Introduction: when is the timing of birth a priority for a species?

Two factors play an important part in determining the optimum number of offspring that any species should produce each pregnancy, the optimum number for the fetuses to develop normally in the uterus, and the ability of the mother to adequately suckle the newborn to the point where they are able to survive the rigors of the environment. The first is of prime importance and is determined by a relatively tight control over the number of pre-ovulatory follicles that develop and ovulate, while the second is determined by when the offspring are born, and how frequently births occur in relation to the development of the young. In some species time of year of birth is not necessarily a major consideration, e.g. where seasonal variations in food supply are not great, but in more extreme climates timing of birth is of crucial importance as there must be sufficient time for the young to be able to withstand harsh climates and variations in food supply. In these situations the time of breeding needs to be controlled to limit the periods of fertility, so that if conception occurs birth can still take place at an optimum time of year. In most species reproduction is very efficient and usually ovulations will result in pregnancy.