In barnacle geese Branta leucopsis, pair-bond members generally remain toge
ther every day, each year, often for life. Geese that maintain long-lasting
pair bonds during their lifetime produce more offspring than those with sh
orter pair durations. This result was shown while statistically controlling
for the birds' life span and the proportion of life spent without a partne
r, two variables that also influence lifetime reproductive success. I argue
that continuous partnerships are maintained in highly competitive goose so
cieties because of the constant need for female-male cooperation, without w
hich acquiring adequate resources for reproduction would be prohibitive for
both sexes.