Acoustics and behavioral contexts of "gecker" vocalizations in young rhesus macaques (Macaca mulatta)

Vocal expression of emotional arousal across two call types in young rhesus macaques

As Darwin first recognized, the study of emotional communication has the potential to improve scientific understanding of the mechanisms of signal production as well as how signals evolve. We examined the relationships between emotional arousal and selected acoustic characteristics of coo and scream vocalizations produced by female rhesus macaques, Macaca mulatta, during development. For coos, arousal was assessed through measures of stress-induced elevations of plasma cortisol exhibited in response to the human intruder test. In the analysis of screams, arousal was evaluated from the intensity of aggression experienced by the vocalizer during natural social interactions. Both call types showed a positive relationship between arousal and overall fundamental frequency (F0, perceived as pitch in humans). In coos, this association was dampened over development from infancy (6 months) to the juvenile, prepubertal period (16 months) and further to menarche (21.3-31.3 months), perhaps reflecting developmental changes in physiology, anatomy and/or call function. Heightened arousal was also associated in coos with increases in an acoustic dimension related to F0 modulation and noisiness. As monkeys matured, coos showed decreases in overall F0 as well as increased noisiness and F0 modulation, likely reflecting growth of the vocal apparatus and changes in vocal fold oscillation. Within screams, only one acoustic dimension (related to F0 modulation) showed developmental change, and only within one subclass of screams within one behavioural context. Our results regarding the acoustic correlates of arousal in both call types are broadly consistent with findings in other species, supporting the hypothesis of evolutionary continuity in emotion expression. We discuss implications for broader theories of how vocal acoustics respond to selection pressures.

Variation in the Mu-Opioid Receptor (OPRM1) and Offspring Sex Are Associated With Maternal Behavior in Rhesus Macaques (Macaca mulatta)

A μ-opioid receptor (OPRM1) single-nucleotide-polymorphism, found in both humans and rhesus macaques mediates the mother-infant attachment bond. Because mothers treat their sons and daughters differently, it is somewhat surprising that the role of infant sex has not been assessed in the context of a maternal-OPRM1-genotype-by-infant-sex interaction. The present study investigates the effect of maternal-OPRM1-genotype and infant sex on mother-infant behaviors. Over the first 6 months of offspring life, mother-infant behavioral data assessing attachment quality was collected twice weekly from a large number of rhesus monkey mother-infant pairs (N = 161 dyads; n = 64 female infants, n = 97 male infants). Mothers were genotyped for OPRM1 variation. Factor analysis of the observed behaviors showed two factors: Attachment (maternal-infant cradling, rejections, and infant approaches and leaves), and Maternal Restraints (mother restrains infant, preventing exploration). Further analyses showed a two-way, maternal-genotype-by-infant-sex interaction for both factors. For Attachment, mothers with the CC genotype cradled and restrained (Maternal Restraints) their female infants more and rejected them less, when compared to female infants of CG mothers. Perhaps as a consequence, female infants of CC genotype mothers approached and left their mothers less often, when compared to female infants of CG mothers, likely an indication that female infants from mothers with CG genotype play a greater role in maintaining the mother-infant bond than do female infants from CC genotype mothers. This finding may also indicate a more secure attachment in infants from CC genotype mothers. Unlike female infants, on average, the mother-infant relationship of dyads with a male infant was largely undifferentiated by maternal genotype. These findings suggest that, in contrast to female infants from CG mothers, CC mothers and their female infants appear to have a closer mother-infant relationship which may portend close life-long bonds, as mothers and female offspring remain together throughout life. Male offspring appear to have a more aloof mother-infant bond regardless of OPRM1-genotype. The results of this study indicate that maternal-OPRM1 variation mediates mother-infant attachment behaviors for female infants and has less effect for male infants. This suggests that offspring sex should be included in studies investigating the effect of maternal-OPRM1 genotype on the mother-infant attachment relationship.

