Female reproductive competition within families in rural Gambia
Ruth Mace
()
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Alexandra Alvergne
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Department of Anthropology, University College London
, London WC1E 6BT,
UK
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Female reproductive competition within
families in rural Gambia
Ruth Mace* and Alexandra Alvergne
Many studies show that the extended human family can be helpful in raising offspring, with maternal
grandmothers, in particular, improving offspring survival. However, less attention has been given to
competition between female kin and co-residents. It has been argued that reproductive conflict between
generations explains the evolution of menopause in cooperatively breeding species where females
disperse, and that older females are related to the offspring of younger females through their sons,
whereas younger, incoming females are unrelated to older females. This means the pattern of help will
be asymmetric, so older females lose in reproductive conflict and become sterile helpers. Here, we
seek evidence for female reproductive competition using longitudinal demographic data from a rural
Gambian population, and examine when women are helping or harming each others reproductive
success. We find that older women benefit and younger women suffer costs of reproductive competition
with women in their compound. But the opposite is found for mothers and daughters; if mother and
daughters reproductive spans overlap, the older woman reduces her reproduction if the younger
woman (daughter) reproduces, whereas daughters fertility is unaffected by their mothers reproduction.
Married daughters are not generally co-resident with their mothers, so we find not only competition
effects with co-resident females, but also with daughters who have dispersed. Dispersal varies across
human societies, but our results suggest reproductive conflict could be influencing reproductive
scheduling whatever the dispersal pattern. A cultural norm of late male marriage reduces paternal grandmother/
daughter-in-law reproductive overlap almost to zero in this population. We argue that cultural norms
surrounding residence and marriage are themselves cultural adaptations to reduce reproductive conflict
between generations in human families.
1. INTRODUCTION
Studies of cooperative and communal breeding in animals
focus strongly on reproductive conflict between
individuals [1], whereas human studies tend to focus more on
the cooperative benefits of communal life, and its
relevance for the evolution of menopause [2]. Human life
history is characterized by a long childhood, followed
by a rapid reproductive phase, and then a long
postreproductive life, at least for females. Human menopause
might be selected for by kin selection favouring older
mothers investing in their grandchildren rather than
continuing to reproduce themselves [2]. There is now
considerable evidence that grandmothers enhance the
reproductive success of their offspring (reviewed in
Sear & Mace [3]), including evidence from our own
study in rural Gambia [4 6]. Evidence that maternal
grandmothers benefit the survival of their daughters
offspring has been found across a wide range of societies,
although kinship norms may influence which residence
patterns are most favourable [7]. However, there is
disagreement about whether the grandmother effect, on its
own, could select for menopause. Verbal models have
emphasized other factors, such as the importance of
mothers survival during childhood [8], or the importance
Electronic supplementary material is available at http://dx.doi.org/
10.1098/rspb.2011.2424 or via http://rspb.royalsocietypublishing.org.
of an extended lifespan when fertile span is
phylogenetically constrained [9]. Some mathematical models
informed by data either fail to predict any fitness benefit
associated with terminating reproduction so long before
death [10] or predict small fitness benefits [11].
Implicit in all the various models of the grandmother
hypothesis is the notion that mothers and daughters are in
reproductive competition, as it is assumed that only by
becoming non-reproductive can a grandmother really help
her daughters reproduction. It is striking in humans how
little human female generations overlap; as a daughter
reaches reproductive age, her mother reaches menopause,
and as she reaches menopause, her mother dies. However,
while reproductive conflict predicts that reproductive
generations should reduce overlap [12], it not does address why it
is the older woman who is foregoing reproduction rather
than the younger one, as is generally the case in most
cooperatively breeding birds or mammals [1,13]. Johnstone &
Cant [14] argue that relatedness to group members, and
hence the power of (...truncated)