Collective action and quid pro quo among chimpanzees
Chimpanzees engage in reciprocal cooperation.
Chimpanzees are remarkably social primates, grooming reciprocally, fighting in groups, and sharing the spoils of hunts. Yet whether chimpanzees engage in collective action and reciprocal cooperation when doing so exacts a toll remains unclear. Kevin Langergraber et al. (pp. 7337–7342) examined whether chimpanzees engage in collective action for group welfare when they incur costs and receive few immediate benefits. The authors analyzed behavioral and genetic data from a 20-year study of more than 140 chimpanzees, 13 years of age or older, at Kibale National Park in Uganda to uncover the motives underlying participation in boundary patrolling. During patrols, which last more than 2 hours and cover up to 2.5 km on average, groups largely composed of male chimpanzees visit the edges of their territories in a show of vigilance, sometimes killing neighbors. Patrolling is associated with heightened stress hormone levels and could exact a physiological toll on individual chimpanzees. Nevertheless, the authors found, males patrolled even when they had no offspring or maternal kin in the group. Overall group patrolling did not decline with group size. Because patrolling can augment group size and future reproductive success, the authors reason, group augmentation might explain chimpanzees’ collective action in the absence of short-term benefits. The findings underscore the latent advantages of living in large groups. In a related article, Martin Schmelz et al. (pp. 7462–7467) tested whether chimpanzees engage in a form of quid pro quo. The authors trained six chimpanzees housed at a primate research center to participate in an experiment that involved choosing between a social option, which delivered food both to themselves and a partner, and a selfish option, which delivered food only to themselves. When players in the experiment had previously witnessed partners appearing to provide assistance in obtaining food, they were more likely to make a social rather than a selfish choice—even when the selfish option would have meant a higher food reward for themselves. Chimpanzees were also sensitive to whether partners that appeared to assist them did so at risk of losing food, suggesting that players’ social acts were a considered response to partners’ perceived social acts. If confirmed in larger studies that include other social primates, such as bonobos, the findings might illuminate the seemingly deep evolutionary roots of human cooperation, according to the authors. — P.N.
Why migraine sufferers are aversive to light
Many migraine sufferers report increased discomfort and headache intensity upon exposure to light. By combining clinical and preclinical studies, Rodrigo Noseda et al. (pp. E5683–E5692) provide evidence that the effects of light during migraine go beyond headache exacerbation to include emotional and autonomic responses regulated by a brain region called the hypothalamus. In the study, 81 migraine patients and 17 healthy participants were exposed to light of various intensities and asked to describe their emotional and physiological responses. Nearly 80% of patients experiencing migraine attacks and a significantly lower proportion of healthy participants reported light-induced, hypothalamus-mediated autonomic responses, such as shortness of breath, rapid heart rate, light-headedness, and nausea. Moreover, light stimulation evoked positive emotions such as happiness and calmness in approximately 80% of healthy participants, compared with approximately 20% of patients who experienced migraine attacks. To explore the neuronal basis of these findings, the authors carried out anatomical and chemical labeling experiments in rats. These experiments revealed that retinal ganglion cells in the eye project to hypothalamic neurons, which in turn directly project to brainstem and spinal cord nuclei that regulate autonomic functions. According to the authors, the findings suggest that the aversive nature of light during migraine is more complex than previously thought. — J.W.
Phonological development in early life
Phonological development might begin early in life. Image courtesy of iStockphoto/Quintanilla.
Prior to 6 months of age, infants exhibit the ability to discriminate between familiar and unfamiliar sounds in language. However, adult-like discrimination of sounds, which is better honed for familiar than unfamiliar sounds, develops around 9–10 months of age, after infants begin to develop word knowledge. Jiyoun Choi et al. (pp. 7307–7312) recruited international adoptees to study the interplay of phonology and vocabulary early in life. The study participants comprised of 29 adult Dutch speakers, 23–41 years of age, adopted from Korea who had retained no conscious knowledge of the Korean language. A similar number of participants were adopted at 3–5 months old and at 17 months or older, a time period after infants have started to learn words. The authors trained the participants, in addition to 29 Dutch control participants, 19–47 years of age, to recognize unfamiliar Korean consonant sounds. Compared with the control participants, the adoptees more rapidly learned the Korean sounds. Moreover, no difference was observed between the learning ability of the younger and older adoptees. According to the authors, infants might acquire phonological knowledge of language within the first 6 months of life, contradicting previous assumptions. — C.S.
Simulation yields insight into sense of touch
The human hand contains thousands of tactile nerve fibers that convey rich information about the shape, size, and texture of objects. Although researchers have extensively studied how individual neurons respond to tactile stimulation, much less is known about the object information represented by neuronal populations. Hannes Saal et al. (pp. E5693–E5702) developed a model that simulates how entire populations of tactile fibers respond to various stimuli applied to the skin of the hand. The model reconstructs the deformations experienced by individual sensory receptors and computes the firing responses of the nerve fibers innervating those receptors. To develop and validate the model, the authors used previously obtained electrophysiological recordings of tactile nerve fiber responses to various types of vibrations imposed on the skin of monkeys. The model accurately reproduced key nerve fiber response properties, including millisecond-precision responses to a wide range of tactile stimuli. Moreover, virtual experiments revealed that activation patterns distributed across different populations of nerve fibers convey synergistic information about various object properties, including edge orientation and direction of motion. According to the authors, the model could be used to address fundamental questions about sensory coding in neural populations, and potentially impart a realistic sense of touch to amputees equipped with bionic hands. — J.W.
Parsing the muscular prowess of chimpanzees
Chimpanzees outstrip humans in muscle strength. Image courtesy of iStockphoto/abadonian.
The notion that chimpanzees (Pan troglodytes) surpass humans in muscle strength is based on anecdotal evidence and few controlled studies. Matthew O’Neill et al. (pp. 7343–7348) reviewed literature on chimpanzee mass-specific muscle performance and found that on average the apes outperform humans by a factor of approximately 1.5 in pulling and jumping tasks. Through direct measurements of the properties of individual muscle fibers and biochemical assessments of muscle proteins, the authors found that the inherent contractile properties of human and chimpanzee skeletal muscle fibers, namely maximum force and shortening velocity, are similar. However, chimpanzee and human muscles differ in the length of fibers as well as the isoform composition of the myosin heavy chain (MHC) protein. Whereas chimpanzee muscle contains a balanced distribution of MHC I, IIa, and IId isoforms, corresponding human muscles are enriched in the MHC I isoform and contain shorter fibers. Computer simulations suggested that the differences increase the maximum dynamic force and power-producing capacity of chimpanzee skeletal muscle by a factor of 1.35, compared with similarly sized human muscle. The authors note that hominins experienced evolutionary retrenchment in maximum dynamic force and power output of skeletal muscles in favor of metabolically inexpensive contractility during the past 7–8 million years. Such a tradeoff might explain the storied difference in human and chimpanzee muscle strength, according to the authors. — P.N.