our findings can be extrapolated to other vertebrate taxa is a matter of greater speculation , and a topic that we will return to in the section  conclusion .
 behavioral and ecological characteristics of estrildid finch species that have been used for studies of grouping behavior . for relevant references , see footnote 1 .
the genus uraeginthus contains three species of blue waxbill that are each known by multiple common names .
thus , although u. angolensis is most typically referred to as simply  blue waxbill ,  we have referred to this species as  angolan blue waxbill  in our publications in order to clearly differentiate them from other species .
this species is commonly encountered in the scientific literature , but under a variety of common names , including spotted munia , nutmeg mannikin , and spice finch .
sociality sensu stricto , as defined by grouping behavior ( alexander , 1974 ) , has broad influences on other important variables such as reproductive behavior , disease transmission , resource exploitation , and defense ( moller and birkhead , 1993 ; krause and ruxton , 2002 ; silk , 2007 ) , but has not previously been examined in neurobiological studies , likely because grouping is difficult to isolate from other aspects of ecology and behavior , such as mating system and patterns of parental care .
for instance , rodent species that differ in their grouping behavior also differ in whether they are monogamous or polygamous , and whether the father contributes to parental care ( king , 1968 ; tamarin , 1985 ) .
however , if we want to examine the neural mechanisms that titrate species - typical group - size preferences , control for such variables is very important , given that shared mechanisms ( particularly neuroendocrine mechanisms ) often regulate numerous aspects of social behavior ( e.g. , pair bonding , parental care , and affiliation ) and related aspects of physiology ( e.g. , hormone levels and stress physiology ; carter et al .
birds offer excellent opportunities to study grouping , since we can identify closely related species that are virtually identical in most aspects of behavior and ecology , but that nonetheless exhibit extreme variation in sociality , ranging from territorial pairs to massive flocks .
the majority of estrildid species ( approximately 141 total ; clements , 2007 ) form small groups of about 612 birds when they are not breeding , and then loosely distribute themselves for nesting without strong territoriality .
defense of a breeding territory has evolved in five to six species across three genera .
colonial breeding has also evolved in several genera , although most colonies are small ( perhaps 510 breeding pairs ) .
available evidence suggests that all estrildids exhibit biparental care and long - term ( typically life - long ) , socially monogamous pair bonds ( immelmann , 1965 ; goodwin , 1982 ) . we have collected five estrildid finch species that exhibit large and seasonally stable differences in grouping , but are otherwise closely matched in behavior and ecology ( table 1 ) .
in addition to being socially monogamous and biparental , as already noted , all of these species live in arid or semi - arid grassland scrub and breed opportunistically or semi - opportunistically in relation to rainfall ( skead , 1975 ; goodwin , 1982 ; zann , 1996 ; goodson et al . , 2006 ) .
these include two territorial african species that live as male  female pairs year - round ( violet - eared waxbill , uraeginthus granatina , and melba finch , pytilia melba ) ; two highly gregarious , colonially breeding species that exhibit modal group sizes of approximately 100 ( zebra finch , taeniopygia guttata , and spice finch , lonchura punctulata ) ; and a moderately gregarious species , the angolan blue waxbill ( uraeginthus angolensis ) , which exhibits a modal group size of approximately 20 .
the two territorial species have evolved their territorial behavior independently and the two colonial species have also evolved their extreme sociality independently ( goodson et al . , 2006 ) .
the gregarious angolan blue waxbill is sympatric with both the melba finch and violet - eared waxbill , and our laboratory population of these three species has been established through the breeding of birds that were caught from a single location in the kalahari thornscrub of south africa in 2001 .
we have obtained zebra finches and wild - caught spice finches ( likely of an indian subspecies ) from commercial suppliers .
the inclusion of the zebra finch is an important asset to our research program because domestic zebra finches are both behaviorally robust and readily available , which makes them an ideal species for extensive laboratory studies .
domestic zebra finches are behaviorally indistinguishable from wild - caught zebra finches ( morris , 1958 ) and virtually their full range of social behavior can be observed in the lab .
