Agonistic Encounters Research Articles

Serotonin, social status and aggression

Serotonin, social status and aggression appear to be linked in many animal species, including humans. The linkages are complex,and, for the most part, details relating the amine to the behavior remain obscure. During the past year, important advances have been made in a crustacean model system relating serotonin and aggression. The findings include the demonstration that serotonin injections will cause transient reversals in the unwillingness of subordinate animals to engage in agonistic encounters, and that at specific synaptic sites involved in activation of escape behavior, the direction of the modulation by serotonin depends on the social status of the animal.

Social Stress in Hamsters: Defeat Activates Specific Neurocircuits within the Brain

During an agonistic encounter, subordinate male hamsters display defensive and submissive postures and show increased secretion of glucocorticoids, whereas dominant males do not. To determine whether specific neuronal pathways are activated during the behavioral and neuroendocrine responses of subordinate males, expression of c-fos mRNA within the brains of subordinate males was compared with the pattern in dominant males after fighting. After 1 week of handling, pairs of hamsters were either swapped between cages (handled control males), or were allowed to interact for 30 min [dominant (DOM) males and subordinate (SUB) males]. A second group of control animals that received no handling or social stimulation (unhandled control males) were also included. After testing, all animals were killed by decapitation, their brains were removed for c-fos in situ hybridization, and trunk blood was collected for analysis of plasma cortisol and corticosterone levels. Exposure of males to their partner's cage for 30 min resulted in increased expression of c-fos mRNA in multiple brain regions. In addition, fighting increased c-fos expression in the medial amygdaloid nucleus of both DOM and SUB males as well as having more selective effects. In DOM males, c-fos expression was elevated within the supraoptic nucleus of the hypothalamus. In SUB males, c-fos expression increased within a multitude of brain areas, including cingulate cortex, lateral septum, bed nucleus of the stria terminalis, medial preoptic area, several hypothalamic nuclei, central amygdaloid nucleus, amygdalohippocampal area, dorsal periaqueductal gray, dorsal raphe, cuneiform nucleus, and locus coeruleus. These findings are discussed in relation to neurocircuits associated with behavioral arousal and stress.

Survival and behaviour of castrated Soay sheep (Ovis aries) in a feral island population on Hirta, St. Kilda, Scotland

The free‐living population of Soay sheep on the island of Hirta, St. Kilda, in the Outer Hebrides, has been intensively studied since 1959. The present study was initiated to throw light on the causes of the high mortality rate of adult rams in comparison to that of ewes. In 1978, 1979 and 1980, a total of 72 male lambs was castrated within a day or two of birth. The survival of these castrates has been much longer than that of the entire rams, marked as controls, and longer than that of ewes of the cohorts of the same years. The daily activity pattern of the castrates was similar to that of ewes rather than that of rams. In particular, during the rut the castrates spent most of the daylight hours grazing, in contrast to the rams who were continuously moving and involved in agonistic and sexual encounters. This study substantiates the earlier assertion that the costs in energy of reproduction are a major cause of mortality in temperate zone ungulates. The social organization of some castrates was similar to that of females in that they remained with the home‐range group of ewes into which they were born, but other individuals resembled males in that these castrates clubbed together in their own groups. These patterns of behaviour persisted throughout life.

Preparing gilts for group housing by increasing their social skills

Former experiments have shown that social experience during nursing in either a group housing system with straw or a very spacious farrowing/nursing pen with straw, increases social skills of growing pigs considerably. Based on experiences in Taiwan, it was postulated that such skills could also be acquired in the period between being taken from flatdecks at 10 weeks of age and the age of 5 months. In an ‘arena’, copied from the one used for introducing new pigs (gilts) in a group housing system, agonistic behaviour and number and severity of lesions of the integument of 384 experimental GY × DUROC hybrid gilts were studied and compared with data of an equal number of control gilts in three different treatments (repeated once). The experimental animals were born in individual farrowing crates with slatted floors. They were weaned at 30 days and kept in flatdeck cages with two other litters, until 10 weeks. At that time they were kept in groups of eight on fully slatted floors and treated with regrouping and repenning several times until the final testing at an age of 5 months. In sequential experiments, this treatment was given twice, three times or four times and effects on social (agonistic) behaviour were studied. The control animals were born in a group farrowing house with straw, where they stayed 30 days (some days after weaning). As for the experimental pigs, two litters were grouped in flatdeck cages. At 10 weeks of age they were moved to pens with fully slatted floors, where they remained in groups of eight undisturbed until final testing at 5 months. At an age of five months, eight unfamiliar experimental animals were grouped in a so-called arena for 24 h. So were 8 control pigs in the same week. Time-lapse video observations during these 24 h (2 frames/s) revealed that the pigs were fighting for social dominance during this period. Experimental pigs that had been regrouped four times only showed 57% of bilateral agonistic actions and 82% of total fighting time, in comparison with control pigs. Three times regrouping still showed a statistically significant lower amount of fighting, but twice regrouping did not. After 24 h in the arena, the number of lesions of the integument was significantly smaller in groups that had experienced regrouping 3 or 4 times. These results proved that social experience by regrouping and repenning prepared gilts for agonistic encounters, by improving their social skills.

