In species with biparental care, the degree of parental investment of each parent may be related to the size of the litter, the parental effort of the partner and the probability of future reproductive success with the same partner. These factors were examined in single females and mated pairs of Peromyscus californicus who were required to 'forage' for food by running in a wheel. Offspring survival and growth, parental behaviour, amount of food obtained by running in the wheel and inter-birth intervals between litters were recorded. Females alone were able to raise litters of two pups, but not four pups. Mated pairs were able to raise a litter of four pups. Litters of one or two pups reared with the mother alone grew as fast as pups reared with both parents. Single females that kept their pups alive had a mean inter-birth interval of 53 days; paired females had a mean inter-birth interval of 37 days. These results show the effect of male parental investment on reproductive success when parents must forage for food: the presence of the male resulted in four times as many pups reared over a 74-day period as a female could rear on her own.
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