Choosing a mate and reproductive strategies are based on methods to succesfully pass on genes

Voles Prairie Monogamous Both parents care for young More receptors Montane Nonmonogamous Mother cares for young briefly Less receptors Same levels of oxytocin

Sexual Dimorphism

Human Females: ~1 egg/month Human Males: 250,000,000 sperm/ ejaculation Fig 46.12

I hope Pat likes my feathers because I am tired of dragging them around everywhere. Fig 23.15

1 2 Which is male?

1 2

Sexual Dimporphism: morphological differences between males and females

Sexual Dimorphism

Male barn swallows have longer tails than females

Why do males have long tails? From “Biological Science” 3 rd ed. by Scott Freeman -fig 51.5

Why do males have long tails? From “Biological Science” 3 rd ed. by Scott Freeman -fig 51.5 feathers cut and reattached captured and released

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. Male elephant seals weigh ~3 tons. From “Biological Science” 3 rd ed. by Scott Freeman -fig 25.14

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” 3 rd ed. by Scott Freeman -fig 25.14

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)

Our closest living relative... bonobos

Stop at 1:53

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

...So when males invest more resources for reproduction, they become the limiting factor and are choosier about mates. Pipefish males care for developing eggs

Different reproductive strategies lead to differences in sexual dimorphism.

In light of differences between male and female resources for reproduction, how does this affect equality between males and females?

Do we want equal treatment of men and women? How should equality be attained, or how should the inequality be regulated?

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