Fighting success in relation to body mass and drumming activity in the male wolf spider Hygrolycosa rubrofasciata

In the wolf spider Hygrolycosa rubrofasciata, males court females by drumming dry leaves with their abdomen. Females prefer to mate with the most actively drumming males, and courtship drumming activity is also positively correlated with male viability. However, body mass of the males seems to have only a minor, if any, effect on female choice or male viability. There is also no correlation between male body mass and courtship drumming activity. We studied the effect of body mass and courtship drumming activity on the outcome of agonistic encounters between male H. rubrofasciata. For this purpose, males and females were randomly placed in a plastic arena, where male courtship drumming activity and agonistic encounters were recorded. Large differences in body mass and drumming activity between two rivals seemed to independently increase the probability of the larger or more active males winning. We conclude that while courtship drumming activity affects the fighting success of the males, and body mass more so, male–male interactions may not be of major importance in sexual selection of H. rubrofasciata.

Courtship song, male agonistic encounters, and female mate choice in the house cricket,Acheta domesticus (Orthoptera: Gryllidae)

Whether female crickets choose among males based on characteristics of the courtship song is uncertain, but in many species, males not producing courtship song do not mate. In the house cricket,Acheta domesticus, we examined whether a female chose or rejected a male based on his size, latency to chirp, latency to produce courtship song, or rate of the high-frequency pulse of courtship song (“court rate”). We confirmed that females mated only with males that produced courtship song, but we found no evidence that the other factors we measured affected a female’s decision to mate. In addition, we investigated whether the outcome of male agonistic encounters affected the subsequent production of courtship song. In one experiment, we observed courtship and mating behavior when a single female was placed with a pair of males following a 10-min interaction period between the two males. Winners of male agonistic encounters had higher mating success. However, winners and losers of agonistic encounters were not different in their likelihood or latency to produce courtship song or in the number of times they were disrupted by the other male in the pair. In a second experiment, we allowed two males to interact for a 10-min period, but following this interaction period, we placed a female with each male separately and observed courtship and mating behavior. The mating success of winners and losers was not different under these circumstances, and we found no differences between winners and losers in any subsequent courtship or mating behavior examined. We conclude that winning agonistic encounters influences a male’s mating success in ways other than his production of courtship song and this effect is lost when winning and losing males are separated and each is given an opportunity to mate.

Behaviour and productivity of pigs in groups composed of disproportionate numbers of littermates

Forty litters of eight pigs each were paired by age and weight and re-grouped at the beginning of a 12-wk grower/finisher period. Each pair of litters was assigned to two pens, with two pigs from one litter (Minority) and six pigs from the other (Majority) in each pen. Four pairs of litters were removed from the experiment due to loss of pigs from injury or poor health. Agonistic encounters following re-grouping were used to rank each Minority pig and to classify it as being within the Dominant or Subordinate half of the social hierarchy. Minority pigs within a pen tended to obtain similar social ranks (within two ranks in 20 of 30 pens; P < 0.05), but were as likely to be Dominant as Subordinate (11 vs. 9 pairs, respectively). The activity and position of all pigs were sampled once daily, 5 days a week. One Minority pig was designated as the focal pig for these observations. The second Minority pig was more likely to be engaged in the same activity as the focal pig than were Majority pigs (P < 0.01), and would lie closer to the focal pig than would the Majority pigs (P < 0.01). There were no differences in the proportion of Majority and Minority pigs which were removed from the experiment due to injuries, nor in the type of injuries requiring their removal. Examination of all pigs at the mid-point of the study revealed some evidence of injury on the head, ears, shoulders, back, flanks, ramp and tail of 76, 75, 87, 90, 91 and 95% of the pigs, respectively, but no differences between Majority and Minority pigs in location or severity of injuries. Average daily gain for the entire experiment was 775 g d−1, and did not differ between Majority and Minority pigs during any 2-wk period of the study. It is concluded that littermates continue to associate with each other after re-grouping and that they attain similar social status. However, the litter with the largest number of pigs present does not consistently achieve social dominance or a competitive advantage manifested in improved weight gain. Key words: Pigs, littermates, re-grouping, social status

Serotonin and aggressive motivation in crustaceans: altering the decision to retreat.

