Reproductive Cycles and Patterns:
Most animals reproduce only at certain times of the year. Unsurprisingly, they generally time their reproductive behaviors so that offspring are produced during the time of year when they’re most likely to survive. Humans are most unusual in that they can and do reproduce at any time of the year.
Animals’
reproductive cycles are generally controlled by chemicals known as
hormones. A hormone is a chemical that is produced in one part of the body, and that travels through the blood to affect one or more “target organs” in some other part of the body. Production of specific hormones can be regulated by changes in such factors as the day length and the temperature. As hormone levels change in response to changes in an animal’s environment, those hormones, in turn, promote changes in the animal’s sexual behaviors, reproductive behaviors, and physiologies.
Consider a songbird, for example. As the day length changes in the early Spring, this triggers increased production of
sex hormones such as estrogen, progesterone and testosterone. These hormones, in turn, trigger sexual behaviors such as courtship and mating, as well as reproductive behaviors such as nest-building.
The effects of changing hormone levels on an animal’s behavior can be subtle, or they can be quite dramatic. Consider some of the effects of the hormone
oxytocin, for instance. In humans, oxytocin is released during orgasm in both males and females. Oxytocin has been shown to promote “bonding behavior” between individuals, so it’s likely that one of the effects of oxytocin release during sexual intercourse is to promote psychological bonding between partners. Needless to say, such bonding is an important factor in successfully raising offspring. Oxytocin, therefore, has been referred to as the “hormone of love.”
Studies in other mammal species have shown that adequate oxytocin levels appear to be necessary for proper bonding between parents and offspring. Accordingly, it has been suggested that conditions such as postpartum depression and failure of mothers (and fathers) to properly bond with their children may be at least partially due to abnormally low oxytocin levels. In the future, such conditions may be treatable through careful manipulation of patients’ hormone levels.