| dc.description.abstracteng | Aging is a universal process that occurs in almost all organisms and is defined as a progressive decrease in physiological functioning. For centuries, researchers have been interested in aging processes and their consequences. This is partly due to the fact that the proportion of older adults in human societies worldwide has increased in recent decades. In addition to research on the physiological basis of aging, it becomes increasingly important to study social aging. With increasing age, the size of the social network often decreases, which can have an impact on health and mortality risk. Psychological theories have been developed that attempt to disentangle the factors that explain the declining sociality in old age. To better understand age-related changes during lifespan from an evolutionary perspective, comparative studies are essential. Due to a similar physiological and cognitive aging process in the absence of societal norms that shape social aging in human societies, non-human primates can provide valuable insights here.
The aim of this dissertation was to establish a link between the psychology of aging and behavioural biology. For that purpose, I studied Barbary macaques (Macaca sylvanus) as a model species to determine whether comparable patterns also occur in non-human primates. Studying non-human primates allowed to explicitly test hypotheses derived from psychological theories and at the same time provides insights for new theory building. The basis of many psychological theories is the assumption that motivational changes lead to changes in the social environment. With age, the motivation to invest resources in maintaining well-being increases, which can lead to a focus on emotionally important social contacts. It is also assumed that older adults are more adept at applying avoidance strategies to minimize potentially negative situations. At the same time, motivational changes also affect cognitive performance. This is true, for example, when goal-oriented behaviour changes from a gain-oriented to a loss-minimizing strategy.
My study animals were semi-free ranging Barbary macaques living in a forested enclosure in Rocamadour, France. Due to the lack of predation pressure and food provisioning, the animals can grow much older compared to wild populations. Therefore, this population is particularly suitable for the investigation of age-specific research questions. I was especially interested in similarities and differences between the social aging process of male and female Barbary macaques. Since social aging in male nonhuman primates has been little investigated, this work makes a significant contribution to the existing literature. Based on the assumptions of psychological theories, I investigated whether Barbary macaques use avoidance strategies similar to those used by older humans to minimize negative and potentially stressful situations. For this purpose, behavioural observations were carried out. To gain a comprehensive insight into social but also cognitive changes with age, I conducted experiments on problem-solving behaviour.
In study 1 (Chapter 2) I demonstrated that males and females show a similar social aging process. For example, the number of partners and the frequency of affiliative interactions decreased with age in both sexes, but this was less pronounced in males as they generally showed less affiliative behaviour. Both sexes had less well-connected partners with increasing age, though males had generally less well-connected partners than females. It also became apparent that older individuals had fewer conspecifics in their proximity and showed less energy demanding behaviours such as climbing or running. The social interest in conspecifics, indicated by commenting on their interactions with others, did not change with age. The results of study 1 suggest that being involved in a smaller social network with increasing age with maintained interest for other conspecifics serves as having a more predictable environment by reducing stress and improving well-being.
Study 2 (Chapter 3) went more into detail on how age influences social behaviour on a communicative level. I show that with increasing age Barbary macaques use less affiliative and aggressive communicative signals such as 'teeth chatter' or 'threat face'. In contrast to the assumptions from psychological theories, I did not find that older individuals apply an avoidance strategy: Older monkeys did not react less to aggressive signals. Furthermore, Barbary macaques were presented with pictures of neutral and aggressive facial expressions. There was no increased attention for neutral facial expressions over aggressive ones. In comparison to younger conspecifics, older Barbary macaques did not react more to affiliative signals. From the sender’s perspective, older individuals reacted less to both aggressive and affiliative signals. Potentially, older Barbary macaques become more adept at pursuing indirect strategies to mitigate negative experience, for example reducing the number of interaction partners or spending more time alone.
The results of study 1 and 2 (Chapters 2 and 3) indicated age-related motivational changes regarding interactions with social partners. How motivation changes with age in relation to problem-solving behaviour was therefore addressed in study 3 (Chapter 4). Barbary macaques were presented with three different problem-solving tasks (transparent objects with a food reward) to investigate the influence of age on cognitive performance (here: inhibitory control and cognitive flexibility) and motivation (persistence). Inhibitory control and cognitive flexibility belong to the executive functions and are important to control goal-oriented behaviour. The likelihood of approaching and interacting with the experimental apparatus decreased with age. Regarding the individuals that explored the objects, older monkeys were on a par with their younger conspecifics in their inhibitory abilities. The assessment of cognitive flexibility was difficult, since only a minority of the monkeys showed flexible behaviour and were successful. Older monkeys were less persistent in exploring the motivation task than younger monkeys. Study 3 shows greater age-related variation in motivation than cognitive functioning in Barbary macaques and highlights the importance of age-related motivational changes as a key factor in cognitive performance.
In summary, age is a determining factor for changes in social behaviour and motivation. My results are consistent with previous findings showing a decrease in social activity in various primate species. The exchange of social signals influences the position in the social network in the sense that older subjects not only have a less central position but also receive less attention from their conspecifics. I was able to show that motivation is often an underestimated influencing factor for age-related cognitive performance and problem-solving behaviour. My results lead to further research questions, which will link current findings with other research fields. For example, endocrinology can help to clarify to what extent age-related hormonal changes in stress response or energy balance have an influence on behavioural changes. The findings from studies on nonhuman primates will contribute to a better understanding of human social evolution. | de |