In crustaceans, as in most animal species, the amine serotonin has been suggested to serve important roles in aggression. Here we show that injection of serotonin into the hemolymph of subordinate, freely moving animals results in a renewed willingness of these animals to engage the dominants in further agonistic encounters. By multivariate statistical analysis, we demonstrate that this reversal results principally from a reduction in the likelihood of retreat and an increase in the duration of fighting. Serotonin infusion does not alter other aspects of fighting behavior, including which animal initiates an encounter, how quickly fighting escalates, or which animal eventually retreats. Preliminary studies suggest that serotonin uptake plays an important role in this behavioral reversal.

Reduced aggressive behaviour in mice with targeted disruption of the oxytocin gene.

Oxytocin (OT) has been reported to mediate aggressive and affiliative behaviours in several species. The behavioural role of OT has been established with physiological manipulations that potentially affected blood pressure, which may have indirectly affected the behaviours under study. To provide converging evidence of the physiological role of OT in aggressive behavior, wild type (WT), heterozygous (OT-/+), and homozygous (OT-/-) mutant mice were tested in two aggression paradigms. In general, there was no significant difference in aggressiveness between WT and OT-/+ mice. However, there were significant reductions in the duration of aggressive behaviors among OT-/- animals, especially in agonistic encounters within neutral arenas. The OT-/- mice did not exhibit any sensorimotor deficits or display any altered general anxiety levels that may have accounted for the observed reduction in aggressive behavior. These data indicate that aggression is mediated in part by OT in mice and that increased aggressiveness is not an obligatory phenotypic result of targeted genetic disruption of any gene.

Effect of repeated experience of victory and defeat in daily agonistic confrontations on brain tryptophan hydroxylase activity

The rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase (TPH), was studied in brain areas of male mice with repeated experience of victory (winners) or defeat (losers) gained in 10 daily agonistic confrontations. A reduction of TPH activity in the midbrain and an increase in the hypothalamus was demonstrated for winners compared with controls. In contrast, repeated defeat in social confrontations was associated with higher TPH activity in the striatum and hypothalamus in losers compared with controls. Agonistic interactions did not affect TPH activity in the amygdala, nucleus accumbens or hippocampus in either winners or losers. The sensory contact technique used in this work for generating winners and losers may be productive in the analysis of TPH gene regulation.

Signals in intra-sexual competition between ring-necked pheasant males

Many sexual ornaments are signals commonly used in both sexual selection contexts: mate choice and intra-sexual competition. Previous studies show that female choice in ring-necked pheasants,Phasianus colchicus, is influenced by features of some male ornaments. Experiments and correlational data were used in this study to investigate the role of male ornaments in male–male agonistic encounters. Some traits used by females in mate choice, namely tail length and black points in the wattle, had no effect in male–male competition, but the length of ear tufts had a role in both contexts. The most important traits appeared to be the head ornaments, which include ear tufts, but also wattle size. Head ornaments function as coverable signals that are displayed to rival males informing about readiness to fight, fighting ability and resource-holding power; and the total time spent displaying correlated with testosterone level, physical condition and rank. Thus, the costs of displaying head ornaments appear to be both inherent and target-receiver dependent.

