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19. Behavior The necessary conflict Is Sex Necessary? The sexes Males: sperm are cheap and plentiful humans: 300 million sperm per ejaculate, make more continuo usly goal: mate with lots of females Females: eggs are in limited supply, offspring costly

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19 behavior

19. Behavior

The necessary conflict

the sexes
The sexes


sperm are cheap and plentiful

humans: 300 million sperm per ejaculate, make more continuously

goal: mate with lots of females


eggs are in limited supply, offspring costly

humans: 400,000 eggs post puberty (many die daily), degrade with time, born with all her sex cells intact

goal: get best male available

Sets up conflict!

operational sex ratio osr
Operational Sex Ratio (OSR)
    • Predicts who will do the fighting
    • Ratio between receptive males to receptive females
    • More males = female choice
    • More females = male choice
  • Males are an extension of their gametes!
  • Males almost always sexually receptive
  • 96% males, 54% female college students said they thought about sex in any hour
tactics of male competition
Tactics of Male Competition

Physical contact: Ibex

Providing resources: Red back spider

Physical display: birds

Although competition occurs among males, females still choose

This choice drives the competition!

Females are choosing genes!

physical contact



Physical contact

Ibex, Mountain Goat from East Africa, Middle East

Males fight for the right to mate

Males usually larger than females

Specialized structure for fighting

providing resources

Hanging flies (males give female insect prey before mating


Providing Resources

Red back spider: male forfeits life to the female to increase sperm transfer

Nuptial gifts: Common in insects, seen in birds

physical display
Physical display

All female choice

Males “displays” his quality (genes)

Good skills – Bower birds

building and selecting objects

Good looks – Bird of Paradise

plumage and dancing

Good performers – Manakins

coordination and dancing

two males are necessary

sexual selection

Irish Elk


Sexual Selection

Competition for mates between individuals of the same sex drives evolution of certain traits

Individuals with desired trait will get to mate

Desired trait = better fitness = offspring will have advantage

Fitness: reproductive output

Genes for favorable trait are passed on to next generation

Runaway selection: trait that females like gets exagarrated

example: Irish Elk

Female choice of large antlers

Antlers got so big they were detrimental to males

Species is now extinct

environmental influence on mating
Environmental influence on mating

Jacana: birds lives in streams, ponds

crocodile predation leads to high loss of eggs, young

females larger

males tend the eggs, young

frees the female to keep laying

mating systems
Mating systems

Polygamy: one individual mates with multiple members of the opposite sex at the same time

97% mammals

about 100% invertebrates

6% birds


Polygyny: 1 female, > 1 male

Polyandry: 1 male, > 1 female

Monogamy: one individual of each sex mates exclusively with one another

3% mammals (humans?)

choose good parenting partners
Choose Good Parenting Partners

Male assistance in raising the young

When do males help?

Environment makes is difficult for female alone to raise offspring (offspring die)

choose good parenting partners22
Choose Good Parenting Partners

Titi monkeys: South America

Father helps raise young

Mother feeds constantly to produce milk

How to ensure monogamy?

Courtship: elaborate behaviors to bond a pair

Courtship ensures shared foraging, nest building, defense, parenting

wandering albatross


Wandering Albatross

Spend most of their life at sea

Courtship lasts several years (prior to sexual maturity)

Chick takes 1 year to fledge

One parent must stay with the young

Other parent finds food

Monogamy assures the success of the offspring


94% of birds, 3% mammals


Life-long: exclusive breeding for entire life

ex: Wandering Albatross

Serial: partners together one or more breeding seasons

Social: partners breed and raise offspring together, but still breed with other individuals (90% of monogamous birds)


Males and females have different goals defined by their gametes

This conflict creates competition

Sexual selection leads to the evolution of sexual traits

Mating systems are diverse and are shaped by the environment

Courtship bonds individuals together

Next time: EXAM 3


Readings: 27.3, 27.4