for instance , we are able to quantify over 20 behaviors while zebra finches are interacting in a colony environment , and can readily track the natural formation of monogamous pair bonds ( goodson et al . , 1999 ;
vertebrates possess a core social behavior network within the basal (  limbic  ) forebrain and midbrain ( figure 1 ) that is evolutionarily conserved across all taxa , but is particularly stable across the amniote clades giving rise to birds and mammals , as demonstrated by a wide range of functional and anatomical studies .
this network includes the medial extended amygdala ( including the medial bed nucleus of the stria terminalis , bstm ) , lateral septum ( ls ) , anterior hypothalamus ( ah ) , ventromedial hypothalamus ( vmh ) , midbrain central gray ( cg ; or periaqueductal gray ) , and the ventral tegmental area , although these structures are not strictly social in function and contribute to other core networks for the regulation of behavior and physiology .
all of these areas express high densities of sex steroid receptors , and in contrast to most other areas of the brain that influence social behavior , they are absolutely essential for basic functions such as the expression of sexual , aggressive , and parental behaviors , and also for the regulation of anxiety , social recognition , and approach  avoidance processes ( newman , 1999 ; goodson , 2005 ) . an evolutionarily conserved suite of brain regions that regulate vertebrate social behavior .
( a ) the core components of the social behavior network include numerous areas of the basal forebrain  the medial extended amygdala ( medial amygdala , mea , or taenial amygdala , tna , plus the medial bed nucleus of the stria terminalis , bstm ) , medial preoptic area ( mpoa ) , anterior hypothalamus ( ah ) , ventromedial hypothalamus ( vmh ) , and lateral septum ( ls ) , as well as areas of the midbrain , most notably the central gray ( cg ; or periaqueductal gray , pag ) and the ventral tegmental area ( vta ) .
( b ) a photomontage of a female zebra finch brain at the level of the anterior commissure ( ac ) .
immunocytochemical triple - labeling for vasoactive intestinal polypeptide ( vip ) , neuropeptide y ( npy ) , and tyrosine hydroxylase ( th ) shows the location of the ah and multiple zones of the ls , bst , and vmh .
. scale bar  =  200  m . modified from goodson , 2005 .
other abbreviations : bstl , lateral bed nucleus of the stria terminalis ; hp , hippocampus ; lh , lateral hypothalamus ; lsc , caudal division of the lateral septum ( dorsal , ventrolateral , and ventral zones denoted as lsc.d , lsc.vl , and lsc.v , respectively ) ; lsr , rostral division of the lateral septum ; me , median eminence ; ms , medial septum ; msib , internal band of the medial septum ; ot , optic tract ; om , occipitomesencephalic tract ; pvn , paraventricular nucleus of the hypothalamus ; sh , septohippocampal septum ; v , lateral ventricle .
birds exhibit mammal - like patterns of immediate early gene ( ieg ) induction in this network across a variety of social contexts , including territorial aggression , a context that is strongly relevant to species differences in sociality .
ieg proteins such as fos and egr-1 are rapidly inducible transcription factors that are proxy markers of neuronal activity . following resident 
intruder encounters in territorial rodents ( kollack - walker et al . , 1997 ; motta et al . , 2009 ) and
simulated territorial intrusion ( sti ; playback of song and presentation of a caged decoy male ) in male song sparrows , significant activation is observed in forebrain areas such as the bstm , ls , paraventricular nucleus of the hypothalamus ( pvn ) , ah , and the lateral portion of the vmh ( goodson and evans , 2004 ; goodson et al . , 2005b ) , as well as midbrain areas such as the cg ( maney and ball , 2003 ) .
ieg results are extensively comparable in winter and summer sparrows ( goodson and evans , 2004 ; goodson et al .
notably , ieg responses of the ah , pvn , and multiple zones of the ls correlate negatively with aggression ( goodson et al . , 2005b ) , and the case is much the same in hamsters and mice , such that fighting - induced fos expression is higher in subordinate versus dominant animals ( kollack - walker et al . , 1997 ; motta et al . , 2009 ) .
this applies to most aggression - related areas in the brain , and we have obtained very similar results in the territorial violet - eared waxbill ( j. l. goodson , s. e. schrock , and m. a. kingsbury , unpublished observations ) .