Social order measured in pasture and pen conditions and its relationship to sexual behavior in Brahman ( Bos indicus) cows

The relation of the social order between cows was determined by recording content and outcome of all agonistic interactions in a herd of 20 non-lactating, non-pregnant Brahman cows ranging from 2–6 years old, under both pasture and yard conditions. The herd was observed at pasture for periods of 1.5 h and under pen conditions for periods of 2 h on the same day. This routine was undertaken every other day from 06.00 to 18.00 between February and June 1995, giving a total observation time of 375 h, after which all cows were given an ear implant of Synchro-mate B (SMB), an estrous synchronization treatment. On the day of implant withdrawal a mature, 5-year-old bull with proven service capacity was placed with the cows. Continuous observation for estrous detection was carried out for 96 h following implant removal. The Kendal rank correlation coefficient was used to determine the relationship between social and sexual variables for the total of 1750 agonistic encounters recorded. The top ranked cow supplanted other animals 225 times. In contrast, the lowest ranked cow never supplanted another animal. Correlation between social order in the yard, in the field, and their sum, ranged from 0.26 to 0.34, with linearity values over 80% for the three variables. Identical values were possible only with respect to the extreme of the ranking scale. Social index values were low and showed little variation, ranging from 0.0 to 0.128. Estrous efficiency expressed as the percentage of SMB treated animals that show estrous was 60%. A low correlation was observed between dominance order and sexual behavior. However, correlation coefficients above 0.9 were observed within the sexual variables such as mounts exhibited, received, number of homosexual participants, or male attention. Our results indicate that dominance order may differ according to the data source (yard, field or their sum), and that there is no relation between social order and sexual behavior.

Altered Excitability of the Crayfish Lateral Giant Escape Reflex during Agonistic Encounters

The excitability of the lateral giant escape reflex of socially dominant and submissive crayfish at rest and during agonistic encounters was studied and compared. During agonistic encounters the excitability of the lateral giant reflex falls, substantially in subordinates and slightly in dominants, whereas at rest excitability seems to be independent of social status. Thus, paradoxically, socially dominant animals are more likely to execute lateral giant escape reactions during interactions than are subordinates. It is suggested that subordinates under threat of attack tend to engage circuitry involved in flexible, nonreflex ("voluntary") types of escape not mediated by giant neurons and therefore inhibit giant neuron-mediated reflex circuitry that produces prompt, but less adaptive, responses. In contrast, dominants go about their business, mainly ignoring their conspecifics and relying on reflex escape to protect them from unexpected attack. Consistent with this view, escape of subordinates during agonistic encounters is mediated by nongiant, not reflex, circuitry. These observations and their interpretation suggest a possible functional role for recently described social status-dependent serotonergic modulation of the lateral giant reflex, which is inhibitory in sign in subordinates and facilitatory in dominants.

Effect of anosmia on the behavior of standard non-aggressive male mice opponents during agonistic encounters

This study assessed whether the differences observed in a previous experiment [Martinez M, Salvador A, Simon VM (1994): Aggressive Behavior 20:441–451] in the behavior of isolated male mice over several agonistic encounters in which they confronted either an “anosmic” or an “intact” non-aggressive “standard opponent” were due to the differences in the behavior of the opponents themselves. Ethologically-inspired analysis was used to assess the behavior of the opponents during the first agonistic encounter. Anosmic opponents spent less time in social investigation and defense and more time in immobility than intact ones. These results suggest that the differences in the behavior of the opponents could be one of the causes of the differences observed in the behavior of the aggressive males confronting them. However, more research is needed to study if other variables, such as the production of odors, are also affected by the state (deprived or intact) of the olfactory sense. These findings provide additional support for the view that the type of opponent used in studies on intermale aggression is of paramount importance.Aggr. Behav. 23:179–181, 1997.© 1997 Wiley-Liss, Inc.

Group Encounters in Wild Gibbons (Hylobates Lar): Agonism, Affiliation, and the Concept of Infanticide

