Social networks may be crucial for the survival and success of individuals that live in groups. Being connected to others may help an individual to access food, avoid predation, disease and parasites, and reproduce fruitfully. The ecological and evolutionary importance of network structure has been recognized for a long time, although the reasons that network structures vary within and between species have not been investigated in depth.
For decades, researchers on Cayo Santiago, an island off the southeastern coast of Puerto Rico, have been studying rhesus macaques (Macaca mulatta), that were introduced from India in 1938. These primates are given supplemental food and water, and are monitored by staff of the Caribbean Primate Research Center (CPRC) on a daily basis to record births, deaths and behavioral patterns.
The macaques are highly social and live in matrilineal kin groups, of which there are 19 on the island. Females groom one another as a way of social bonding, and this can also be used as a measure of connectedness. Previous research has shown that the social networks of older females change as they age, with more mature females prioritizing relationships with their closer partners and kin, while reducing their contact with less favored females. These older females do not spend any less time grooming others, or being groomed themselves, but they do focus their attentions on a smaller circle of partners.
In a new study, led by scientists from the University of Exeter, the same groups of macaques were investigated to find out whether this change in social connectedness of older females has any effect on the cohesion and structure of the social network as a whole. The researchers stress that older people also tend to reduce the number of social relationships they have with others, and focus their attention and effort on close friends and family. The effect that this has on human society has also not been investigated.
“For both humans and macaques, focusing on close friends and family in later life may bring a variety of benefits,” said Dr Erin Siracusa, from Exeter’s Centre for Research in Animal Behavior. “Our study aimed to find out what knock-on effect these individual age-related changes have for how well connected a society is overall.”
“We had information on six monkey groups collected over eight years, and representing, in total, 19 social networks,” she explained.
The 19 social networks contained differing proportions of old females (over 18 years of age), although none had more than 20 percent. The researchers hypothesized that networks with a greater proportion of old individuals would be more sparsely connected due to the older animals having fewer social partners (ie., the presence of more old females would have an effect on the overall structure of the network).
“The first thing we found is that that older female macaques are poor influencers,” said Siracusa. “By having fewer friends, older females are less able to transmit knowledge and experience outside their immediate social circles.”
However, the researchers did not find any differences between the structure of networks containing a greater proportion of old females and those containing a greater number of young adults. This was despite the fact that the older females have reduced social networks.
The team proposed that, since no more than 20 percent of monkeys were old in any given group that was studied, it was possible that this was too few individuals to affect the overall network structure. They decided to use a computer model to simulate the possible effects if a greater proportion of the network was made up of old individuals, and also if the old monkeys were even older than 18 years of age.
By modeling age-based differences in two interaction patterns (number of social partners and the tendency to link with kin) using data collected in the field, the researchers found that the age composition of a group can have important consequences for its cohesiveness and connectedness. Interestingly, these effects did not necessarily scale in the linear manner that might be expected in response to a linear increase in proportion of old individuals in the network.
“We found really substantial consequences for network structure, which could affect useful things like information transmission and cooperation, and could also limit the spread of disease,” said Professor Lauren Brent, also from the University of Exeter.
“In humans, population aging is poised to be one of the most significant social transformations of the 21st Century. Our findings suggest this could have far-reaching effects on the structure of our societies and the way they function.”
With the global human population of over-60s expected to double by 2050, the findings suggest social structures, cohesion and connectedness could all change significantly.
The findings, published in the Philosophical Transactions of the Royal Society B, Biological Sciences, also indicate that any animal group containing disproportionately large numbers of old, or large numbers of young individuals, is likely to experience a shift in the social network structure. This is because young and old individuals differ in the level of their social activities.
For example, the removal of older individuals through trophy hunting could alter important processes in the remaining group, such as communication, cooperation and access to resource-related knowledge. Younger individuals may also be more aggressive and less socially cohesive, which could have repercussions for network connectivity and structure.
“Consequently, through its effects on network structure, shifting age demography may have broader implications than previously appreciated for group dynamics and persistence, and thus warrants further research,” concluded the study authors.
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