neural circuitry that regulates grouping is substantially less well known . as an initial approach to this issue
, we examined the neural responses of territorial and gregarious estrildids to same - sex conspecifics ( goodson et al . , 2005a ) .
subjects were isolated in a quiet room and then exposed to a same - sex conspecific through a wire barrier .
importantly , this manipulation elicits very little overt behavior and thus species differences in neural response should primarily reflect species differences in perceptual or motivational processes . despite the simplicity of this paradigm , fos and egr-1 responses clearly distinguish the territorial from gregarious ( flocking ) species , including differences between the two sympatric uraeginthus species described in the previous section .
overall , territorial birds exhibit significantly greater ieg responses in the medial extended amygdala ( especially the bstm ) , ventrolateral ls , ah , lateral vmh , and cg ( and adjacent  intercollicular  territory that is now known to be homologous to the dorsal and dorsolateral cg of mammals ; goodson et al .
this pattern is virtually identical to a pattern associated with aversive social stimulation in rodents ( sheehan et al . , 2001 ; providing a good example of deep functional conservation in the social behavior circuits of vertebrate brains ) , but possible mechanisms of gregariousness were not evident at this level of analysis .
this may partially reflect the fact that functionally distinct neurons are intercalated with one another in the various brain areas that influence social behavior ( e.g. , choi et al . ,
for instance , as described in the next section , fos induction within arginine vasotocin ( vt ) neurons of the bstm is strictly associated with affiliation - related stimuli , and following same - sex exposure , the gregarious species show greater fos induction in the vt neurons than do territorial species ( goodson and wang , 2006 ) . however , as just noted , the overall fos response of the bstm is greater in the territorial birds ( goodson et al . , 2005a ) , suggesting that the vt neurons are intermingled with other neurons that exhibit opposing profiles of response .
these hypothetical  aversion  neurons have not yet been identified , although aromatase - expressing neurons , which are numerous in the bstm and mostly negative for vt ( kabelik et al . ,
2010 ) , are possible candidates . consistent with this idea , recent findings in mice suggest that estrogen signaling within the bstm is important for the regulation of resident  intruder aggression ( trainor et al . , 2006 , 2007 ) and inhibits affiliative behavior in male prairie voles ( lei et al . , 2010 ) .
nonapeptides ,  are among the most interesting modulators of social behavior identified to date .
although many anatomical and functional properties of nonapeptide systems are strongly conserved across all vertebrate taxa ( and in the case of neurosecretory magnocellular neurons , perhaps across all bilaterians ; tessmar - raible et al . , 2007 ) , there is substantial diversity in receptor distributions and nonapeptides have been extensively linked to behavioral variation across individuals , sexes , and species ( goodson and bass , 2001 ; de vries and panzica , 2006 ; donaldson and young , 2008 ; goodson , 2008 ) .
duplication of the vt gene in early jawed vertebrates gave rise to two nonapeptide lineages , which include the mammalian peptides arginine vasopressin ( vp ; homolog of vt ) and oxytocin ( ot ) .
most non - mammalian vertebrates express vt and an ot - like peptide , such as isotocin , found in ray - finned fishes , or mesotocin ( mt ) , which is ubiquitously expressed in non - mammalian tetrapods ( acher , 1972 ; hoyle , 1999 ) .
all jawed vertebrates express their two nonapeptide forms in both magnocellular and parvocellular neurons of the preoptic area and hypothalamus , which in amniotes are located primarily within the supraoptic nucleus of the hypothalamus and the pvn ( moore and lowry , 1998 ; goodson , 2008 ) . in rats ,
the parvocellular neurons of the pvn give rise to widespread projections in the brain ( de vries and buijs , 1983 ) , and this is almost certainly the same in other vertebrates .
lesions of the pvn in rats virtually eliminate vp projections to the caudal brainstem , but not other areas , and eliminate ot projections throughout the brain ( de vries and buijs , 1983 ) .
thus , given the strong similarities of mt and ot systems , it is likely the case that extrahypothalamic mt projections in birds are exclusively or almost exclusively derived from the pvn .
