Human Reproductive Strategies

Question 1

Secondarily Altricial

Answer 1

Human infants are “secondarily altricial” than most other primates - they can’t hold onto their mothers independently and are completely dependent on them

Question 2

Primate Precocial Babies

Answer 2

• Most newborn primates must be able to maintain close contact w/mother w/in a social group, however, a considerable amount of brain growth after birth allows them to learn about both the social and ecological environment which is important for survival
• Because most haplorrhine monkeys are born w/ brains about 65% of adult brain size there is an emphasis on brain development over body mass and dentition where the juvenile period (after weaning) in primates is a time of change and challenges that lead to high mortality rates in juveniles due to malnutrition and disease

Question 3

The Cost of Human Brains

Answer 3

• human infants require a lot of maternal help as they are born with 30% of their brain developed - ultrasound reveals human fetus brain growth is much greater and faster than chimps as chimps taper off w/ the size of brain devel. in the uterus, while human brains continue to develop in the uterus - chimp brains reach adult size ~2yrs w/ most brain growth b4 birth; human brain growth both b4 birth and w/in first 5 yrs of life
• Human brain is very costly, that at birth, the brain is most energy extensive organ in body, more BMR supports brain during early growth which creates cost for child and mother supplying energy where time between 0-5 is most important as brain reaches adult size by age 5 - humans emphasize brain growth first, then dentition, body and finally reproduction- brain size 2% of body and requires 20% of energy

**Altogether has implications for human birth and human form of child-rearing

Question 4

Human Reproductive Strategies

Summary 1 - Human infants are more expensive than other primates

Answer 4

• secondarily altricial
• costly brain
• slow to mature

Question 5

Male investment in offspring

Answer 5

-Less than 5% of mammals show direct male care
Average wt of newborns in primates = 8% of mothers body wt while tamarins/marmosets = 18% of mothers wt
-Twins
-Costly for fathers to carry offspring
-by helping mother, males are also increasing infant survival & their own reproductive success
-Males highly responsive to infant stimuli & known to begin carrying infants moment they born

Question 6

Male Hormone Profiles : Prolactin:

Answer 6

promotes milk production but also increases infant care

• > experienced fathers have higher levels
• > facilitates interest w/infant
• prolactin levels also higher in some non-primates

In Human fathers:
Prolactin rose 20% during the 3 wks after their partners gave birth

Question 7

Male Hormone Profiles : Cortisol:

Answer 7

higher levels associated w/ internal stress. Experienced fathers have lower levels than other male adults

In Human fathers:
Cortisol is lower when they held their newborn & linked to higher direct & indirect parenting effort

Question 8

Importance of Male Hormonal Profiles

Answer 8

These hormonal shifts may enhance male care of their infants

Question 9

Human Reproductive Strategies

Summary 2

Answer 9

• Human infants are more expensive than other primates

- Male parental care may be especially important for humans

Question 10

Increasing reproductive success:

Parental Investment

Answer 10

The risks taken by a parent & the time & energy invested in an existing offspring

Question 11

Mothering Styles:

Answer 11

• Some mothers invest little more than an egg (repitles)
• For many mammals mothering has a strong hormonal basis that guides good mothering behavior
• For Primates: Learning to be a good mother is essential as well

Question 12

Chimps vs Humans life span and interbirth intervals:

Answer 12

Chimps have a shorter lifespan & longer interbirth interval than humans but they both have similar reproductive time-spans:

• Chimps take care of offspring for ~60 mths
• Chimps in a lifespan can produce 3 offspring
• Result: Humans can produce MORE offspring during the same period than apes can

Question 13

Other Helpers:

Alloparenting:

Answer 13

Help from individuals who are not parents; kin such as grandparents, aunts, uncles, older siblings = appears key

**In some social mammals, primates & especially humans the mothers always have potential helpers. In humans these are often grandmothers -> Humans do it to a much greater degree

Question 14

Fathering Styles:

Answer 14

In most mammals males dont provide direct care for their offspring

• > Humans DO !
• male relatives & fathers are important care givers in hunter-gatherer societies

Question 15

Increasing Human Reproductive Success:

Answer 15

• Human mothers have helpers (grandmothers)
• Human fathers & male relatives participate as well
• Higher potential investment: support of juveniles
• Together humans can support multiple children at a time
• • Humans increase reproductive success by long term support of juveniles AND by mothers having helpers to raise offspring & fathers /male relatives providing care

RESULT: reduction** in juvenile mortality in humans

Question 16

mortality rate between wild chimps, captive chimps and Ache Forages tells us:

Answer 16

Wall of Death in Wild animals suggests animal lifespan- wall is when a lot of death occurs usually around 24 yrs

• Wild chimps have highest probability of death across all ages
• Captive chimps are “buffered” & have a pattern closer to Ache
• Ache have lower age specific mortality even in old age

**Human cultural adaptations may buffer us from normal wild primate mortality rate

Question 17

Compared to other Primates:

Overal Life History traits (especially long lives that allows extensive alloparenting) is an effective strategy to:

Answer 17

1. Increase offspring survival

2. Have more than one dependent offspring at the same time