I analyse and summarize the empirical evidence in mammals supporting
alternative benefits that individuals
may accrue when committing nonparental infanticide. Nonparental infanticide
may provide the perpetrator
with nutritional benefits, increased access to limited resources, increased
reproductive opportunities, or it
may prevent misdirecting parental care to unrelated offspring. The possibility
that infanticide is either a
neutral or maladaptive behaviour also is considered. I devote the second
half of this article to reviewing
potential mechanisms that individuals may use to prevent infanticide. These
counterstrategies include the
early termination of pregnancy, direct aggression by the mother against
intruders, the formation of coalitions
for group defence, the avoidance of infanticidal conspecifics, female promiscuity,
and territoriality. I evaluate
the support for each benefit and counterstrategy across different groups
of mammals and make suggestions
for future research.