in addition to these hypothalamic cell groups , most tetrapods exhibit a very unique vt / vp cell group of the bstm , which in some mammals extends into medial amygdala proper . with only a few exceptions , these neurons and their projections to the ls and other basal forebrain areas
have been found to be seasonally variable , exquisitely dependent on sex steroids , and sexually dimorphic ( males  >  females ) . in each of these respects ,
the extrahypothalamic vt / vp circuitry arising in the bstm is among the most extremely plastic systems in the brain . indeed , as demonstrated for many species of mammals , amphibians , birds , and reptiles , this circuitry typically disappears in animals that are in non - reproductive condition or otherwise deprived of sex steroids ( goodson and bass , 2001 ; de vries and panzica , 2006 ) .
however , in estrildids that exhibit opportunistic breeding and have no apparent endogenous reproductive cycles , no such collapse is observed , although basal transcriptional activity [ as measured by constitutive expression of fos protein in vt - immunoreactive ( -ir ) neurons ] is nonetheless regulated by androgens ( kabelik et al . ,
thus , of the finches that we have examined , large seasonal fluctuations in vt immunoreactivity are found only for the spice finch , a species that exhibits photorefractoriness and endogenous rhythms of reproductive physiology that correlate with monsoon cyclicity ( goodwin , 1982 ; chaturvedi and prasad , 1991 ; sikdar et al . , 1992 ;
the vt / vp neurons of the bstm project to numerous other areas of the basal forebrain where vt / vp modulates aggression , parental behavior , social recognition , and various affiliative and anxiety - like behaviors ; and at least in male prairie voles , vp release in the ventral pallidum promotes partner preference ( donaldson and young , 2008 ; veenema and neumann , 2008 ; goodson and thompson , 2010 ; insel , 2010 ) .
although these basal forebrain sites likely receive vt / vp from multiple hypothalamic cell groups in addition to the bstm population , the relative contributions of the different cell groups are difficult to ascertain , given the extensive evidence for paracrine signaling and volumetric release from dendrites and soma ( ludwig and leng , 2006 ; goodson and kabelik , 2009 ) . in order to determine the kinds of stimuli that different vt / vp cell groups respond to ( particularly those of the bstm ) , we have conducted several experiments in which we have exposed animals to social stimuli or control conditions , and then sacrificed the animals 90  min later for immunohistochemical colocalization of vt and fos ( see table 2 for a summary of studies ) .
experimental induction of fos is still robust at 90  min , but the half - life of fos protein is only 45  min ( herdegen and leah , 1998 ) , and thus if cellular activity is depressed by our social manipulations , we can detect this as a reduction in fos expression .
fos colocalization , since mt behavioral functions were unknown at the time , but we are now conducting these analyses with alternate tissue series from the same animals . a summary of main findings from studies of nonapeptide systems in territorial ( t ) , moderately gregarious ( mg ) , and highly gregarious ( hg ) species of estrildid finches . goodson and wang ( 2006 ) , also see goodson et al .
goodson ( 1998 ) , goodson and adkins - regan ( 1999 ) , goodson et al .
goodson et al . ( 2004 ) , kelly et al . ( 2011 ) .
our experiments have demonstrated a remarkable sensitivity of the bstm vt / vp cells to social valence ( figure 2 ) .
relative to handled controls , vt - ir neurons in the territorial estrildid species decrease their fos expression in response to same - sex stimuli ( exposed through a wire barrier as described above ) , but increase fos expression in response to their pairbond partner .
in contrast , in colonial birds that form mixed - sex flocks , vt - ir cells in the bstm increase their activity in response to both same - sex stimuli and competitive courtship interactions , but not following intense subjugation ( goodson and wang , 2006 ) .
no sex differences were observed in any of these results and similar effects were not observed for vt 
fos colocalization in the pvn ( j. l. goodson and y. wang , unpublished observations ) .
notably , although the modestly gregarious angolan blue waxbill shows only a small ( and not significant ) increase in vt 
fos colocalization following exposure to a same - sex conspecific , the response profile of this species is still significantly different from its territorial congener , the violet - eared waxbill . species differences in vt signaling may be further magnified based on vt - ir cell numbers , which are approximately 10 times more abundant in the colonial species relative to other species ( goodson and wang , 2006 ) .
finally , adding onto the remarkable response profile of this cell group , recent experiments in male zebra finches demonstrate that the bstm vt neurons increase their fos expression selectively in response to a positive social stimulus , but show no response to a positive non - social stimulus ( goodson et al .
valence sensitivity of vasotocin ( vt ) neurons in the medial 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 .
scale bar  =  20  m . ( 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 both males and females .
fos colocalization in the territorial violet - eared waxbill , a species that does not naturally exhibit same - sex affiliation , but exposure to the subject 's pairbond partner ( a presumably positive stimulus ) , produces a robust increase in neuronal activity .
