Volatile social environments can favour investments in quality over quantity of social relationships

Cooperation does not occur in a vacuum: interactions develop over time in
social groups that undergo demographic changes. Intuition suggests that
stable social environments favour developing few but strong reciprocal
relationships (a 'focused' strategy), while volatile social
environments favour the opposite: more but weaker social relationships (a
'diversifying' strategy). We model reciprocal investments under
a quality-quantity tradeoff for social relationships. We find that
volatility, counterintuitively, can favour a focused strategy. This result
becomes explicable through applying the theory of antagonistic pleiotropy,
originally developed for senescence, to social life. Diversifying
strategies show superior performance later in life, but with costs paid at
young ages while the social network is slowly being built. Under volatile
environments, many individuals die before reaching sufficiently old ages
to reap the benefits. Social strategies that do well early in life are
then favoured: a focused strategy leads individuals to form their first
few social bonds quickly and to make strong use of existing bonds. Our
model highlights the importance of pleiotropy and population age structure
for the evolution of cooperative strategies and other social traits, and
shows that it is not sufficient to reflect on the fate of survivors only,
when evaluating the benefits of social strategies.