In a monogamous mating system one male and one female are selected and paired together for the duration of their breeding life. This system simplifies record keeping and, once the proper pairing has been established, lends itself well to maintaining inbred or outbred rodent colonies.

The harem system is also known as the colony or polygamous mating system. It is a system in which one male is housed with two or more females. It results in the largest number of young from the least number of breeder animals. It is the most economical method of laboratory animal production. It can be difficult to keep accurate records when using the harem system because females often share nursing responsibilities. This causes uncertainty as to which female gave birth to which offspring, therefore use only when it is not critical to know which female is the mother. The cage must be large enough to permit proper housing ** of adults and young until offspring are old enough to wean (21 days).

Inbreeding

Inbreeding is a mating scheme used to produce genetically similar animals. They share most characteristics and are identified as members of a specific strain. Strains are considered inbred after a minimum of 20 generations of brother to sister or parent to offspring breedings.

Outbreeding

Outbreeding, sometimes called random breeding, is a carefully planned scheme in which only unrelated animals of the same stock are mated. The result is a maximum amount of genetic differences. This mating scheme usually produces more vigorous animals and larger litter size.

Consideration should be given to the selection of good breeding stock. The animals should be proven healthy, young, and non-aggressive. Females should exhibit good mothering characteristics such as care of the young and adequate milk production. Separation of weanlings from their parents is often stressful for the young, therefore only wean at the appropriate stage of development. Minimal handling of newborn animals is a general rule. Animals disturbed by frequent observations or unusual environmental changes commonly desert, kill, or cannibalize their young. If newborns must be manipulated, they should not be touched directly. Rather, they should be manipulated with gloved hands and moved with the bedding they are in. Caging should provide comfort, privacy, and space for the developing young, while permitting routine observation. Nestlets should be provided 2-3 days prior to parturition. A lighting system that provides 14 hours of light each day is best for breeding colonies, it helps females establish consistent estrous cycles.

The intensive breeding method requires that the male remain with the female(s) continuously. If parentage is unimportant, several males may be housed in a large cage with many females. An advantage of this system is that it occupies less space than a monogamous system, since it requires fewer males than females. However there is likely to be more fighting between adults and more injuries to young. Males remain with the females so that the postpartum mating can take place. This may be stressful to the females since they are almost always pregnant and nursing young.

The continuous pair or trio mating system is used to help avoid fighting among some strains of mice. These animals are housed together from weaning age. This helps minimize aggressive behavior and encourages all adults to participate in caring for the litter. No more than two females should be housed with one male. The presence of only females in a cage may depress the estrous cycle of all females in that cage. The addition of a male to the cage causes all such females to initiate estrus in about three days. This phenomenon is called the Whitten Effect. Mouse breeders often take advantage of the Whitten effect to enhance timed pregnancies.

In non-intensive breeding systems, the male and female are housed separately while the female is pregnant. The female is not permitted to mate again until the young are weaned.

A quiet area away from busy traffic areas is usually more harmonious with good colony production. Cannibalism or desertion of the newborn is seen among all lab animals. It is usually caused by an inexperienced female, overcrowding, poor environmental conditions, disturbing of the newborn by staff, or stress.

If animals are brought together only at breeding time, it is better to bring the female to the male’s cage. A male often refuses to breed with a female that spends a lot of time defending her territory. Also, in the female’s cage the male may be “preoccupied” with territorial marking of the new environment.

In general, the first few litters born to most female lab animals are smaller in number than later litters. There is also a tendency for females to offer poorer maternal care to earlier litters. This may result in a higher mortality rate among early litters than among later ones. As these animals approach the end of their reproductive lives, litter size again tends to be smaller.

Mice have a postpartum estrus that occurs within 24 hours after giving birth. In continuous breeding pairs females are usually fertilized during this period and remain pregnant while nursing their previous litter. Some breeders provide females with a resting period after each litter by removing the pregnant female from the male to permit her to improve her physical condition between litters. After weaning the female is re-mated. Breeders usually remove females and/or their mates from the breeding program as soon as litter size begins to decline. Some do this at a specific age that is known to correspond with decreased fecundity.

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