Attention to social stimuli is modulated by sex and exposure time in tufted capuchin monkeys

No immediate attentional bias towards or choice bias for male secondary sexual characteristics in Bornean orang-utans (Pongo pygmaeus)

Primate faces provide information about a range of variant and invariant traits, including some that are relevant for mate choice. For example, faces of males may convey information about their health or genetic quality through symmetry or facial masculinity. Because perceiving and processing such information may have bearing on the reproductive success of an individual, cognitive systems are expected to be sensitive to facial cues of mate quality. However, few studies have investigated this topic in non-human primate species. Orang-utans are an interesting species to test mate-relevant cognitive biases, because they are characterised by male bimaturism: some adult males are fully developed and bear conspicuous flanges on the side of their face, while other males look relatively similar to females. Here, we describe two non-invasive computerised experiments with Bornean orang-utans (Pongo pygmaeus), testing (i) immediate attention towards large flanges and symmetrical faces using a dot-probe task (N = 3 individuals; 2F) and (ii) choice bias for pictures of flanged males over unflanged males using a preference test (N = 6 individuals; 4F). In contrast with our expectations, we found no immediate attentional bias towards either large flanges or symmetrical faces. In addition, individuals did not show a choice bias for stimuli of flanged males. We did find exploratory evidence for a colour bias and energy efficiency trade-offs in the preference task. We discuss our null results and exploratory results in the context of the evolutionary history of Bornean orang-utans, and provide suggestions for a more biocentric approach to the study of orang-utan cognition.

The Dot-Probe Attention Bias Task as a Method to Assess Psychological Well-Being after Anesthesia: A Study with Adult Female Long-Tailed Macaques (Macaca fascicularis)

Understanding the impact routine research and laboratory procedures have on animals is crucial to improving their well-being and to the success and reproducibility of the research they are involved in. Cognitive measures of welfare offer insight into animals' internal psychological state, but require validation. Attention bias - the tendency to attend to one type of information over another - is a cognitive phenomenon documented in humans and animals that is known to be modulated by affective state (i.e., emotions). Hence, changes in attention bias may offer researchers a deeper perspective of their animals' psychological well-being. The dot-probe task is an established method for quantifying attention bias in humans (by measuring reaction time to a dot-probe replacing pairs of stimuli), but has yet to be validated in animals. We developed a dot-probe task for long-tailed macaques (Macaca fascicularis) to determine if the task can detect changes in attention bias following anesthesia, a context known to modulate attention and trigger physiological arousal in macaques. Our task included the following features: stimulus pairs of threatening and neutral facial expressions of conspecifics and their scrambled counterparts, two stimuli durations (100 and 1,000 ms), and counterbalancing of the dot-probe's position on the touchscreen (left and right) and location relative to the threatening stimulus. We tested 8 group-housed adult females on different days relative to being anesthetized (baseline and 1-, 3-, 7-, and 14-days after). At baseline, monkeys were vigilant to threatening content when stimulus pairs were presented for 100 ms, but not 1,000 ms. On the day immediately following anesthesia, we found evidence that attention bias changed to an avoidance of threatening content. Attention bias returned to threat vigilance by the third day postanesthesia and remained so up to the last day of testing (14-days after anesthesia). We also found that attention bias was independent of the type of stimuli pair (i.e., whole face vs. scrambled counterparts), suggesting that the scrambled stimuli retained aspects of the original stimuli. Nevertheless, whole faces were more salient to the monkeys as responses to these trials were generally slower than to scrambled stimulus pairs. Overall, our study suggests it is feasible to detect changes in attention bias following anesthesia using the dot-probe task in nonhuman primates. Our results also reveal important aspects of stimulus preparation and experimental design.

Testing the two sides of indirect reciprocity in tufted capuchin monkeys

We addressed two different aspects of indirect reciprocity in tufted capuchin monkeys (Sapajus spp.) studying two common cooperative behaviours, grooming and food sharing. In an observational study, we tested whether capuchin monkeys were more likely to groom an individual that had just groomed a group mate than an individual that had not groomed anybody. In an experimental study, we tested whether capuchin monkeys were more likely to share their food with a partner when in the presence of a bystander (or of an image of the eyes of a conspecific) than when alone with their partner. In the observational study, we found an increase in the likelihood of receiving grooming after giving grooming, but this effect seemed to depend on social facilitation rather than on indirect reciprocity, as we found a similar effect after receiving (rather than giving) grooming. In the experimental study, the presence of a bystander or of an image of eyes did not affect the amount of food transferred to a group mate. Overall, these results suggest capuchin monkeys do not engage in indirect reciprocity.