( 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 71210 times during a 10-min interaction , demonstrating that social arousal alone does not increase vt 
( 2009b ) ; ( b  d ) are modified from goodson and wang ( 2006 ) .
our results in birds are consistent with other recent findings in mice , and may therefore represent a common feature of bstm vt / vp neurons across all tetrapods . in male c57bl/6j mice ,
bstm vp neurons exhibit robust fos responses to copulation ( which is clearly a positive , affiliation - related stimulus ) and very modest responses to non - aggressive same - sex chemoinvestigation , but show no greater fos response to aggressive interactions than simple chemoinvestigation ( ho et al . , 2010 ) .
notably , the sensitivity of amygdala neurons to valence has been extensively characterized through neurophysiological and neuroimaging studies in mammals ( nishijo et al . , 1988 ;
also see goodson and thompson , 2010 ) , but specific cell types that process valence have not previously been identified .
neurophysiological studies in monkeys demonstrate that neurons which exhibit stable preferences for positive or negative stimuli are intercalated ( paton et al . , 2006 ) , and thus the neurochemical identification of valence - sensitive populations represents an important step toward understanding how social value is encoded and used by the brain to regulate behavior .
species differences in the response profiles of various brain areas and specific cell group are likely coordinated , at least to an extent , by the differential expression of receptors for neuromodulators .
in fact , using the five finch species already introduced , we have obtained good evidence that the distributions of binding sites for vasoactive intestinal polypeptide ( vip ) , vt , and mt all exhibit convergent and divergent evolution in relation to sociality ( based on quantitative autoradiography using i  vip and iodinated antagonists of ot and v1a receptors ; goodson et al . , 2006 , 2009c )
. remarkably , binding sites for all three peptides exhibit sociality - related evolution within the ls , indicating that the ls plays a special role in grouping . in order to be interpreted as  sociality - related ,
 we require that binding densities differ significantly between the two territorial and three flocking species ( defined as contrast a ) and differ significantly between the two territorial and two colonial species ( contrast b ) , with the modestly gregarious species being intermediate or falling in line with the two colonial species .
the most convincing comparisons are those that meet these criteria , with additional significant differences between the territorial and gregarious uraeginthus species , which are sympatric ( contrast c ) .
figure 3 shows one such pattern for the density of v1a - like receptor density in the dorsal ls , and similar abc species differences are observed in several other ls zones for both v1a - like , ot - like and vip binding sites ( goodson et al . , 2006 , 2009c ) .
binding densities are mostly biased toward the gregarious species , although sociality - related differences in ot - like binding densities reverse along a dorso - ventral gradient , and the relative density between the pallial and subpallial ls provides the clearest differentiation of territorial and flocking species ( goodson et al . ,
2009c ; figure 4 ) , suggesting that sociality is reflected in a complex neuromodulatory balance across the ls subnuclei .
outside of the ls , sociality - related differences in the finches are observed for vip binding in the bstm , which are biased toward the gregarious species ( goodson et al . , 2006 ) .
v1a antagonist binding in the lateral septum ( ls ) reflect evolutionary convergence and divergence in flocking and territoriality .
( a  e ) representative i  v1a antagonist binding in the ls of the territorial melba finch [ mf ; ( a ) ] , territorial violet - eared waxbill [ vew ; ( b ) ] , moderately gregarious angolan blue waxbill [ abw ; ( c ) ] , colonial spice finch [ sf ; ( d ) ] , and colonial zebra finch [ zf ; ( e ) ] .
the scale bar in ( e ) corresponds to 500  m in ( a  e ) .
