Lecture 10- Human life histories 2 Environmental effects on life history Flashcards
How does environmental variation in space and time affect human life histories?
• Geographic variation in risk (survival prospects): do humans discount againstfutureprospects? • Geographic variation in resources and offspring fitness: effects on brood size? • Temporal variation in resources and offspring fitness: is early investment important? • Socioeconomic variation and the probability of full-term pregnancy
What is the classic example of geographic variation in risk and life history?
- Classic example: differences in predation risk across stream communities shape guppy life history patterns in Trinidad
- Downstream: many predators present (cichlid fish), thus probably high- predation communities
- Upstream: only one potential predator present (kilifish), thus probably low- predation communities
- high predation= faster maturity, higher number of offspring in 1st litter (might not survive to the next one), have shorter time between litters
What is the mortality and reproductive timing like in Chicago?
-Wilson and Daly (1997) examined the relationship between risk-taking, life expectancy, reproductive timing and economic inequality in 77 neighbourhood units -poor neighbourhoods= high homicide risk, the opposite in rich neighbourhoods -poor neighbourhood= more risk, should favour discounting future prospects -Prediction: in neighbourhoods with poor economic opportunities and low life expectancy, risk-taking behaviour should be elevated -Discounting of future prospects should favour early onset of reproduction -took 10 lowest and 10 highest neighbourhoods -poor= lower age of mother when have the first child -low economic group= birth rate is much higher, more children per 1000 women per year other factors= education, understanding plus access to contraception -Negative correlation between high homocide rates and life expectancy -Women with low life expectancy appear to reproduce early
What is the study looking at resource availability and human twinning?
-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 -Pre-industrial era (1752-1850) -• compared 20 archipelago & mainland parish pairs Twinning frequencies: • archipelagos: 21.3% • mainland: 14.9% • twin mothers compared with controls of same age • total number of offspring calculated • total reproductive success the same, but mothers of twins did worse on mainland -didn’t distinguish between mono and dizygotic twins -only dizygotic have a genetic component -Analysis restricted to mothers with genetic tendency to produce dizygotic twins (different sexes) Differences in profitability of twinning in two areas probably related to food availability -high reproductive effort=in areas where more constant resources -total reproductive success was comparable
What was the study looking at early investment and offspring fitness?
-In Western fence lizards, experimental removal of yolk from eggs reduces the quality of offspring -food has impact, is it when the food is offered? when it is available? = really important for future development -remove yolk from the egg during early development so the embryo has fewer resources, = the offspring were smaller at birth and slower runners! -Young hatched from yolk- reduced eggs were smaller and could not run as fast -Investment early in life has important consequences for fitness
What was the study looking at early somatic investment in humans?
-Two birth seasons in rural Gambia (sub-Saharan Africa): 1. Hungry period: wet season (Jun-Oct) Staple foods from harvest depleted,Heavy adult workload, malaria/diarrhoea Stress effects on foetal development 2. Harvest period (dry season): mor ebaundant resources -High-energy biscuits given 20 weeks before delivery significantly improve birth weight, particularly during the hungry season -similar pattern in humans: -have seasons of feast and famine -famine time= mother in suboptimal condition -give high energy biscuits to mothers at 20 weeks before delivery -huge improvement to the birth rate! -Seasonality documented over 40 years Detailed records of births and deaths kept since 1949 in three subsistence-farming villages (popn=3,000) Post 35, odds of premature death rise almost tenfold for individuals born in hungry season -long -lasting effects= affects mortality in the adult offspring -red= if born during hungry season -blue= during harvest season -there is not much difference in probability of survival when young but big discrepancy later on (particularly disease etc.)
What is the effect of poor maternal nutrition during pregnancy?
-Poor maternal nutrition during pregnancy is associated with a range of metabolic disorders later in the life of the offspring -In rats, maternal diet alters balance of activity of key liver enzymes, a phenomenon known as ‘metabolic programming’ or ‘metabolic imprinting’ -Glucose tolerance in human adults aged 50-55 years in relation to exposure to famine (high values = impaired glucose tolerance) -impact not only on survival but other aspects: -maternal nutrition makes changes to enzymes in liver that affect the foetus
What is compensatory growth and the associated issues?
-Where poor early foetal growth is offset by compensatory growth late in pregnancy (or shortly after), there are often severe long-term costs (e.g. risk of adult obesity; low glucose tolerance) -poor foetal nutrition may permanently reduce number of kidney cells Rapid growth may disrupt function, or excessively stress smaller cell mass Frequent cell division and free radical damage during compensatory growth may shorten telomeres and hasten cell death, organ degradation -highest risk of coronary disease as an adult for small birth size followed by compensatory growth to relatively large age by 7 years -the offspring may catch up but still effects -fewer kidney cells -3 groups, small, medium and large -there is compensatory growth in the small group, it increases their risks of diseases (coronary disease)
Are there intergenerational effects of a bad start?
-Downstream effects of starvation events on offspring birth weight and survival can last over generations -Example: Dutch famine of World War II -During winter of 1944-45, urban centres of western Netherlands experienced acute famine as a result of German embargo on rail transport, and severe winter that led to freezing of canals -Normally well-fed population affected over short and precisely defined period – similar to experimental protocol -it is not just you that is affected= what happens in previous generations can affect several subsequent generations
What was the study looking at the Dutch famine?
-Women who became pregnant during famine lost weight and produced smaller babies than those fertilised before or after the famine -Women whose mothers were exposed to famine were at higher risk of losing their own offspring -much higher risk of mortality in offspring exposed to the famine conditions in utero. -worst the ones that were fertilised during famine and born after famine -this was carried on to the next generation, if mother’s mother was exposed during third semester it has a huge effect
What is the relationship between parental investment and abortion rates?
-In many human societies, marital status correlates with resource availability - single women have less social and material support for child rearing -Costs of postnatal investment may select for reproductive management in utero -Prediction 1: single women should be more likely to abort a pregnancy than married women (less social/material support) -Prediction 2: single females should place increasing value on current reproduction compared to future reproduction as they age (less likely to abort with age)
What is the probability of an abortion in British women affected by?
-age and status -when young and pregnant= more likely to abort if unmarried -when older the single ones less likely to abort a pregnancy -married women abort more as the age -probability of future marriage= inverse of age, as you age less likely to marriage -if very likely to be married more likely to abort! (competition in human males for paternity of offspring)
Is there a relationship between seasons and births in the human population?
-Since survival probabilities of children vary seasonally, timing of birth should be a life-history trait under natural selection -Fitness can theoretically be maximised by timing birth to occur at the ‘right’ time -Lummaa et al examined seasonal distribution of births and survival in rural Finland during 1769-1850
What is the pattern of seasonality and the number of births+ weight at birth?
• significant seasonal variation in both birth rate and survival (similar to patterns found in other European populations) • mortality higher for male infants in all seasons (growth and development less buffered against env. disturbances) • but NO correlation between birth rate and survival -Either humans are incapable of timing reproduction to season of highest infant survival probability, or other things than infant survival are being maximised What factors DID correlate with birth rate? - holidays and the ‘Christmas’ effect - spring birth peak exhibited by countries of diverse cultural and religious traditions -timing of conception appears to be an example of a life- history trait that is not obviously moulded by natural selection
Summary:
- Environmental variation in time and space may select for variation within human life histories 2. Geographic variation in a) risk (survival prospects) and b) resources has influenced both the timing and degree of investment in offspring 3. Effects early in development appear to have disproportionate impact on fitness, and may even affect subsequent generations 4. There is some evidence that human females are able to control reproductive investment in utero, but not via seasonably favourable timing of conception