Ultrasonic vocalizations in golden hamsters (Mesocricetus auratus) reveal modest sex differences and nonlinear signals of sexual motivation

Vocal signaling is one of many behaviors that animals perform during social interactions. Vocalizations produced by both sexes before mating can communicate sex, identity and condition of the caller. Adult golden hamsters produce ultrasonic vocalizations (USV) after intersexual contact. To determine whether these vocalizations are sexually dimorphic, we analyzed the vocal repertoire for sex differences in: 1) calling rates, 2) composition (structural complexity, call types and nonlinear phenomena) and 3) acoustic structure. In addition, we examined it for individual variation in the calls. The vocal repertoire was mainly composed of 1-note simple calls and at least half of them presented some degree of deterministic chaos. The prevalence of this nonlinear phenomenon was confirmed by low values of harmonic-to-noise ratio for most calls. We found modest sexual differences between repertoires. Males were more likely than females to produce tonal and less chaotic calls, as well as call types with frequency jumps. Multivariate analysis of the acoustic features of 1-note simple calls revealed significant sex differences in the second axis represented mostly by entropy and bandwidth parameters. Male calls showed lower entropy and inter-quartile bandwidth than female calls. Because the variation of acoustic structure within individuals was higher than among individuals, USV could not be reliably assigned to the correct individual. Interestingly, however, this high variability, augmented by the prevalence of chaos and frequency jumps, could be the result of increased vocal effort. Hamsters motivated to produce high calling rates also produced longer calls of broader bandwidth. Thus, the sex differences found could be the result of different sex preferences but also of a sex difference in calling motivation or condition. We suggest that variable and complex USV may have been selected to increase responsiveness of a potential mate by communicating sexual arousal and preventing habituation to the caller.

Latent variables affecting behavioral response to the human intruder test in infant rhesus macaques (Macaca mulatta)

The human intruder test is a testing paradigm designed to measure rhesus macaques' behavioral responses to a stressful and threatening situation. In the test, an unfamiliar human positions him/herself in various threatening positions relative to a caged macaque. This paradigm has been utilized for over 20 years to measure a variety of behavioral constructs, including fear and anxiety, behavioral inhibition, emotionality, and aggression. To date, there have been no attempts to evaluate comprehensively the structure of the behavioral responses to the test. Our first goal was to identify the underlying latent factors affecting the different responses among subjects, and our second goal was to determine if rhesus reared in different environments respond differently in this testing paradigm. To accomplish this, we first performed exploratory and confirmatory factor analyses on the behavioral responses of 3- to 4-month-old rhesus macaques, utilizing data from over 2,000 separate tests conducted between 2001-2007. Using the resulting model, we then tested to see whether early rearing experience affected responses in the test. Our first analyses suggested that most of the variation in infant behavioral responses to the human intruder test could be explained by four latent factors: "activity," "emotionality," "aggression," and "displacement." Our second analyses revealed a significant effect of rearing condition for each factor score (P < 0.001); most notable socially reared animals had the lowest activity score (P < 0.001), indoor mother-reared animals had the highest displacement score (P < 0.001), and nursery-reared animals had the highest emotionality (P < 0.001) and lowest aggression scores (P < 0.001). These results demonstrate that this standardized testing paradigm reveals multiple patterns of response, which are influenced by an animal's rearing history.

Silky sifaka (Propithecus candidus) "zzuss" vocalizations: sexual dimorphism, individuality, and function in the alarm call of a monomorphic lemur

Vocalizations of Madagascar's lemurs have generally been less investigated than those of other primate groups, with virtually no information available about calling in the silky sifaka (Propithecus candidus), a large rainforest species. Current work examined the "zzuss" vocalization, one of the most common and loudest sounds produced by this monomorphic species, and included 160 calls from nine adults (five males, four females) in three groups. Analyses focused on overall acoustic features, individual and sex differences, call usage, and likely function. Acoustically, the calls included separable turbulent noise and tonal components, with the later often marked by frequency jumps and dramatic frequency modulation. Male and female zzuss calls differed most in F0- and amplitude-related features, characteristics that are relatively unconstrained by overall body size. All measures differed among individual callers, with F0-related variables again playing the largest role. Based on usage, these calls most likely function both as generalized alarm and group-coordination signals. The sounds were thus of interest in several regards, including showing sexual differentiation in the absence of other dimorphisms, exhibiting primarily F0-based differentiation in both sex- and individual-based comparisons, and combining apparent alarm and coordination functions across a variety of contexts.

