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Evolution III - contents
• Sexual selection vs. natural selection
• Male and female strategies
• Alternative mating tactics
Sexual dimorphism
• The large, male California sea lion, is distinctive
from the surrounding, smaller females (from
Kardong).
Sexual dimorphism
• e.g. in peacock
•
Sexual selection
• Differential reproduction owing to variation
in the ability to obtain mates (as opposed to
natural selection, which is related to the
differential survival of individuals in nature,
depending on non-sex related traits) – see
Futuyma – Evolutionary Biology
Males are often less selective than females
male
toad
Australian
jewel
beetle
Biologists exploiting animal desire
• sea
elephant
Anisogamy - Sperm and oocyte of
hamsters (enlarged 4000 times – from
Alcock)
Differential reproductive success
in males and females
• Female oocytes are a limited reproductive resource,
whereas male sperm is not
• Therefore, males have the potential to sire many more
offspring than females
• In humans, the male world record is held by Moulay Ismail
the Bloodthirsty (Sultan of Morocco 1672-1727), who is
said to have fathered 888 children
• Men produce between
100 and 300 million
sperm per day.
Female reproductive success in
humans
• According to Guinness World Records 2001, the highest
officially recorded number of children born to one mother
is 69. This was the first wife of Feodor Vassilyev (17071782) of Shuya, Russia. Between 1725 and 1765, in a total
of 27 confinements, she gave birth to 16 pairs of twins,
seven sets of triplets, and four sets of quadruplets. 67 of
them survived infancy.
Females must be more choosy
because
• costs of producing female gametes are
higher
• costs of pregnancy are higher (e.g. in
mammals)
• females often invest more into parental care
• costs of mating with partner of poor genetic
quality are higher in females
Thus, a general pattern across the
animal kingdom is
• males compete for females and try to mate
with as many as possible (emphasis often on
quantity)
• females are choosy and try to find the
highest quality partners (emphasis on
quality)
How to impress females
• 1. look nice
How to impress females
• 2. bring them presents
(as e.g. in scorpion flies (right)
and Bittacus (below))
How to impress females
• 3. offer yourself as a present (e.g. preying
mantids, some spiders: females cannibalise
males during or after mating –
thus detracting attention
from other possible sex
partners. also you give your
body as a resource for offspring)
How to impress females
• 4. sing them songs (picture from Alcock)
European warblers
Days to
finding
partner
Number of songs in repertoire
If none of this works - cheat
• Satellite strategies –
younger or “weaker”
males often position
themselves
strategically near an
attractive male, then
sneaking up on
females when the
large male is unattentive or otherwise
engaged
Great Plains toads,
longhorn sheep,
horseshoe crabs
Alternative male mating tactics
• different males in the same species use
different strategies to gain access to females
• satellite strategies are one example
Alternative male mating tactics – “rape”
• In scorpion flies, some
males will not bring
presents, but mate with
unwilling females.
Usually “losers”,
fertilisation success
typically low.
Females often prefer exaggerated male traits
FIGURE 7.12 Barn Swallows (Kardong, after Andersson, Moller)
Why do females prefer
exaggerated male traits?
• “Good genes hypothesis” – extravagant traits
indicate biological fitness (because less fit males
would be less able to afford such traits).
• One special case of this hypotheses is that such
traits indicate ability to cope with parasites.
• “Runaway selection hypotheses” – females prefer
sexually attractive traits which they pass on to
sons, which makes sons more fit (but only because
the sons in turn will also be more attractive to
females).