Lecture 9- Introduction to human life histories: Evolution of the human lifespan

Question 1

What are some of the life history questions we will be looking at?

Answer 1

1. Why do we grow old and die? Senescence 2. Effects of the environment Geographic and temporal variation in life histories 3. Optimal scheduling of reproduction The puzzle of menopause 4. Sons or daughters? Sex allocation

Question 2

What is a life history?

Answer 2

The stages of growth, reproduction, and dispersal that an individual goes through, during its life from birth to death

Question 3

What are the stages of human life history?

Answer 3

1.Gestation/Childhood GROWTH and maintenance 2.Early adulthood Gestation/Childhood GROWTH and maintenance REPRODUCTION and maintenance 3.Post-reproductive age MAINTENANCE -childhood lasts 0-15 (large amounts of energy into growth) early adulthood (we are maintaining, we invest in reproduction) -then we stop reproducing, post-reproductive age, only maintenance (45-until death)

Question 4

What is the problem with studying human life history?

Answer 4

-Rarely or never opportunities to conduct experimental studies -and correlation is not causation! eg. -looked at number of births (in stalks) and number of newborn babies -as the no. of stalks grew so did the number of babies! what? -however … Remarkable records of reproduction and life span for many societies - perspective not available for animal societies

Question 5

What are some of the key resources for studying human life history?

Answer 5

-Historical parish registers maintained by churches (Finland, Canada, Lapland) -Contemporary rural (non-industrialised) communities (Gambia)

Question 6

What are the 3 prerequisites for natural selection?

Answer 6

1. Variation among individuals in some attribute or trait 2. Consistent relationship between trait and fitness 3. Heritability of the trait in question

Question 7

What is the ideal organism: a wish list?

Answer 7

• reproduces during its entire, long life • produces large number of large offspring • lavishes parental care on all offspring • outcompetes its competitors • always escapes predation -Even the most successful organism can’t be good at everything!

Question 8

What is the fundamental concept for understanding life histories?

Answer 8

-Allocation of finite energy and resources -Trade-offs among resources to maximise fitness Reproduction: ! How many offspring? ! How much parental care invested into each? Survival: ! Storage? ! Growth? -trading off things to maximise fitness -eg. reproduction: how many offspring, how much care? there must be a balance -survival= do you invest a lot into storage? (if immune system attacked) -often these tradeoffs are negatively correlated to one another

Question 9

What are the trade-offs among resources?

Answer 9

• Negative correlation between two life history activities
• Increased allocation to one activity usually results in reduced allocation to another activity
• To analyse trade-offs, we use life history theory
• allocate more resources in one activity decreases the availability of resources put into another life history activity

Question 10

Can we look at one individual and estimate life history of the species?

Answer 10

-NO -cannot take a life history from one person, it is an average -single individual is not informative, need patterns from the population eg: - Died at 122 years, 164 days (longest confirmed lifespan) - Smoked from age of 21 (gave up at 117 because of cataracts) - “Interested in everything; passionate about nothing” - Took up fencing at 85; still rode a bicycle at 100 - Drank port wine and ate about a kilo of chocolate every week - One daughter

Question 11

Why do we age?

Answer 11

-disposable soma (cells) theory -Reproduction is costly; investment in reproduction diverts resources from maintenance and repair of cells, causing ageing -Prediction: shortage of food should exacerbate this trade-off But … ! Caloric restriction experiments find animals live longer when fed less ! Males invest much less into reproduction than females, yet females live longer - Male Drosophila melanogaster males that mate frequently have reduced lifespans

Question 12

What is the relationship between longevity and reproduction in humans?

Answer 12

-Are longevity and reproduction negatively correlated in humans? -Anecdotal accounts of non- reproducing individuals living for longer (celibate nuns and priests; neutered pets)

Question 13

What are the costs of reproduction in humans?

