Differential effects of multiplex and uniplex affiliative relationships on biomarkers of inflammation

Social relationships profoundly impact health in social species. Much of
what we know regarding the impact of affiliative social relationships on
health in nonhuman primates (NHPs) has focused on the structure of
connections or the quality of relationships. These relationships are often
quantified by comparing different types of affiliative behaviors (e.g.,
contact sitting, grooming, proximity) or pooling affiliative behaviors
into an overall measure of affiliation. However, it is unclear how the
breadth of affiliative behaviors (e.g., how many different types or which
ones) a dyad engages in impact health and fitness outcomes. We used a
novel social network approach to quantify the breadth of affiliative
relationships based on two behaviors: grooming and sitting in contact.
Dyadic relationships were filtered into separate networks depending on
whether the pair engaged in multiple affiliative behaviors (multiplex
networks) or just one (uniplex networks). Typically, in social network
analysis, the edges in the network represent the presence of a single
behavior (e.g., grooming) regardless of the presence or absence of other
behaviors (e.g., contact sitting, proximity). Therefore, to validate this
method, we first compared the overall structure of the standard network
for each affiliative behavior: all grooming interactions regardless of
contact sitting, and all contact sitting interactions regardless of
grooming. We then similarly compared the structure of our filtered
multiplex vs. uniplex networks. Results indicated that multiplex networks
were more modular, reciprocal, and kin-based while connections in uniplex
networks were more strongly associated with social status. These
differences were not replicated when comparing networks based on a single
behavior alone (i.e., all grooming networks vs. all contact sitting
networks). Next, we evaluated whether individual network position in
multiplex vs. uniplex (novel approach) or grooming vs. contact sitting
(traditional approach) networks differentially impact inflammatory
biomarkers in a commonly studied non-human primate model system, the
rhesus macaque (Macaca mulatta). Being well connected in multiplex
networks (networks where individuals both contact sat and groomed) was
associated with lower inflammation (IL-6, TNF-alpha). In contrast, being
well connected in uniplex grooming networks (dyad engaged only in grooming
and not in contact sitting) was associated with greater inflammation.
Altogether, these results suggest that multiplex relationships may
function as supportive relationships (e.g., those between kin or strong
bonds) that promote health. In contrast, the function of uniplex grooming
relationships may be more transactional (e.g., based on social tolerance
or social status) and may incur physiological costs. This complexity is
important to consider for understanding the mechanisms underlying the
association of social relationships on human and animal health.