Studying the evolution of cooperative breeding and group living requires
simultaneous quantification of both helping benefits
and competitive costs within groups. Although such
research has traditionally focused on the fitness benefits of helping
behavior,
increasing evidence now highlights reproductive
competition in cooperatively breeding animals including humans. Human
groups
consist of cooperative individuals of varying
relatedness, predicted to lead to conflict when resources are limited
and relatedness
low. However, few studies exist that determine the
costs of co-breeding to both parties sharing resources. Here, we studied
female reproductive competition in historical
Finnish joint-families where brothers stayed on their natal farms and
sisters
married out, so that several unrelated women of
reproductive age co-resided in the same households. Using detailed
parish
registers we quantified the effects of simultaneous
reproduction of these women on their offspring mortality. We found that
the risk for offspring mortality before adulthood
was increased by 23% if co-resident women reproduced within 2 years of
each
other, a risk that was not associated with the
overall numbers of co-resident reproductive-aged women or children. Such
costly
competition may have promoted the evolution of
birth scheduling, dispersal patterns and life-history traits including
menopause
that avoid resource competition with other
reproductive females.