Left-handedness is Correlated with CSF Monoamine Metabolite and Plasma Cortisol Concentrations, and with Impaired Sociality, in Free-ranging Adult Male Rhesus Macaques (Macaca mulatta)

In this research we examined biological and behavioural correlates of handedness in free-ranging adult male rhesus macaques (Macaca mulatta). Specifically, we examined relationships between handedness and cerebrospinal fluid (CSF) concentrations of the monoamine metabolites 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), and 3-methoxy-4-hydroxyphenylglycol (MHPG), plasma concentrations of the hormones cortisol and adrenocorticotropin (ACTH), and multiple indices of social behaviour, including occurrences of proximity to other animals, grooming, submission, and aggression. We determined handedness through systematic observation of animals reaching for food in their unrestricted home environment. The frequency of right- versus left-hand use was significantly positively correlated with CSF 5-HIAA, CSF MHPG, and plasma cortisol concentrations, and with social proximity and the frequency and duration of bouts in which animals received grooming. The frequency of right- versus left-hand use was significantly negatively correlated with the frequency of submissive behaviour, and with the frequency and intensity of bouts in which animals received aggression. We conclude that handedness is associated with an array of biological and behavioural processes in free-ranging adult male rhesus macaques and that left-handedness may be used to identify individuals at increased risk for impaired functioning of the serotonin, norepinephrine, and hypothalamic-pituitary-adrenal systems, and for social isolation and susceptibility to violent attack.

CSF 5-HIAA concentration as an early screening tool for predicting significant life history outcomes in female specific-pathogen-free (SPF) rhesus macaques (Macaca mulatta) maintained in captive breeding groups

We examined relationships among cerebrospinal fluid (CSF) concentrations of the major serotonin metabolite (5-HIAA, 5-hydroxyindoleacetic acid) and significant medical and behavioral outcomes for female rhesus macaques. Based on earlier findings with males we predicted that low CSF 5-HIAA concentrations would be associated with a range of negative life history outcomes in our captive specific-pathogen-free (SPF) breeding colony. We found that the mean CSF 5-HIAA concentration among animals that died over the course of the study period was significantly lower than among animals that survived. Further examination indicated an inverse relationship between CSF 5-HIAA concentration and number of treatments for illness, further suggesting a link between serotonergic functioning and overall animal health. Examination of behavioral data indicated that individuals with low CSF 5-HIAA concentrations were more often the targets of aggressive bouts than were individuals with high CSF 5-HIAA concentrations. Finally, we found a positive relationship between CSF 5-HIAA concentration and infant survivorship. These results suggest negative life history consequences of impaired serotonergic functioning in captive female rhesus macaques, and indicate that CSF 5-HIAA concentration sampled early in life may provide a useful tool in facilitating colony management decisions concerning utilization of scarce and increasingly valuable non-human primate resources.

Familial influences on hand preference: genotypic variation between closely related primate species

The emergence of hemispheric specialization has important implications for the development of higher order cognitive processes, including language and spatial skills. In this research we sought to further understand psychobiological processes associated with the development of hemispheric specialization by examining and comparing familial influences on hand preference in two closely related macaque species: rhesus macaques (Macaca mulatta) and pigtailed macaques (Macaca nemestrina). The results of our study indicate contrasting patterns of familial influence on hand preference in each species. For the rhesus macaque we found a positive correlation in the direction of hand preference between mothers and their juvenile offspring, and for the pigtailed macaque we found a negative mother-offspring correlation in the direction of hand preference. Fathers did not contribute significantly to the direction of hand preference in either species. There was a trend toward a positive correlation for strength and consistency of hand preference between parents and offspring in rhesus macaques but not in pigtailed macaques. These findings indicate that maternal influences on offspring hand preference vary between closely related primate species and lead us to question the generalizability of universal single-factor theories used to explain intergenerational transmission of hand preference in humans.

Between-species variation in the development of hand preference among macaques

This research examined between-species variation in the development of hand preference among Macaca. Specifically, we examined hand preference using juveniles and adults of three macaque species that differ in social and reactive tendencies in order to examine whether the correlation between temperament and handedness that has been noted within Macaca mulatta occurs between closely related species. Each of the species studied exhibited a different pattern of hand preference development. Both juvenile and adult M. mulatta exhibited group-level left-hand bias. Juvenile Macaca nemestrina were not biased towards either hand at the group-level, whereas adults exhibited a group-level left-hand bias. Neither juvenile nor adult Macaca fascicularis exhibited manual bias at the group-level. Analysis of variance indicated statistically significant main effects of species and age class on hand preference measures. Post-hoc analysis indicated greater use of the left- versus right-hand, and greater hand preference strength independent of direction, among M. mulatta and M. nemestrina than among M. fascicularis, and among adults than among juveniles. These results indicate significant between-species variation in the development of hand preference within the genus Macaca, and are inconsistent with any one single-factor theory yet offered to explain the etiology of primate laterality. We hypothesize that the relationship between handedness and temperament that has been shown within M. mulatta may generalize across closely related primate species.

