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Reboreda, J. C., Clayton, N. S., & Kacelnik, A. (1996). Species and sex differences in hippocampus size in parasitic and non-parasitic cowbirds. Neuroreport, 7(2), 505–508.
Abstract: To test the hypothesis that selection for spatial abilities which require birds to locate and to return accurately to host nests has produced an enlarged hippocampus in brood parasites, three species of cowbird were compared. In shiny cowbirds, females search for host nests without the assistance of the male; in screaming cowbirds, males and females inspect hosts' nests together; in bay-winged cowbirds, neither sex searches because this species is not a brood parasite. As predicted, the two parasitic species had a relatively larger hippocampus than the non-parasitic species. There were no sex differences in relative hippocampus size in screaming or bay-winged cowbirds, but female shiny cowbirds had a larger hippocampus than the male.
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Hemelrijk, C. K., & Wantia, J. (2005). Individual variation by self-organisation. Neurosci Biobehav Rev, 29(1), 125–136.
Abstract: In this paper, we show that differences in dominance and spatial centrality of individuals in a group may arise through self-organisation. Our instrument is a model, called DomWorld, that represents two traits that are often found in animals, namely grouping and competing. In this model individual differences grow under the following conditions: (1) when the intensity of aggression increases and grouping becomes denser, (2) when the degree of sexual dimorphism in fighting power increases. In this case the differences among females compared to males grow too, (3) when, upon encountering another individual, the tendency to attack is 'obligate' and not conditional, namely 'sensitive to risks'. Results resemble phenomena described for societies of primates, mice, birds and pigs.
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Kurvers, C. M. H. C., van Weeren, P. R., Rogers, C. W., & van Dierendonck, M. C. (2006). Quantification of spontaneous locomotion activity in foals kept in pastures under various management conditions. Am J Vet Res, 67(7), 1212–1217.
Abstract: OBJECTIVE: To describe spontaneous locomotion activity of foals kept under various management conditions and assess the suitability of global positioning system (GPS) technology for recording foal activity. Animals-59 foals. PROCEDURES: During the foals' first 4 months of life, 921 observation periods (15 minutes each) were collected and analyzed for locomotion activities. The GPS system was evaluated by simultaneously carrying out field observations with a handheld computer. RESULTS: Foals spent 0.5% of total observed time cantering, 0.2% trotting, 10.7% walking, 32.0% grazing, 34.8% standing, and 21.6% lying down. Total observed daytime workload (velocity x distance) in the first month was approximately twice that in the following months. Locomotion activity decreased with increasing age. Colts had more activity than fillies in certain periods, and foals that were stabled for some portion of the day had compensatory locomotion activity, which was probably insufficient to reach the level of foals kept continually outside. The GPS recordings and handheld-computer observations were strongly correlated for canter, trot, and walk and moderately correlated for standing and lying. Correlation for grazing was low. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicated that domestically managed foals, when kept 24 h/d at pasture, will exercise at a level comparable with feral foals. High workload during the first month of life might be important for conditioning the musculoskeletal system. The GPS technique accurately quantified canter, trot, and walk activities; less accurately indexed resting; and was unsuitable for grazing because of the wide array of velocities used while foraging.
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Dunbar, R. I. M. (2007). Male and female brain evolution is subject to contrasting selection pressures in primates. BMC Biol, 5, 21.
Abstract: The claim that differences in brain size across primate species has mainly been driven by the demands of sociality (the “social brain” hypothesis) is now widely accepted. Some of the evidence to support this comes from the fact that species that live in large social groups have larger brains, and in particular larger neocortices. Lindenfors and colleagues (BMC Biology 5:20) add significantly to our appreciation of this process by showing that there are striking differences between the two sexes in the social mechanisms and brain units involved. Female sociality (which is more affiliative) is related most closely to neocortex volume, but male sociality (which is more competitive and combative) is more closely related to subcortical units (notably those associated with emotional responses). Thus different brain units have responded to different selection pressures.
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Koba, R., & Izumi, A. (2006). Sex categorization of conspecific pictures in Japanese monkeys (Macaca fuscata). Anim. Cogn., 9(3), 183–191.
Abstract: We investigated whether monkeys discriminate the sex of individuals from their pictures. Whole-body pictures of adult and nonadult monkeys were used as stimuli. Two male Japanese monkeys were trained for a two-choice sex categorization task in which each of two choice pictures were assigned to male and female, respectively. Following the training, the monkeys were presented with novel monkey pictures, and whether they had acquired the categorization task was tested. The results suggested that while monkeys discriminate between the pictures of adult males and females, discrimination of nonadult pictures was difficult. Partial presentations of the pictures showed that conspicuous and sexually characteristic parts (i.e., underbellies including male scrotums or breasts including female nipples) played an important role in the sex categorization.
