Today: Sexual Dimorphism

Why does sexual reproduction exist? Cons: Need two individuals Hard to find mate Diseases/Competition Pros: Genetic diversity or 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

Sexual Dimporphism: differences between males and females

Sexual Dimorphism

Male barn swallows have longer tails than females Fig 51.4

Fig 51.5 Why 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 Male elephant seals weigh ~3 tons.

Competition between males is fierce, only a few males father most of the offspring. Fig % 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 Weight Testicle size (oz) per body weight (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, 45:874–884 Adam G. Jones, Gunilla Rosenqvist, Anders Berglund, and John C. Avise

Jones et al. Fig. 1 frequency number of mates When there are more females than males, many females cannot mate.

frequency number of mates Jones et al. Fig. 1 When the sex ratio is even, most males and females mate.

frequency number of mates Jones 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.

number of individuals body length (mm) Jones et al. Fig. 4 Excess females: only medium size females can mate

number of individuals 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 23 in humans X 23 in humans X 23 in humans Inheritance = The interaction between genes inherited from Mom and Dad. Next: Inheritance