It is widely assumed that the development of male secondary sexual traits in birds and mammals is testosterone-dependent. In birds, however, masculinity has dual origins. Male-type behaviour and morphology, such as spurs and wattles, are usually testosterone-dependent. However, showy male-type plumage is, generally, the neutral state of development. For… (More)
The present review explores sexual differentiation in three non-conventional species: the spotted hyena, the elephant and the tammar wallaby, selected because of the natural challenges they present for contemporary understanding of sexual differentiation. According to the prevailing view of mammalian sexual differentiation, originally proposed by Alfred… (More)
While pinealectomy (Px) has little effect on mammalian circadian rhythms, daily injections of the pineal hormone melatonin in rats have profound effects. These can be classified according to their effects under three categories of desynchronization: external desynchronization, internal desynchronization and phase-shift of the zeitgeber (Aschoff, 1969).… (More)
The classical view of mammalian sexual differentiation is that a gene on the Y chromosome transforms the indifferent gonad into a testis. The Leydig cells then secrete androgen which stimulates the development of the male reproductive tract, and the Sertoli cells secrete Mullerian inhibitory substance which inhibits the development of the female… (More)
BACKGROUND Monozygotic (Mz, identical) twinning occurs at a rate of around three per 1000 maternities in all populations, whereas dizygotic (Dz, fraternal) twinning is highly heritable, and varies with age and race. The Dz/Mz twinning ratio reflects the frequency of twin ovulations, and can provide a useful measure of human fertility. METHODS 1625 pairs… (More)
It has long been known that breast feeding inhibits female fertility and that it is a factor in restricting population growth. But just how important is it?