Size of Rabbit Litters

No matter for what purpose rabbits are kept, the size oflitters is a matter of some concern. In the commercial fieldthe largest litters consistent with good growth are obviouslydesirable.

Litter size is controlled by two different sets of factors.The first set are those of an inherited nature. The secondinclude those of environment. Obviously the environmentalfactors will influence the doe only, but the inherited characterswill affect both the buck and the doe.

Litter size is affected at ovulation, at fertilisation, andduring pregnancy. At ovulation a certain number of eggsare shed. Some of these may not be fertilised, and some ofthe fertilised eggs may die during pregnancy. The potentialfecundity of the doe is therefore the maximum number ofeggs sited at ovulation, whilst the actual fecundity is thenumber of young born in each litter.

There are two basic hereditary influences. In general thelarger the breed, the higher is the number of young producedin each litter. But at the same time, in strains of the samesize, some hereditary factors occur for high or low fecundity.Thus in the stud where management and breed are the samefor all animals, and large litter size is desired, preferenceshould be given when selecting breeding stock to animalswhich have a high average litter size. This applies to thebucks as well as the does, for inheritance of fecundity canbe transmitted through the male line. Inheritance offecundity is not controlled in a simple way by one or twogenes. Thus the inheritance of this characteristic blendsover a wide range. Occasionally when extra large animalsare desired the breeder may unconsciously select breedingstock for smaller and smaller litter size. This is because hewill often select for future breeding on the basis of size, and most frequently the size of young in small litters is greaterthan that of those in large litters.

Apart from inherited characteristics, nutrition, age ofdoe, number of previous litters and disease all have an effecton the final litter size.

Of these factors nutrition is the most important. A risingcondition will produce the largest litter, and poor nutrition,overfatness, a falling condition and so on will lead to smalllitters. Litters born during the spring and summer are oftenlarger than those born at other times of the year. But thisis probably correlated with the better feeding condition then,for when nutrition is standardized throughout the year thesevariations tend to disappear.

There is almost invariably an increase in litter size betweenthe first and second litters, but thereafter depending on thestrain, variation occurs. In some strains the litter sizecontinues to increase for some litters, in others it remainsmore or less stationary, in others it decreases. In somestrains an increase may be noted up to an age of some threeyears, but eventually litter size decreases.

During the course of pregnancy, embryos may die forseveral reasons. There are one or two diseases which causedeath, but the most important factor is an inherited one whichcauses foetal atrophy. This factor is recessive, and whenpresent in a homozygous state in the doe (for the characteroccurs in the doe arid not in the embryos) the majority ofembryos die.

Following the death of the embryo it is either absorbedor aborted, depending mainly on the stage of pregnancy.In the early stages the embryos are absorbed whilst later onthey are aborted.

Fertility, the ability to produce living young, and fecundity,a measure of the number of young produced, are closelyconnected. Strains showing high fertility are usually alsohighly fecund. But this general rule does not of courseapply to the relationship between weight of adult and littersize, for in this relationship the small breeds (early maturingsexually) have smaller litters than do large breeds (latematuring sexually).