( f , g ) representative sections for a male angolan blue waxbill and male spice finch ( colonial ) , respectively , showing species differences in binding for the nidopallium ( n ) and other areas of the forebrain .
the scale bar in ( g ) corresponds to 1  mm in ( f , g ) .
v1a antagonist binding in the dorsal ( pallial ) portion of the ls , shown as decompositions per min / mg ( dpm / mg ; means    sem ) .
different letters above the error bars denote significant species differences ( fisher 's plsd following significant anova ; p  <  0.0001 ) .
abbreviations : e , entopallium ; ha , apical part of the hyperpallium ; lsc , caudal division of the lateral septum ( dorsal , ventrolateral , and ventral zones denoted as lsc.d , lsc.vl , and lsc.v , respectively ) ; lsr , rostral division of the lateral septum ; lst , lateral striatum ; ms , medial septum ; n , nidopallium ; sh , septohippocampal septum ; teo , optic tectum .
species - specific distributions of oxytocin - like binding sites reflect evolutionary convergence and divergence in flocking and territoriality .
( a  c ) representative autoradiograms of i  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 ) .
different letters above the boxes denote significant species differences ( mann  whitney p  < 
( 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 ( 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 .
densities of nonapeptide receptors in the ls are also highly variable in mammals , although the relevance of these species differences to behavior has largely been a matter of speculation ( insel et al .
however , recent findings suggest that ot receptor densities may reflect species differences in the provision of alloparental care by juvenile females , and alloparental care in female prairie voles is negatively correlated with ot receptor densities in the ls ( olazbal and young , 2006 ) . in male prairie voles , social investigation behavior is positively correlated with v1a receptor densities and negatively with ot receptor densities in the ls ( ophir et al . ,
the relevance of nonapeptide receptors to grouping behavior has been directly confirmed by a series of recent experiments using central antagonist and antisense manipulations in zebra finches . based on the well known effects of ot and ot receptors on affiliation in mammals ( carter et al . , 2008 ; goodson and thompson , 2010 ) , we began by blocking ot - like receptors ( as in mammals , a single ot - like receptor has been identified in birds , vt3 ; baeyens and cornett , 2006 ) . using the choice apparatus shown in figure 5a , in which subjects can choose between groups of 10 or 2 same - sex conspecifics ( or not spend time near either )
, we first showed that subcutaneous and lateral ventricle infusions of an ot antagonist reduce preferences for the larger group ( figures 5b  e ) without influencing the amount of time that subjects spent in close proximity to other birds ( hereafter  contact time  ) .
the peripheral effect was female - specific and ventricular infusions of mt produced a female - specific increase in the amount of time that subjects spent with large group , again with no effect on contact time .
infusions of the ot antagonist directly into the ls replicated the peripheral and ventricular antagonist effects in females , whereas control infusions into the medial striatum ( adjacent to the ventricle ) did not ( goodson et al .
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 ) .
( b  e ) relative to vehicle treatments , subcutaneous ( s.c . ) or intracerebroventricular ( i.c.v . )
nh2 , d(ch2)5[tyr(me ) , thr]ovt ( ota ; 250  ng ) , 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 .
as described in the previous sections , gregarious species show a relatively greater social induction of fos within vt neurons , more vt - ir neurons in the bstm , and a higher density of vt v1a - like receptors in the ls than do territorial species ( goodson and wang , 2006 ; goodson et al . , 2006 ) , suggesting the hypothesis that vt projections from the bstm to the ls promote sociality .
indeed , recent experiments in male zebra finches demonstrate that knockdown of vt production in the bstm by antisense oligonucleotides potently reduces gregariousness relative to subjects infused with scrambled oligonucleotides , with a median reduction of 80% .
 gregariousness  in this experiment was defined as the percent of contact time that was spent next to a group of 10 conspecifics versus a group of 2 ( kelly et al . , 2011 ) .
surprisingly , this same manipulation increased contact time modestly relative to scrambled oligonucleotide controls ( median difference of 25% ) , although intraseptal infusions of a v1a antagonist produce no effects on contact time ( and convincingly so ) .
as with the vt antisense , intraseptal v1a antagonist infusions virtually eliminated subjects preferences for the large group while concomitantly increasing the amount of time that subjects spent with the small group .
