Influence of polygamous versus monogamous mating on embryo production in four different strains of mice after superovulatory treatment

Efficiency of timed pregnancies in C57BL/6 and BALB/c mice by mating one male with up to four females

For a wide range of biomedical approaches, an accurate estimate of the age of embryos or pups is important. Overnight mating is the method that is mostly used to establish timed pregnancies. The oestrus cycle in mice repeats every four to five days. So, not all females will get pregnant because they are not in oestrus. Therefore, the aim of this study was to analyse whether polygamous mating could increase the rate of timed pregnancies per breeding cage and female. We compared overnight timed mating regimes with up to four females per male, using C57BL/6 and BALB/c mice as well as F1 hybrids of these two strains. The number of vaginal plugs, number of females that gave birth and weaned litter (including size and weaning weight) were recorded. Our results showed that the plug and pregnancy rate decreased, but the productivity per breeding cage increased for polygamous mating regimes. The proportion of females with vaginal plugs and females that gave birth was significantly higher in monogamous mating. The proportion of plugged females that gave birth, as well as litter size and weaning weight, were not influenced by the mating regime. After analysing 513 breeding cages with a total of 1090 females, we found that polygamous mating with up to three females per male can increase the number of timed pregnancies. However, in the mating regime with more than three females, the rate of timed pregnancy as well as number of pups per female declined.

Effect of Ground Transportation on Adrenocortical Activity in Prepuberal Female Mice from Five Different Genetic Backgrounds

Specific experimental protocols necessitate transportation, a potentially stressful event that could confound results. We determined adrenocortical activity by measuring fecal corticosterone metabolites (FCMs), as a stress marker, in prepuberal (three-week old) female C57BL/6J, C57BL/6NCrl, FVB/NCrl, Crl:CD1(ICR), and BALB/cAnCrl mice. On each transport day, five female cage mates per genetic background were weaned and transported in stable groups via truck from the breeding to the research facility. Fecal pellets were collected on Days 0, 1, and 4. Mice were superovulated for embryo production to determine if repeated fecal collection impacts this procedure. The average duration of transportation over 600 km and from packing to unpacking of mice was 7.24 and 22.62 h, respectively. FCM levels increased from Day 0 to Day 1 and decreased on Day 4 in all genetic backgrounds except in FVB/NCrl, but only B6N showed significantly higher FCM levels on Day 1. Furthermore, embryo production was not affected by repeated feces collection. The results show that weaning and immediate transport of prepuberal mice from the breeding to the research facility led to temporal and genetic background-dependent increases of adrenocortical activity in four of the five genetic backgrounds investigated, which returned to baseline levels within four days.