Determinate growth
- Mammals like humans have distinct life phase that is devoted to growing to adult size, and then growth stops
- During this time, energy is not spent on reproduction
Age of Maturity
- Childhood Strategy ~ Growth and Maintenance - Adult Strategy ~ Reproduction and Maintenance - This transition is assumed to optimize the BENEFITS of growth larger versus the COST of delaying reproduction
How does decreased energy availability affect growth?
- Later age of maturity (slow) - or - smaller adult body size (stop earlier) - or - Both
What happens with an increase of energy and growth during childhood?
When the body has an intake of energy and growth and finally hit the stopping point they tend to develop sexual maturity earlier in life.
Two major Hypothesis for Transition from Growth to Reproduction (Age of Menarche) in Females
- Critical Fat Hypothesis
- Skeletal Maturation Hypothesis
Critical Fat Hypothesis
- Menarche occurs when sufficient body fat has been accumulated
~ Fat is necessary to reproduce - Menarcheal age has reduced in accordance with increasing BMI in populations
- Underweight girls have late menarche
- However, there is only a weak relationship between age and weight (or BMI) at the time of menarche
- Often, being obese delays menarche!
Skeletal Maturation Hypothesis
- Manarche is delayed until the pelvis is large enough to support a live birth
- If the pelvic canal is not big enough to accommodate the infant brain, mother and infant can die during childbirth
- Constraint that is unique to humans - Big Brains + bipedal-ism ?
In girls
- Reproductive ability occurs after growth spurt and appearance of secondary sexual characteristics
In boys
- Reproductive ability occurs before growth spurt and appearance of secondary sexual characteristics
Stages of Human development
- Embryo/ fetus ~ birth - Infancy ~ weaning - Juvenility ~ Onset of puberty - Adolescence ~ completion of social and physical development (puberty) - Adulthood ~ senescence - Old age
Juveniles
- can often survive without their mother, but require support from others
Long juvenile periods
- Are rear but found in highly social animals (primates, whales, lions, hyenas, elephants, etc.)
We cannot afford the cost of grow up faster
- Cost of brain growth constrains body growth
- MOthers divide thier efforts by investing by multiple offspring
Ecological risk aversion
- If the environment is unpredictable, spreading growth over a long time minimizes the risk of starvation
Slow development is necessary for learning the skills to be an adult
- social learing hypothesis
~ need to learn social skills and develop alliances - learning to forage
A special case of learning to forage hypothesis for humans
_ humans foraging skills (hunting, cooperation, extraction foraging cooking) take many years to learn
Learning is ‘capital’
- Early investments in learning
- lead to dramatic payoffs in productivity later
- Allows humans not just to feed themselves but to subsidize others (juveniles, reproductive females, elders, sick, etc)
