Reproductive performance can be measured according to several different parameters including age at first mating, number of litters sired, number of pups per litter, and the frequency with which a strain has productive matings. Table 4.1 shows the values obtained for these different parameters with the most commonly used inbred strains of mice. As the table shows, inbred strains vary widely in their reproductive fitness.
The first important measure of reproductive fitness is the frequency with which a mating pair will produce any offspring at all. 22 With some strains, such as C3H/HeOuJ, CBA/CaJ and FVB/N, over 90% of all matings that are set up will produce offspring. The C3H/HeOuJ strain is at the extreme end of this group with a 99% frequency of productive matings. At the opposite extreme, among the most well-characterized strains, is BALB/cJ with a frequency of non-productive matings that is over 50%. A second measure of fitness is the age at which females first become pregnant. This can vary from an early 5.9 weeks for C3H/HeOuJ to a late 8.0 weeks for BALB/cJ. The third measure of reproductive fitness is litter size. Once again, BALB/cJ performs the worst in this category with an average litter size of just 5.2. All but one of the remaining inbred strains have average litter sizes in the range of 5.4 to 7.0. The one strain that outperforms all others in this category is FVB/N with a much larger average litter size of 9.5. The final measure of reproductive fitness is the average number of litters that a single female can produce in a lifetime. This varies from a low of 2.2 litters with AKR/J females to a high of 4.8 litters with FVB/N females.
Three of the easily quantitated measures of reproductive performance — frequency of productive matings, litter size, and number of litters — have been multiplied together to give a sense of the overall fecundity associated with any one inbred strain in comparison to the others (Table 4.1). Far and away, the highest fecundity (41.0) is associated with the relatively new FVB/N strain. It is for this reason, as well as others, that FVB/N has become the strain of choice for use in the production of transgenic animals (see Section 6.2). Among the traditional inbred strains, C57BL/6J (B6) and C3H/HeOuJ show a fecundity (23.5 and 23.4) that is significantly above all others. The lowest fecundity (9.3) is associated with the BALB/cJ strain.
The fecundity of female mice declines with both age and number of prior pregnancies. Few inbred females of any strain, with the exception of FVB/N, will produce more than five litters (Green and Witham, 1991). Irrespective of their past reproductive history, most inbred females exhibit greatly reduced fecundity by the age of 8 to 10 months. Male mice, like male humans, can remain fertile throughout their lives. However, older males that have become obese or sedentary are unlikely to breed.
Reproductive performance is among the characteristics most affected by inbreeding. Outbred animals and F1 hybrids of all types will routinely surpass the inbred strains in all of the categories listed in Table 4.1 as a consequence of "hybrid vigor". With non-inbred animals, the frequency of productive matings is close to 100%, the age of first mating can be as early as five weeks, and litters can have as many as 16 pups. Finally, non-inbred females can sometimes remain fertile up to 18 months of age, and bring as many as 10 litters successfully to weaning.