we have further replicated this effect in both males and females using a novel v1a antagonist that crosses the blood  brain barrier ( jnj-17308616 ; j. l. goodson and s. e. schrock , unpublished observations ) . in conjunction with both the antisense and central antagonist studies , we also conducted tests of anxiety - like behavior ( novelty - suppressed feeding and exploration of a novel environment ) , which produced particularly intriguing results : whereas septal vp is usually found to be anxiogenic in rodents , it appears to be strongly anxiolytic in zebra finches ( kelly et al . , 2011 )
whether this difference is the result of unique receptor distributions and/or is an important factor in the social evolution of zebra finches remains to be determined .
however , it is interesting in this light to note that septal vp may promote active stress coping in rats ( ebner et al . , 1999 ) , which are more social than laboratory mice and hamsters .
using five finch species that are all socially monogamous and biparental , we have shown that ( 1 ) receptor distributions for multiple neuropeptide systems ( vt / mt , vip , and crf ) exhibit divergent and convergent evolution in relation to species - typical group size , particularly within the ls , ( 2 ) vt cells in the bstm exhibit an exquisite sensitivity to the valence of social stimuli , thereby creating differences between gregarious and territorial species in the response of their bstm vt neurons to same - sex conspecifics , ( 3 ) endogenous nonapeptide signaling via v1a- and ot - like receptors in the ls promotes preferences for larger groups in zebra finches without effects on social contact time , and ( 4 ) in male zebra finches , antisense knockdown of vt production in the bstm profoundly reduces gregariousness . although not discussed above , we have also found that the three flocking finch species exhibit relatively more dopamine neurons than do territorial species in a caudal subpopulation of cells in the ventral tegmental area that has also been implicated in appetitive courtship behavior ( goodson et al . , 2009a ) .
all of the neurochemical systems just mentioned influence myriad behavioral and physiological functions , thus we might expect that those systems may not evolve in relation to grouping , and in an estrildid - like manner , if other species - specific behavioral and physiological functions constrain the evolutionary process .
this may occur if a given neural mechanism is under strong selection in relation to something other than grouping . at the same time , nonapeptides influence basic social behaviors across a wide range of vertebrates , suggesting that they may be common or even ubiquitous targets of selection during social evolution .
the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
 species - typical patterns of grouping have profound impacts on many aspects of physiology and behavior . however , prior to our recent studies in estrildid finches , neural mechanisms that titrate species - typical group - size preferences , independent of other aspects of social organization ( e.g. , mating system and parental care ) , have been wholly unexplored , likely because species - typical group size is typically confounded with other aspects of behavior and biology . 
 an additional complication is that components of social organization are evolutionarily labile and prone to repeated divergence and convergence . 
 hence , we can not assume that convergence in social structure has been produced by convergent modifications to the same neural characters , and thus any comparative approach to grouping must include not only species that differ in their species - typical group sizes , but also species that exhibit convergent evolution in this aspect of social organization . using five estrildid finch species that differ selectively in grouping ( all biparental and monogamous ) 
 we have demonstrated that neural motivational systems evolve in predictable ways in relation to species - typical group sizes , including convergence in two highly gregarious species and convergence in two relatively asocial , territorial species . 
 these systems include nonapeptide ( vasotocin and mesotocin ) circuits that encode the valence of social stimuli ( positive  negative ) , titrate group - size preferences , and modulate anxiety - like behaviors . 
 nonapeptide systems exhibit functional and anatomical properties that are biased toward gregarious species , and experimental reductions of nonapeptide signaling by receptor antagonism and antisense oligonucleotides significantly decrease preferred group sizes in the gregarious zebra finch . combined , these findings suggest that selection on species - typical group size may reliably target the same neural motivation systems when a given social structure evolves independently . 
Introduction: Evolution and Diversity in Social Phenotypes
A Comparative Approach to Grouping
Neural Circuits of Territorial Aggression, Aversion, and Sociality in Songbirds
Nonapeptide Systems: Phenotypic Diversity and the Species-Specific Assignment of Social Valence
Evolutionary Convergence and Divergence in Neuropeptide Receptor Distributions: A Special Status for the Lateral Septum?
Nonapeptides Promote Flocking
Conclusion
Conflict of Interest Statement