910

Reproduction in many species is seasonal. This seasonality in many species of vertebrates involves mechanisms of photoperiodism. Briefly, changes in day length enable animals to anticipate the best time of year for the rearing of young and therefore time their reproduction and birth accordingly. How do animals measure day length? As we will discuss in more detail in Chapter 52, animals have endogenous daily rhythms called circadian (circa, “day,” + dia, “length”) rhythms. As day length changes, sunrise and sunset occur at different phases of the circadian rhythm. The release of melatonin from the pineal gland is controlled by the circadian rhythm but is inhibited by light. Therefore as days get longer, the nightly pulse of melatonin gets shorter, and conversely as days get shorter, the melatonin pulse gets longer. Day length determines when many species come into reproductive condition and mate. That is not true for the tammar wallaby that was introduced at the beginning of this chapter. The wallaby mates at any time of year soon after it gives birth. However, diapause in the wallaby is sensitive to night length—that is, to the duration of the melatonin pulse. A non-lactating wallaby will break diapause when night length reaches 12 hr. Similarly, when non-lactating wallabies are held at a night length of 9 hr but given melatonin injections 2.5 hr before sunset, they break diapause. These results support the hypothesis that short daily melatonin pulses prolong diapause and that long daily melatonin pulses are a signal to break diapause. The significance for the nocturnal wallaby is that birth tends to be at the time of year when nights are longest.

Humans are not seasonal breeders, but there is a timing component of childbirth. Most women go into labor at night. However, it is quite common for labor to cease when women arrive in the brightly lit hospital. Bright light interrupts the nightly melatonin pulse in humans. The work with the data exercise (Circadian Timing, Hormone Release, and Labor) explores whether melatonin is involved in the timing of human births.