Relationship between platelet-activating factor concentration in rhesus monkey (Macaca mulatta) spermatozoa and sperm motility

Platelet-activating factor (PAF) is a potent signaling phospholipid that has been implicated in a number of biological activities. PAF concentration in primate spermatozoa has a positive correlation with fertility. While PAF is present in rhesus spermatozoa, there are no relational reports on its concentration and the cell's motility. The study objective was to determine if PAF concentration in rhesus spermatozoa was correlated with motility indices (percent motility and forward progression). Semen was collected from sexually mature males and cell counts, and percent motilities and forward progressions were recorded prior to PAF measurement by radioimmunoassay. Spermatozoa-derived PAF concentration ranged from a low of 0.9 picomoles/10(6) cells to a high of 13.0 picomoles/10(6) cells. The overall mean (+/-SEM) PAF concentration was 4.6 (+/-1.6) picomoles/10(6) spermatozoa. Regression analysis revealed a positive and significant relationship between PAF concentration in the spermatozoa and percent motility (R2 = 0.914; P < 0.01) as well as forward progression (R2 = 0.849; P < 0.05). A receiver-operator characteristic curve and the calculation of the probability that a positive forward progression will be predicted indicated a cutoff limit of 1.5 picomoles/10(6) cells for PAF concentration in rhesus sperm. Rhesus monkey spermatozoa motility was significantly greater (P < 0.01) in the high-PAF (> or =2 picomoles/10(6) cells) group (31.0 +/- 7.6) than in the low-PAF (<2 picomoles/10(6) cells) group (6.8 +/- 2.1). Rhesus monkey spermatozoa forward progression was significantly greater (P < 0.05) in the high-PAF (> or =2 picomoles/10(6) cells) group (3.0 +/- 1.0) than in the low-PAF (<2 picomoles/10(6) cells) group (0.7 +/- 0.3). The data demonstrate that PAF concentration in rhesus spermatozoa has a significant relationship with percent motility and the cell's forward progression. Determining PAF concentration in spermatozoa may be a significant predictor of fertility in the primate. Additional studies will elucidate the role of PAF in spermatozoa function and the significance PAF plays in primate fertility.

Stress correlates of hand preference in rhesus macaques

In this research we examined stress-related correlates of hand preference in monkeys. Specifically, we tested the hypothesis that stress reactivity and plasma levels of the stress hormone cortisol are developmentally related to handedness in rhesus macaques (Macaca mulatta). We found a significant positive correlation between cortisol levels sampled in juveniles and the frequency of right- versus left-hand use sampled in these same animals during adulthood. Right-hand preference was negatively correlated with stress reactivity. These data are consistent with the view that stress functioning and reactivity are associated with the development of hemispheric specialization in primates.

Hormonal correlates of hand preference in free-ranging primates

In this research we examined hormonal correlates of hand preference in free-ranging primates. Specifically, we tested the hypothesis that levels of the stress hormone cortisol and the male sex hormone testosterone are correlated with handedness in male rhesus macaques (Macaca mulatta). We found significant positive relationships between cortisol and testosterone levels sampled during adolescence and the frequency of right- versus left-hand use sampled during adulthood. These data indicate that adolescent measures of cortisol and testosterone are correlated with hemispheric specialization in adult free-ranging primates.

Left-handedness and longevity in primates

In the present research we used the rhesus macaque to model the decline in the incidence of left-handedness that has been noted in the aged human population. We found a significant group-level bias towards use of the left hand among young-adult macaques, and a significant group-level bias towards use of the right hand among aged macaques. The distribution of hand preference across age classes cannot be explained through simple elimination of left-handed subjects in the aged population. Rather, our data are consistent with a maturational view positing increased use of the right hand with increased age. Similar findings across phylogenetically diverse primate taxa suggest that this phenomenon is an evolutionarily ancient trait. Greater use of the left hand in male versus female macaques indicates that sex differences in hand preference are also deeply rooted in our primate origins.