Vocal cues indicate level of arousal in infant African elephant roars

Arousal-based physiological changes influence acoustic features of vocalizations in mammals. In particular, nonlinear phenomena are thought to convey information about the caller's arousal state. This hypothesis was tested in the infant African elephant (Loxodonta africana) roar, a call type produced in situations of arousal and distress. Ninety-two percent of roars exhibited nonlinear phenomena, with chaos being the most common type. Acoustic irregularities were strongly associated with elevated fundamental frequency values. Roars produced in situations of highest urgency, based on the occurrence of behavioral indicators of arousal, were characterized by the lowest harmonics-to-noise ratio; this indicates low tonality. In addition, roars produced in these situations lasted longer than those produced in contexts of lower presumed urgency. Testing the infant roars for individual distinctiveness revealed only a moderate classification result. Combined, these findings indicate that infant African elephant roars primarily function to signal the caller's arousal state. The effective communication of this type of information may allow mothers to respond differentially based on their infant's degree of need and may be crucial for the survival of infant African elephants in their natural environment.

Acoustic variability and social significance of calls in female Campbell's monkeys (Cercopithecus campbelli campbelli)

Although the vocal repertoire of nonhuman primates is strongly constrained by genetic, a growing number of studies evidence socially determined flexibility. According to Snowdon et al. [Social Influences on Vocal Development (University Press, Cambridge, 1997), pp. 234-248], calls with a higher social function (affiliative or agonistic) would be expected to show more flexibility than lesser social calls. Owren and Rendall [Evol. Anthropol., 10, 58-71 (2001)] nuanced this by defending a structure-function relationship. Calls with particular acoustic properties, which directly influence the listener's affect, would be less individually distinctive than calls involved in an affective conditioning process. These hypotheses were tested in Campbell's monkeys using telemetric recordings. This is the first detailed description of female Campbell's monkeys' vocal repertoire emphasizing a possible relationship between social function and flexibility level. The vocal repertoire displayed an "arborescent" organization (call type, subtype, and variants). The highest number of subtypes and the greatest acoustic variability, within and among individuals, were found in calls associated with the highest affiliative social value. However, calls associated with agonism were the most stereotyped, whereas less social alarm calls were intermediate. This only partially validate the hypothesis of Snowdon et al. In accordance with Owren and Rendall's hypotheses, the level of individual distinctiveness was minimum for noisy pulsed calls and maximum for calls involved in affiliative interactions.

Elasticity and stress relaxation of rhesus monkey (Macaca mulatta) vocal folds

Fundamental frequency is an important perceptual parameter for acoustic communication in mammals. It is determined by vocal fold oscillation, which depends on the morphology and viscoelastic properties of the oscillating tissue. In this study, I tested if stress-strain and stress-relaxation behavior of rhesus monkey (Macaca mulatta) vocal folds allows the prediction of a species' natural fundamental frequency range across its entire vocal repertoire as well as of frequency contours within a single call type. In tensile tests, the load-strain and stress-relaxation behavior of rhesus monkey vocal folds and ventricular folds has been examined. Using the string model, predictions about the species' fundamental frequency range, individual variability, as well as the frequency contour of 'coo' calls were made. The low- and mid-frequency range (up to 2 kHz) of rhesus monkeys can be predicted relatively well with the string model. The discrepancy between predicted maximum fundamental frequency and what has been recorded in rhesus monkeys is currently ascribed to the difficulty in predicting the behavior of the lamina propria at very high strain. Histological sections of the vocal fold and different staining techniques identified collagen, elastin, hyaluronan and, surprisingly, fat cells as components of the lamina propria. The distribution of all four components is not uniform, suggesting that different aspects of the lamina propria are drawn into oscillation depending on vocal fold tension. A differentiated recruitment of tissue into oscillation could extend the frequency range specifically at the upper end of the frequency scale.

Asymmetries in the individual distinctiveness and maternal recognition of infant contact calls and distress screams in baboons

A key component of nonhuman primate vocal communication is the production and recognition of clear cues to social identity that function in the management of these species' individualistic social relationships. However, it remains unclear how ubiquitous such identity cues are across call types and age-sex classes and what the underlying vocal production mechanisms responsible might be. This study focused on two structurally distinct call types produced by infant baboons in contexts that place a similar functional premium on communicating clear cues to caller identity: (1) contact calls produced when physically separated from, and attempting to relocate, mothers and (2) distress screams produced when aggressively attacked by other group members. Acoustic analyses and field experiments were conducted to examine individual differentiation in single vocalizations of each type and to test mothers' ability to recognize infant calls. Both call types showed statistically significant individual differentiation, but the magnitude of the differentiation was substantially higher in contact calls. Mothers readily discriminated own-offspring contact calls from those of familiar but unrelated infants, but did not do so when it came to distress screams. Several possible explanations for these asymmetries in call differentiation and recognition are considered.