W5L2 human life history 2

Question 1

what is special about human menopause

Answer 1

-Human stop reproducing long before they die

Question 2

Menopause: three evolutionary hypotheses

Answer 2

1. Attentive mother hypothesis
   Birth gets riskier as mothers grow older; early reproductive termination allows females to provide marginal benefit to existing offspring.
2. Helpful grandmother hypothesis
   As women age, benefits of care-giving to first or second-order relatives outweigh those of independent reproduction.
3. Reproductive conflict hypothesis
   The cost of inter-generational reproductive conflict between younger and older females in the same social unit promotes early menopause.

Question 3

Maternal grandmothers and survival of firstborns

Answer 3

-data from a village in Gambia show that the present of grand mother increase survival after 20 months

Question 4

Tests of the grandmother hypothesis

Answer 4

 Data from 18th & 19th century Finnish and Canadian women
 Historical farming communities
 Grandparents integral part of family, usually residing in same house as offspring
 Probable benefits: transfer of knowledge, participation in household tasks and childcare
 similar relationship of more grandchildren and age despite social, cultural & life-history differences
 post-reproductive lifespan unrelated to the number of children produced before age 50
 ~2 g/children/10y

Question 5

Effect of grandmother presence

Answer 5

-In presence of a living mother, offspring reproduce earlier and are more fecund
-grandchildren survive better if grandmother alive at birth
-effect depends on age of grandmother

Question 6

study on grandfather and grandchildren

Answer 6

Finnish data set - men who weremarried only once
 Reductions in offspring age at first reproduction and birth intervals
 No increases in reproductive tenure lengths.
 No extra fitness (> grandchildren) through grandfathering

Question 7

Other spp with long post-reproductive lifespans: killer whales

Answer 7

 Unusually long post-reproductive lives in females
 Females produce calves from 10 to ~40; oldest female around 90 years
 Matrilineal social structure in pod
 Benefits of food sharing, care and transfer of information

Question 8

Does the grandmother effect operate in Orcas

Answer 8

Data: sightings of individually recognisable whales off US and Canadian waters over 40 yrs
378 individuals with known maternal grandmothers
Post reproductive grandmothers provide significant survival benefits to their grand offspring above that provided by reproductive grandmothers

Question 9

Other evidence of grandmother effect in orca

Answer 9

• normally, reproductive female would be the leader of the pods
• during food crisis, post reproductive female become the leader (using exp)
  -if reproduction of two female generation overlap, older generation calf is 1.7 times more likely to die compare to the one from younger gen

Question 10

Grandmother effect in Asian Elephants

Answer 10

if grandmother close by:
* 8 times lower mortality risk for grandcalves from young mothers (<20 years)
Daughters’ inter-birth intervals reduced by one year

Question 11

Offspring number - twinning: why do humans typically produce only one offspring

Answer 11

 Physiological limits to rate of milk production?
 Inability to carry two foetuses to term at optimal body size?
 Caring for one infant interferes with successful care of another?
Strongest determinant of conception is nursing of infant

Question 12

current rate of twining

Answer 12

Rate of twinning has increased from 1980 then levelled off from ~2010 – pattern similar across developed countries

Question 13

Cause of increase in twining?

Answer 13

-in developed country, age of mother are higher
-twinning is more common in older mother

Question 14

study on twinning rate reveal; interesting information about twinning

Answer 14

Rate of twinning increases then decreases with maternal age (peak after 30)
Probability of double ovulation increases with maternal age
Probability of live births per zygote decreases with maternal age

Question 15

Resource availability and human twinning study

Answer 15

Frequency of twinning varies across human populations
Highest twinning rates for caucasian populations have been recorded on archipelagos in south-west Finland
Lumaa et al. compared lifetime reproductive success of females producing singletons and twins on the archipelagos versus mainland
* compared 20 archipelago & mainland parish pairs
Twinning frequencies: * archipelagos: 21.3% * mainland: 14.9%
-total reproductive success the same
Differences in profitability of twinning in two areas probably related to food availability

Question 16

Inter-birth intervals (IBI) in primates

Answer 16

• we have shorter IBI when compare to other primate of similar size

Question 17

IBI in !Kung

Answer 17

 Nomadic hunter-gatherer lifestyle
 IBI average around 4 years - maladaptive
 Shorter birth intervals require a mother to carry higher backloads (baby + food) on foraging trips
 Sharply increased backload might lead to such a severe increase in mortality of offspring that 4-year interval is optimal

Question 18

A graphical model of optimal fertility rate

Answer 18

Presumed trade-offs between investment in one offspring and survival of another
As birth rate goes up, so does the rate of mortality
α = net difference between fertility and mortality (# of surviving offspring)
Fo = fertility rate that maximises offspring survival