3.4 Sensitive periods
The steroid hormone testosterone plays a major role in the development of mammals. In particular it is instrumental in causing differences between males and females. One well explored difference concerns play-fighting in young rodents. In the rat, play-fighting is a sequence which begins when one animal pounces on another. The pounce is followed by wrestling and/or boxing and the play-fight usually finishes with one animal on top of the other. A similar sequence of play-fighting is seen in young Rhesus monkeys. Throughout adolescence, the males of both species initiate and become involved in play-fights more frequently than females. The females do play-fight, and the behavioural components of their play-fighting are the same as those used by males, but the female adolescents play-fight less frequently.
To examine the role of testosterone in the development of play-fighting two main procedures are used. The first involves the removal of the principal source of the hormone. In this case the testes are removed from males. The second involves the administration of the hormone of interest, testosterone in this case. The hormone can be administered either to females, who naturally produce relatively little testosterone, or to castrated males, i.e. males whose testes have been removed.
In general terms, what does the first procedure, the removal of the principal source of the hormone, reveal?
The first procedure reveals how development proceeds in otherwise normal males in the absence of the hormone of interest.
The results of this procedure are clear. The play-fighting of male rats castrated soon after birth was reduced in frequency, to a frequency similar to that seen in female rats. This result is highly suggestive of the role of testosterone, but it is not conclusive.
Why is the result from the first procedure not conclusive?
It is not conclusive because there may be other factors produced by the testes that affect play-fighting. Removal of the testes also removes these other factors.
Many developmental studies take as a starting point the effect of the removal of a key stimulus on the development of a particular behaviour or character (sometimes called a trait). However, such studies must be treated with caution because of the very real possibility that several stimuli are removed, along with the stimulus in question.
How can the role of testosterone be confirmed?
The role of testosterone can be confirmed by administering testosterone, i.e. by using the second procedure.
Testosterone administered soon after birth to female rats or to castrated males results in a level of play-fighting similar to that of normal males.
In both the procedures described above the phrase ‘soon after birth’ has been used. The phrase is important because if the procedures are carried out too long after birth, they have no effect on play-fighting. Gestation lasts 21 days in the rat. Testosterone can only exert its effects on play-fighting from about three days before to six days after birth. Once this neonatal period has passed, castration of males or testosterone injection of females has no effect on play-fighting. So the answer to the question posed at the end of the previous section, Section 3.3, is clear – there is a particular period in development which is sensitive to particular environmental stimuli. The time restriction or developmental window during which a stimulus can exert an influence on the organism is referred to as a sensitive period.
The effect of testosterone, or lack of it, on the developing organism during the sensitive period is permanent.
Testosterone is said to masculinise the organism, because it promotes those behaviours predominantly seen in males. In its absence, the organism is de-masculinised. There are also feminine behaviours, such as the preceptive behaviours seen in the rat, of hopping, darting and ear wiggling prior to sexual activity. They are seen in males castrated neonatally. These behaviours are not seen in the intact male, nor in the female treated with testosterone neonatally. In this case testosterone de-feminises, by reducing the frequency of preceptive behaviours.
The examples discussed so far show that rather dramatic changes in environmental factors, be they external factors, outside the body, or internal factors, inside the body, exert an effect on the course of development, and have a permanent effect. However, dramatic environmental events during development, with the notable exception of birth, are rare. Much more common are subtle changes in environment. The next section considers whether subtle differences in environmental factors can also have discernible effects on development.