Abstract 1. Gibbons are the least studied apes and traditionally thought to live in nuclear families of 2-6 individuals including a pair of breeding adults who maintain lifelong, sexually monogamous relationships and vigorously defend territories against neighbours. The present paper challenges this view. 2. During a long-term study on white-handed gibbons (Hylobates lar) in Thailand's Khao Yai rainforest, 162 encounters were recorded between 3 habituated and 8 non-habituated groups. Encounters characterized 9% of the activity day (mean 8.31 h). Those between habituated groups were nine times more frequent (0.7/day) and lasted significantly longer (median 70 min) than encounters with non-habituated neighbours (median 38 min). It was also found that gibbon group home ranges (mean 24 ha) overlap extensively (64%) with neighbours', all of whom were met. However, most previous studies centered on single groups surrounded by unhabituated neighbours. This produced underreporting of the importance of inter-group interactions, particularly the affiliative aspects observed presently. 3. Encounters included many elements which seem to have a 'defensive' function. Chases occurred during 61%, contact aggression during 8-9%; each adult and subadult male chased all others and was chased by all others. Moreover, encounters occurred often in or near food trees and rates peaked during the early morning when ripe fruit were most abundant. However, a seasonal correlation between the rate of encounters and (crude) measures of resource availability could not be detected. 4. The study indicates that gibbon groups are structured by female resource-defense and male mate-defense because adult females led 76% of all travels into and out of fruit trees, whereas males moved to the front as soon as neighbours were encountered. Male-male interactions constituted 90% of all inter-group chases. This pattern is compatible with the idea that conflicts over food sources (which can be shared) will rarely provoke contact aggression. Control of mate access, on the other hand, has a much higher relative value for males. These explanations are more parsimonious than the traditional concepts of 'pairbonding' and 'territoriality'. 5. Contrary to earlier assumptions, encounters were nevertheless not always dominated by aggression. Groups fed, traveled or rested together for prolonged times (35% of encounters). Inter-group play between immatures was likewise common (21 % of encounters; 55% dyadic, 45% triadic and quartetic play). Same-aged play partners are not present in a gibbon's natal group. However, gibbon youngsters clearly preferred age-mates during inter-group play which may indicate that play is ontogenetically important. 6. The adult female of one group copulated not only with her pair-mate, but also with two neighbouring males. The overall proportion of extra-pair-copulations (EPC) was 12% and they occurred during a period when the female conceived. Encounters thus provide opportunities for alternative mating strategies. However, philandering males are faced with the dilemma to lower their mate guard which creates a risk of cuckoldry for themselves. This could explain why subadults are often tolerated in natal groups beyond sexual maturity, because they assisted the resident adult male during agonistic encounters. Moreover, females gave solo great calls during a quarter of all encounters. These calls increase the costs of philandering for the paired male (who cannot answer without giving away that his female is unguarded) and may at the same time attract neighbouring males. 7. For the first time, close spatial proximity and body contact between intruding adult males and infants of neighbouring groups are reported, including play (during 6% of all inter-group play sessions). Immatures were at times relaxed but at other times frightened in the presence of neighbouring males. A near-zero mortality of infants at Khao Yai shows that infanticide is absent or at least not a regular occurrence. EPCs and a long period of pre-conception copulations could be strategies of females to confuse paternity and forestall infanticide. In any case, the non-monogamous mating pattern makes it likely that kin-relationships extend well into neighbouring groups. A reduced level of inter-group competition and aggression is therefore not surprising.

Comparative analysis of anxiety-like behavior in partition and plus-maze tests after agonistic interactions in mice

The behavior of C57BL/6J male mice with experience of repeated victories (winners) or defeats (losers) in daily agonistic interactions, was examined in the plus-maze and partition tests. The latter procedure assesses the reactivity of mice to another conspecific in the neighboring compartment of a common cage, communicative behavior or level of sociability. The behavior of mice after 10 days (T10) and 20 days (T20) of agonistic confrontations, as well as in controls (5 days of individual housing) was analyzed. Significant differences were found between T10 and T20 losers and controls in both tests. In the partition test, a decrease in the number of approaches and total and average time spent near the partition was found in T10 and, more pronounced, in T20 losers. In the plus-maze, losers showed fewer open and total entries than controls. Moreover, rarely did they pass from one enclosed arm to another, and also showed a decreased number of ‘peepings’ from enclosed arms. Percentage of open time did not differ significantly in losers compared to controls. It is suggested that the level of losers' sociability, estimated as low by the partition test, can include anxiety as a component, which is confirmed, at least partly, by some parameters of the plus-maze test. There were no differences in the partition or plus-maze tests between T10 winners and controls. However, all parameters of partition behavior were significantly different between T20 winners and controls. In the plus-maze, similar to losers, T20 winners displayed fewer open arm entries, number of ‘peepings’, and passages than controls. It was concluded that parameters of the partition and plus-maze tests correlate in mice with the alternative experience of agonistic confrontations, positive or negative. A combination of these two tests may be used for estimation of developing anxiety in mice.

