Secondary Sexual Characteristics
Sexual Reproduction
• Why sex?
 Increases variation
 If parents are involved in bringing up offspring,
it increases the chances of offspring surviving
• This is the point of all animal courtship
rituals- to select the best possible male and
increase participation in bringing up offspring
Sex
• Animals take several strategies:
 External fertilisation (sperm & eggs released into water).
There may or may not be parental care of offspring.
 Internal fertilisation (sperm introduced into female).
Usually parental care of offspring.
•
• We are going to examine human reproduction firstremember that humans are placental mammals. They also
invest a lot of parental care, ensuring a high survival rate
for offspring. Thus, there are complex rituals which aim at
increasing bonding and involvement with offspring.
Primary and Secondary Characteristics
• Primary sexual characteristics
– These are the organs directly involved in reproduction.
– Males- penis, testes, prostate gland & other seminal
glands
– Females- vagina, uterus, ovaries, fallopian tubes
• Secondary sexual characteristics
– The physical characteristics associated with a person’s
sex but not directly involved in reproduction.
Differences between males & females fall into three
categories:
 Reproductive
 Different physical roles
 Signalling differences
• Secondary sexual
characteristics are produced
by sex hormones.
• • In males, the main sex
hormone is testosterone
(there are other androgens).
• • In females, the sex
hormones are oestrogens &
progesterone.
• Note: both males & females
produce small amounts of
both hormones in the
adrenal glands.
•
Male and Female Secondary Sexual
Characteristics
• Males
–
–
–
–
–
body size & muscular development
Enlargement of the larynx (vocal cords) - deeper voice
red blood cell count
body & facial hair
The skull is more angular- heavier lower jaw, stronger brow ridges to
protect eyes
– Armpit & pubic hair- these areas have different sweat glands which
may produce pheromones.
• Females
– Breast development- breast tissue is composed of glandular tissue &
fat
– Widened pelvis- the pelvis becomes wider & more bowl shaped
– body fat
– Armpit & pubic hair
• What controls the hormones producing these
effects?
• Answer: the brain!
• 1. The hypothalamus (area of the brain) releases
Gonadotrophin Releasing Hormone (GnRH).
•
• 2. GnRH stimulates cells in the anterior pituitary
gland to release Follicle Stimulating Hormone
(FSH) and Lutenising Hormone (LH) into the
bloodstream.
• 3. FSH & LH stimulate the gonads to produce the
appropriate hormones.
Questions
• 1.
What is the function of the increased body fat in women?
• 2. Why have males evolved to have a larger body size &
musculature?
• 3. What are some secondary sexual characteristics in other
mammals? Give at least three examples.
• 4. What roles do the secondary sexual characteristics play in
other mammals? Give at least two examples.
• 5. Other mammals do not have breast development except
for when lactating (breastfeeding). Why might humans have
evolved with breast development?