Biology 12

hormones. The steroid hormone testosteroneis
the major androgen and is responsible for the
development of the male secondary sexual
characteristics. The secondary sexual
characteristics include the enlargement of the
primary sexual characteristics (the penis and testes)
and the enlargement of the larynx (the Adam’s
apple). Testosterone also inhibits fat while
promoting the development of muscle tissue, and
stimulates the formation of hair on the face, chest,
under the arms, and around the genitals. These
characteristics begin to appear during puberty, with
an increase in testosterone levels around ages 10 to

12. As Figure 6.25 shows, the level of testosterone
    in the blood inhibits the production of LH, forming
    the second negative feedback loop.
    The following Thinking Lab looks at how
    testosterone levels vary in dominant and
    subordinate baboons.

Figure 6.25GnRH (gonadotropin-releasing hormone)
stimulates the anterior pituitary to secrete the gonadotropic
hormones FSH and LH. FSH stimulates the testes to
produce sperm, and LH stimulates the testes to produce
testosterone. Testosterone and inhibin exert negative
feedback control over the hypothalamus and the anterior
pituitary; this regulates the level of testosterone in the blood.

hypothalamus

anterior

pituitary

LH FSH

interstitial

cells

produce

testosterone

seminiferous

tubules

produce

sperm

plus inhibin

testis

GnRH

THINKING LAB

Testosterone and

Baboon Behaviour

Background

In baboon tribes, a social structure of dominant and
subordinate males exists. The dominant males have better
access to food, the best resting spots, and the female
baboons. In contrast, the subordinate male baboons must
laboriously search for food, often only to have it stolen by a
dominant male.

In males, the hormone testosterone regulates sexual
behaviour and aggression and increases the rate at which
glucose reaches the muscles. This graph shows
testosterone levels of dominant and subordinate male
baboons. When the male baboons are at rest, the
testosterone levels are essentially equal. After being
exposed to the same stress, however, the reactions of the
dominant and subordinate males differ sharply for the first
few hours.

You Try It

Explain the adaptive advantage of higher levels of

testosterone in the dominant male during times of stress.

(^0135)

1

2

3

2

4

5

6

7

4 6 7 8

Testosterone

(nanograms per millilitre)

Hours after stress

subordinate

dominant

Average testosterone levels in baboons

Chapter 6 The Endocrine System • MHR 195