Seasonality and variability of the interestrous interval in the bitch.

Records from two breeding colonies (A and B) located near each other were analyzed for this experiment. Colony A consisted of 19 bitches (8 Maltese, 5 Yorkshire, 3 Lhasa Apso, and 3 Bouvier des Flandres), while Colony B consisted of 48 Beagle bitches. A total of 126 interestrous intervals (141 estrous cycles) from Colony A were reviewed to quantitate the variability of the interestrous interval. Analysis of variance showed that the degree of variation of the estrous cycle length within bitches (65%) was about twice the degree of variation of means of the estrous cycle length among bitches (35%). It was found that the estrous cycle length is extremely variable, and it cannot be used to predict the next estrus in a single bitch, although some bitches were very consistent. The seasonal and monthly distribution of estrous cycles throughout the year was also analyzed from bitches kept in Colonies A and B for a total of 210 estrous cycles. The data were collected over a four-year period. A seasonal pattern was observed when the cumulative distributions over years were analyzed. A higher frequency of estrous cycles was observed during winter and summer. This seasonality pattern was not observed when individual years were analyzed separately. However, the overall probability that an estrus would occur at any month of the year was the same for each month (1/12) when cumulative distribution over years were analyzed.