Today: Sexual Dimorphism

Why does sexual reproduction exist?

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

Pros:• Genetic diversityor• Replace damage DNA

The male perspective on reproduction

Eggs require large resource input.

A clutch of bird eggs can be ~20% of bird’s weight.

Sperm are cheap.

A female kiwi ready to lay an egg. Fig 25.13

Human Females:~1 egg/month

Human Males:250,000,000 sperm/ ejaculation

Fig 48.3

I hope Pat likes my feathers because I am tired of dragging them

around everywhere.

1

2

Which is male?

1

2

Which is male?

Sexual Dimporphism: differences between males and females

Sexual Dimorphism

Male barn swallows have longer tails than females Fig 51.4

Fig 51.5Why do males have long tails?

Female grizzly bears cannot reproduce while lactating. Therefore they can only

reproduce every once every 3 years…

…this leads to competition between males

Male elephant seals compete for access to females in limited beach territories. They are on average 4 times bigger than females.

Fig 25.14Male elephant seals weigh ~3 tons.

Competition between males is fierce, only a few males father most

of the offspring.

Fig 24.14

90% of males never successfully reproduce

60% of females do not successfully reproduce

Average height/weight by sex in U.S.

Ratio of Weight Male/Female

M/F

Primates have different reproductive stratagies

Testes Compared to Overall WeightTe

stic

le si

ze (o

z) p

er b

ody

wei

ght (

lbs)

Bonobos

Pipefish: related to seahorses

Pregnant Male:Males care for developing eggs

If differences in reproductive behavior and morphology are based on sex roles, what happens when the roles are reversed?

Males care for developing eggs

The Measurement of Sexual Selection Using Bateman’s Principles: An Experimental Test in the Sex-Role-Reversed Pipefish Syngnathus typhle (2005)Integrative and Comparative Biology, 45:874–884Adam G. Jones, Gunilla Rosenqvist, Anders Berglund, and John C. Avise

Jones et al. Fig. 1

freq

uenc

y

number of mates

When there are more females than males, many females cannot mate.

freq

uenc

y

number of matesJones et al. Fig. 1

When the sex ratio is even, most males and females mate.

freq

uenc

y

number of matesJones et al. Fig. 1

When there are more males than females, most males can mate.

Jones et al. Fig. 1

The sex ratio affects who will successfully reproduce. Pipefish females are more affected by competition.

num

ber o

f ind

ivid

uals

body length (mm)Jones et al. Fig. 4

Excess females: only medium size females can mate

num

ber o

f ind

ivid

uals

body length (mm)Jones et al. Fig. 4

Excess males: many different size males can mate

Jones et al. Fig. 4

medium sized female pipefish are most successful at reproducing while size is less critical for male pipefish

Different reproductive strategies lead to differences in sexual dimorphism.

haploid

diploid

X 23in humans X 23

in humans

X 23in humans

Inheritance = The interaction between genes inherited from Mom and Dad.

Next: Inheritance