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Heath-Lange, S., Ha, J. C., & Sackett, G. P. (1999). Behavioral measurement of temperament in male nursery-raised infant macaques and baboons. Am. J. Primatol., 47(1), 43–50.
Abstract: We define temperament as an individual's set of characteristic behavioral responses to novel or challenging stimuli. This study adapted a temperament scale used with rhesus macaques by Schneider and colleagues [American Journal of Primatology 25:137-155, 1991] for use with male pigtailed macaque (Macaca nemestrina, n = 7), longtailed macaque (M. fascicularis, n = 3), and baboon infants (Papio cynocephalus anubis, n = 4). Subjects were evaluated twice weekly for the first 5 months of age during routine removal from their cages for weighing. Behavioral measures were based on the subject's interactions with a familiar human caretaker and included predominant state before capture, response to capture, contact latency, resistance to tester's hold, degree of clinging, attention to environment, defecation/urination, consolability, facial expression, vocalizations, and irritability. Species differences indicated that baboons were more active than macaques in establishing or terminating contact with the tester. Temperament scores decreased over time for the variables Response to Capture and Contact Latency, indicating that as they grew older, subjects became less reactive and more bold in their interactions with the tester. Temperament scores changed slowly with age, with greater change occurring at younger ages. The retention of variability in reactivity between and within species may be advantageous for primates, reflecting the flexibility necessary to survive in a changing environment.
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de Waal, F. B. (1999). The end of nature versus nurture. Sci Am, 281(6), 94–99.
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Lilienfeld, S. O., Gershon, J., Duke, M., Marino, L., & de Waal, F. B. (1999). A preliminary investigation of the construct of psychopathic personality (psychopathy) in chimpanzees (Pan troglodytes). J Comp Psychol, 113(4), 365–375.
Abstract: Although the construct of psychopathy has received considerable attention in humans, its relevance to other animals is largely unknown. We developed a measure of psychopathy for use in chimpanzees (Pan troglodytes), the Chimpanzee Psychopathy Measure (CPM), and asked 6 raters to complete this index on 34 chimpanzees. The CPM (a) demonstrated satisfactory interrater reliability and internal consistency; (b) exhibited marginally significant sex differences (males > females); (c) correlated positively with measures of extraversion, agreeableness, and observational ratings of agonism, sexual activity, daring behaviors, teasing, silent bluff displays, and temper tantrums, and negatively with observational ratings of generosity; and (d) demonstrated incremental validity above and beyond a measure of dominance. Although further validation of the CPM is needed, these findings suggest that the psychopathy construct may be relevant to chimpanzees.
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Parr, L. A., Hopkins, W. D., & de Waal, F. B. (1997). Haptic discrimination in capuchin monkeys (Cebus apella): evidence of manual specialization. Neuropsychologia, 35(2), 143–152.
Abstract: Two experiments investigated the effects of haptic and visual discrimination on hand preference in 22 brown capuchin monkeys (Cebus apella). The percentage of left-handed subjects in Experiment 1 were 63.6%, 45.5%, and 18.2% for haptic, bipedal, and quadrupedal reaching, respectively. In Experiment 2, the haptic demands of the task were manipulated by using additional food types and another tactile medium. Left-hand preferences were further strengthened when reaching into water compared to pineshavings in Experiment 1. Reaching with no tactile interference resulted in equal numbers of lateralized and nonlateralized subjects. These results show that when reaching demands the use of haptic cues, as opposed to visual ones, monkeys shift towards greater left hand use. This is consistent with what is known about right hemisphere superiority for haptic discrimination in humans.
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Harkins, J. D., Kamerling, S. G., & Church, G. (1992). Effect of competition on performance of thoroughbred racehorses. J Appl Physiol, 72(3), 836–841.
Abstract: The effect of competition and the influence of age and sex on performance were examined in a study of 18 Thoroughbred racehorses. The horses performed two solo and two competitive runs at 1,200 and 1,600 m for a total of eight runs. No group ran faster during competition, which may have been a reflection of the quality of horses used for this study and their susceptibility to stress-induced impairment of performance. Males showed no significant difference between competitive and solo run times, whereas females were consistently slower during competition. Males ran significantly faster than females in all runs. There was no difference in run times due to age, which may have been due to the high mean age (5.9 yr) of the group. The slower competitive run times may have occurred because of an earlier onset of fatigue when compared with solo runs. Plasma lactate was significantly greater for the 1,200-m competitive than for the solo runs.
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