Today: Sexual Dimorphism

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Today Sexual Dimorphism
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Why does sexual reproduction exist?

• Cons
• Need two individuals
• Hard to find mate
• Diseases
• Competition

• Pros
• Genetic diversity
• or
• Replace damage DNA

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The male perspective on reproduction
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Eggs require large resource input. A clutch of
bird eggs can be 20 of birds weight. Sperm
are cheap.
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A female kiwi ready to lay an egg.
From Biological Science 3rd ed. by Scott
Freeman -fig 25.13
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Fig 46.12
Human Males 250,000,000 sperm/ ejaculation
Human Females 1 egg/month
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I hope Pat likes my feathers because I am tired
of dragging them around everywhere.
Fig 23.15
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Which is male?
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Which is male?
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Sexual Dimporphism differences between males
and females
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Sexual Dimorphism
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Male barn swallows have longer tails than females
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From Biological Science 3rd ed. by Scott
Freeman -fig 51.5
Why do males have long tails?
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Female grizzly bears cannot reproduce while
lactating. Therefore they can only reproduce
every once every 3 years
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this leads to competition between males
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Male elephant seals compete for access to females
in limited beach territories. They are on
average 4 times bigger than females.
Male elephant seals weigh 3 tons.
From Biological Science 3rd ed. by Scott
Freeman -fig 25.14
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Competition between males is fierce, only a few
males father most of the offspring.
90 of males never successfully reproduce
60 of females do not successfully reproduce
From Biological Science 3rd ed. by Scott
Freeman -fig 25.14
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Average height/weight by sex in U.S.
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Ratio of Weight Male/Female
M/F
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Primates have different reproductive stratagies
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Testes Compared to Overall Weight
Testicle size (oz) per body weight (lbs)
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Pipefish related to seahorses
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Pregnant Male Males care for developing eggs
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If differences in reproductive behavior and
morphology are based on sex roles, what happens
when the roles are reversed? Males care for
developing eggs
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The Measurement of Sexual Selection Using
Batemans Principles An Experimental Test in
the Sex-Role-Reversed Pipefish Syngnathus typhle
(2005) Integrative and Comparative Biology,
45874884 Adam G. Jones, Gunilla Rosenqvist,
Anders Berglund, and John C. Avise
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When there are more females than males, many
females cannot mate.
frequency
number of mates
Jones et al. Fig. 1
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frequency
When the sex ratio is even, most males and
females mate.
number of mates
Jones et al. Fig. 1
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frequency
When there are more males than females, most
males can mate.
number of mates
Jones et al. Fig. 1
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The sex ratio affects who will successfully
reproduce. Pipefish females are more affected by
competition.
Jones et al. Fig. 1
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Excess females only medium size females can mate
number of individuals
body length (mm)
Jones et al. Fig. 4
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Excess males many different size males can mate
number of individuals
body length (mm)
Jones et al. Fig. 4
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medium sized female pipefish are most successful
at reproducing while size is less critical for
male pipefish
Jones et al. Fig. 4
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Different reproductive strategies lead to
differences in sexual dimorphism.
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Next Inheritance
haploid
X 23 in humans
X 23 in humans
diploid
X 23 in humans
Inheritance The interaction between genes
inherited from Mom and Dad.