Reproduction and Life Cycle

Commensal rodents can breed even more efficiently than rabbits and sizeable infestations can develop very quickly from a single pair. Conditions that suit a rapid population increase are even temperatures, surplus food and harbourage with adequate water and undisturbed cover for rearing young and escaping from enemies. Under these optimum conditions rats and mice may breed throughout the year. In less favourable habitats commensal rodent breeding takes place mainly in the summer and autumn. In rural areas where rats may be exposed to more extreme winter conditions they may be less likely to breed in the winter than urban rats that may be sheltered from the extremes of the winter conditions. The life cycle of rats and mice is similar although cycle times differ slightly. It should also be remembered that times will vary with environmental conditions. Norway Rats are capable of reproducing at the age of about 3 months. Pair-bonds are not formed as mating is carried out on an opportunistic and promiscuous basis. When a female becomes sexually receptive, her scent attracts all the local males. The female is receptive to the male for only a relatively short time (about 12 hours) each cycle. Mating is brief and can take place with a number of male many times. After mating and conception there is a relatively short period of pregnancy (gestation). Birth is followed by a similar time period after which the litter is weaned. Female mice become sexually mature at a younger age and the cycles are slightly shorter than for rats. As mentioned earlier the female is most likely to be covered by the male dominating their territory. There is however evidence that female house mice mate more frequently with males outside their territory than might be expected. A Breeding Advantage... Commensal rodents exhibit one characteristic that enables them to breed more efficiently; they exhibit post-partum oestrus. Unlike many mammals, commensal rats and mice do not have to wait until the original litter is weaned before coming back into oestrus. The females can come back into oestrus and be willing to mate and conceive within 24/48 hours of giving birth. This is most likely to occur if conditions are favourable and will not occur if the environmental conditions are unfavourable or the population under stress. Commensal rodents may give birth every 24 to 28 days and this can give rise to very rapidly increasing populations. At very high densities reproduction rates decrease to take account of the reducing availability of resources and the increasing competition within the population. This table give you the facts about breeding activity by species: Breeding activity Common Rat Ship Rat House Mouse Sexually mature 8-12 weeks 12-16 weeks 8-12 weeks Gestation period 21-24 days 21-22 days 17-20 days Litter size (average) 6-11 babies 6-10 babies 5-6 babies Weaning period 3-4 weeks 3-4 weeks 3 weeks In the next bite, we'll go on to look at nesting in more detail and talk about rodent predators who may well aid the control process, but can also be vulnerable to poisoning themselves.