Reproduction

Babies #

Two types:

Altricial: born undeveloped #

• long post-hatching period for brain to develop
• competitive ecological niche that requires skill
• low juvenile mortality

Precocial: #

• born immediately able to move, eat, etc after birth
• abundant, low-skill food (eg chickens)

K-selected species: fewer offspring with long-term care (K = limited by carrying capacity) R-selected species: lots of offspring that are not specialized (R = limited by reproductive rate)

Humans are highly altricial #

• brain 30% of adult size
• 9 mo gestation period vs 18-21 for other primates
• many years of development after birth
• primary limitation: energy needed from mother — unable to sustain >9 mo
  • underdeveloped baby only able to survive w/ help from others (cooperative breeding)
    • selected for by unpredictability of environment, predators
    • present in other species (wolves, eagles, meerkats…) → cooperative breeding
    • Hamilton’s Rule rB>C: helping others’ offspring beneficial for survival of genes

Egg Hyp.: not enough energy

Starlings only cooperative in plains (unpredictable) Little Mama lived to 35 yrs old

Obstetric Hypothesis: (disproven) pelvis size is limiting factor for why humans are so altricial

Life History Theory #

Why do humans live so long? #

• even without medicine, forager societies are very long-lived (excluding infant mortality)
• women live far more than reproductive life, compared to chimpanzees

Expensive Brain Hypothesis #

The following changes are required to support a large brain:

• Energy turnover ↑ — cooking, fire
• Locomotion ↓ — bipedalism, running
• Growth ↓ — decreased muscle mass
• Reproduction ↓ — cooperative breeding
  • Reproduction critical for survival: cooperative breeding allows us to reproduce faster, so we don’t die first
    • recent development of nuclear family in industrial times is maladaptive

Role of Others in Reproduction #

• big variation of role of men in different societies
  • ex. Aka pygmy men highly involved, hold babies 20% of time in camps
• Hrdy Hypothesis: alloparents (not direct parents) necessary for raising children
• Grandmother Hypothesis: postreproductive females (grandmothers) care for

Grandmother Hypothesis #

post-reproductive females (grandmothers) aid in children, teach skills

• selection for longevity: increased inclusive fitness
• daughters decrease birth intervals → increase individual fitness
• ability for children to have a long childhood — collective supports education
• also present in 4 species of whale: more unpredictable food supply → increased cooperation
  • post-reproductive grandmothers have larger effect
• Grandmothers don’t reproduce since they don’t want to compete w/ daughters

Yanomamo — independent male hunters #

Orcas are matrilineal #