Overlapping litters and reproductive performance in the domestic rabbit

Rabbits (Oryctolagus cuniculus), like many small mammals, have a marked postpartum estrus, and during the breeding season, are often both pregnant and lactating. We examined reproductive performance and allocation of resources by rabbit mothers with overlapping litters under presumably optimal conditions of unrestricted food, standardized litter size, and young reproductive age. Even under these conditions, females raising overlapping litters incurred higher costs in terms of greatly increased food intake and pup mortality than females raising nonoverlapping litters. Despite such costs, females with overlapping litters raised a larger total number of young to weaning than females without the load of simultaneous lactation and pregnancy. By more than doubling food intake during lactation, females maintained a stable baseline body weight throughout the study, suggesting that this, in a species with low fat reserves, might have priority over the short-term raising of a maximum number of young. Contrary to the expectation that females with overlapping litters would allocate more resources to their senior young, no appreciable difference in number, growth, or survival between pups of senior and junior litters was found either at birth or weaning. While this might reflect a strategy in this opportunistic breeder of distributing reproductive chances similarly between senior and junior litters, it is now necessary to investigate females' breeding strategy when resources are limited.