Pronounced heterochely in the ghost shrimp, Neotrypaea californiensis (Decapoda: Thalassinidea: Callianassidae): allometry, inferred function and development

To understand their function and ontogeny better, we conducted a morphometric analysis of claw size and shape variation in the strikingly heterochelous, north‐eastern Pacific ghost shrimp, Neotrypaea (formerly Callianassa) californiensis. Master claws approached 25% of total body weight in mature males, but rarely exceeded 10% in females. Minor claws were less than 3% of body weight in both sexes. The proportions of right and left master claws did not differ significantly from 50:50. Males exhibited a greater positive allometry than females in both master and minor claw size, though master claws differed more than minor claws. Sexual dimorphism was also observed in master but not minor claw shape: compared to females, mature male master claws: a) were proportionally higher relative to their length; b) exhibited a deeper propodal notch and consequently a larger gape; c) developed a more slender and more distally hooked dactyl; and d) exhibited more well‐developed teeth about the periphery of the claw gape.The shape of the conspicuous gape in mature male master claws bore a close resemblance to the cross‐section of similar‐sized master claws. The shape of this gape, and the presence of fine teeth about its periphery, strongly suggests that master claws function in a highly stereotyped form of grappling during agonistic encounters or perhaps during mating between similar‐sized conspecifics. In addition, a landmark morphometric analysis of relative growth suggested that the pronounced propodal notch develops via localized deformations near the base of the fixed finger rather than via a more generalized contraction of the ventral manus region. Finally, a preliminary survey suggests that the distinctive propodal notch, which may be diagnostic of the hypothesized grappling function, has evolved at least twice in the Callianassidae, once in the Callianassinae and once in the Callichirinae. Sexual selection may have significantly influenced the evolution of these unusual master claws.

Gargle vocalizations of black-capped chickadees: test of repertoire and video stimuli

The gargle call is uttered by male black-capped chickadees, Parus atricapillus, during agonistic encounters; in fights between two birds, the gargler nearly always wins. Individuals vary in the number of different types of gargles they have in their repertoires. An experiment was conducted on captive chickadees to determine whether they had a greater aversive response to a larger gargle repertoire. The measure of aversiveness was the time spent away from the stimulus. Video images of a gargling chickadee or of no bird were paired with different gargle repertoire sizes as the test stimuli. The treatment causing the most aversive response was a video of a stimulus bird paired with a repertoire of three gargle types broadcast from a loudspeaker. Only low response was found when a vocal stimulus of three gargle types was presented without the video of the stimulus bird, or when the video was presented with no gargling. The treatment in which the video of a chickadee was paired with a repertoire of a single gargle type was of intermediate aversiveness. Black-capped chickadees find the presence of a potential rival that is giving a multiple-gargle repertoire to be more threatening than one giving a single-gargle repertoire.

Absence of social recognition in laboratory-reared cuttlefish,Sepia officinalisL. (Mollusca: Cephalopoda)

Five experiments were performed to determine the level of social recognition in captive-reared adult cuttlefish,Sepia officinalisL. No evidence of discrimination of familiar from unfamiliar individuals was found in either females or males. Despite good evidence for mate guarding, no recognition of individual mates was found. Within sex classes, associations between freely moving animals were not different from random (f–f, f–m and m–m). Male dominance, measured by displacement success, was consistent from day to day, was related to size and was consistent with number of copulations obtained. Dominance ranks were not learned or recognized and did not result in energy savings through a reduction in agonistic encounters. Social interactions of these marine invertebrates depend upon relative size, internal motivation and the behaviour patterns of conspecifics, rather than upon any direct recognition of social partners.

Organization of Agonistic Vocalizations in Black-Chinned Hummingbirds

We describe vocalizations of Black-chinned Hummingbirds (Archilochus alexandri) recorded during agonistic encounters at feeders. Calls are composed of one to five different note-types that comprise a recombinatorial system exhibiting syntax. A Markov analysis revealed non-random ordering of note-types. The distribution of call-types (unique combinations of notes) illustrates openness; the number of call-types increases as more calls are sampled. Constraints on call length occur that are related to the length of individual note-types; shorter note-types are more common in calls with more notes. No sex differences occurred in the call-types with the exception of the Z note which occurred more often in male calls. The agonistic vocalizations of these hummingbirds demonstrate a level of vocal complexity comparable to songs of many passerines. We compare the vocalizations of the Black-chinned Hummingbird with studies of Anna’s Hummingbird (Calypte anna) and point out major differences in repertoire organization. Marked similarities occur between organization of calls in certain chickadees (Parus) and that of the Black-chinned Hummingbird. This finding is surprising in view of their phyletic differences, but may reflect certain underlying constraints on the organization of avian vocalizations.