Answer 13

-Costs of reproduction in humans -Study of 18th & 19th century -British aristocracy -Minimised effects of socio-economic conditions on reproduction and lifespan by restricting study to individuals that were relatively well-off -Studied the relationship between reproductive success and longevity in married women in cohorts up to 1876 -33,497 individuals (Peerage cd) -Early child bearing is associated with a shorter life span -is there evidence for a general pattern?: studies of other populations have not found similar effects - Women that reach a ripe old age are not more likely to be childless … -Women that have more children do not seem have an increased risk of mortality

Question 14

What is the mutation accumulation and its connection to mortality?

Answer 14

-Animals tend to die of extrinsic causes (predation, disease, accidents) well before they reach old age -Weak selection for weeding out random, detrimental mutations that don’t show up until late in life (not disadvantageous to majority) -The older we become, the more we accumulate these mutations – these eventually kill us

Question 15

What is the idea of antagonistic pleiotropy?

Answer 15

-Many genes are pleiotropic (affect two or more traits) ! The same gene can have positive as well as negative effects ! For instance, a gene that promotes calcium deposits in bones but also in arteries has early fitness benefits, but exacts a cost later on Enhanced early survival will be selected for (even if it comes with a price tag later in life) ! because so many more individuals are alive at earlier ages - natural selection will be stronger So, negative effects in old age may result from selection for pleiotropic genes with beneficial effects early in life

Question 16

What is the connection between extrinsic mortality and ageing?

Answer 16

-If extrinsic mortality is high, ageing should be rapid natural selection promotes traits that promote early reproduction at expense of delayed ageing -If extrinsic mortality is low & reproduction at older ages likely natural selection promotes traits that protect against ageing and promote survival -Prediction: populations living in ‘safer’ environments (with less extrinsic mortality) should age more slowly, because natural selection wanes more slowly

Question 17

What is the safety and senescence in possums?

Answer 17

-Austad compared senescence in two populations of Virginia opossums – one on Sapelo Island off east coast of Georgia, other in South Carolina ! Same species – genetically isolated ! Previous work in Venezuela: poor defenses against predators, short-lived, early reproduction ! Signs of senescence start at 18 months (cataracts, arthritis and hair loss), no individuals survived past 2 years -Prediction: ageing should be slower in the ‘safer’ island population

Question 18

What was the study with possums?

Answer 18

-Approach: ! Females captured and tracked with radio collars ! Mortality sensors allowed accurate determination of time of death ! Tendon fibres collected from tails each time an opossum was caught ! Amino acid chains of collagen become cross linked (free radical damage) and less flexible with age (break more easily) -! Island opossums lived 25-50% longer than their mainland counterparts ! They reproduced later, and aged more slowly ! Island population showed reduced rate of damage in collagenous -Could other factors beside extrinsic mortality be responsible for the pattern? ! (climate, food availability, parasites) ! Ideal experiment? Replication, experimental introductions (but problems of generation time) ! Austadtestedforalternativeexplanations: ” “ “ climate identical (slightly higher temps on island) more ticks on the island higher density on island, but BMI same

Question 19

What are some of the other trade-offs with the possums?

Answer 19

-offspring quality vs quantity -I = R/C (investment = resources/children) Problem: variation in parental condition may mask the trade-off (parents with more children have more resources and therefore are also able to invest more in each child) To deal with this problem, can calculate ‘energy- adjusted’ fertility rate, which takes into account body mass of the mother Prediction: energy-adjusted fertility rate should correlate negatively with offspring size -Expected relationship from life history theory holds for both primates, and for ‘natural fertility’ human societies

Question 20

Summary?

Answer 20

1. Trade-offs are central to life-history theory, and helpful for understanding differences between species 2. The study of trade-offs in humans is plagued by the general inability to conduct manipulative experiments; however, long-term datasets of pre-industrial humans offer a useful alternative 3. The evidence for trade-offs between reproduction and lifespan is mixed; but recent comparative evidence suggests offspring quantity is traded off against quality.