life history: the stages of growth, reproduction, and dispersal that an individual goes through...

Life History: the stages of growth, reproduction, and dispersal that an individual goes through during its life from birth to death.

Stages of human life history

GestationChildhood

(3) Reproductive period

(4) Post-reproductive period

GROWTH, MAINTENANCE AND REPRODUCTION ARE SUBJECT TO TRADE-OFFS

TO ANALYZE SUCH TRADE-OFFS, WE USE LIFE-HISTORY THEORY

GROWTH and maintenance

REPRODUCTION and maintenance

MAINTENANCE

-The amount of energy available is finite, and energy devoted to one function, such as growth or tissue repair, cannot be devoted to other functions – such as reproduction

-Biological processes take time. An individual growing to a large size before maturing risks dying from disease or other causes before ever reproducing

-Fundamental trade-offs involving energy and time mean that every organism’s life history is an evolutionary compromise.

All organisms face fundamental tradeoffs

Simplest, mostgeneral life-historytradeoff

Given strong tradeoffs, can excellent health truly ever be achieved?

Other fundamental life-history tradeoffs:

Natural selection shapes the timing of life histories, and patterns of allocation to growth, maintenance, and reproduction throughout the lifespan

Humans: ‘SLOW’ life history: large babies, long childhood, late age at 1st reproduction, large adult size, low mortality after mid-childhood, long adult lifespan (but early weaning and ‘stacking’ of babies)

Overview of allocation patterns and trade-offs in humans

The expression of trade-offs may depend upon overall levels of available resources

-> Car-house ‘paradox’: with monetary trade-offs, why do peoplewith fancier cars have bigger houses?

Resolution: Because people vary in levels of overall resources, and people lower in overall resources are subject to stronger tradeoffs

Implication for health: tradeoffs are stronger, and affect health more, in individuals more subject to resource stress

Some life-history trade-offs in humans

(1) Trade-off between size and numbers of offspring, across primates, and across human cultures

PRSLB2007

(2) PNAS 2007

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(3)

Tradeoffs between current and future reproductive investment

PRSLB 2007

What are the proximate and ultimate causes of trade-offs in human health and disease?

CENTRAL ROLE OF HORMONES

(1) Hormones mediate flow, distribution, rate of consumption of glucose and fat among competing physiological functions

(2) Hormones implement tradeoffs between growth, reproduction and survival

Simple example: castration in domestic animals reduces effectsof testosterone on reproductive effort, results in more fat and protein deposition

Hormones and trade-offs in males

(1) Androgens guide prenatal development as a male via effects in body and brain; males also suffer higher early mortality

(2) Hormonal quiescence in childhood; precocious puberty leads to short stature

(3) Adolescence: primary and secondary sexual traits develop (genitalia, hair, voice, muscles); increase in risky behavior and male mortality spike, which may be related to sexual selection(high-risk behavior may augmentcompetitiveness and attractiveness, in some individuals and conditions)

Hormones and trade-offs in males

Benefits and costs of testosterone in adulthood

(4) Benefits - support of spermatogenesis, secondary sexual traits,augmentation of body weight, muscle, cognitive benefits; supplementation may provide benefits especially for older men not subject to energetic constraints

(5) Costs - higher metabolic rate, reduced immune function under stress (and lower survivorship), reduction in fat (may involve reduced survivorship), possible links with cancer, especially prostate cancer, and may involve shorter lifespan

(7) Other effects: lower testosterone in context of paternal behavior, mate bonding -- cause and effect?

Testosterone builds muscle and a faster BMR

Testosterone levelsincrease as infection is cleared

Model of effects of testosterone on life-history tradeoffs in males

Muehlenbein & Bribiescas 2005

Are pleiotropic effects of genes underlying testosterone production and response - sex vs death, take your pick

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Hormones and trade-offs in females

(1) Female reproductive success is limited by direct metabolic investments, and investments from mate and kin; in pre-modern societies, infant survival (strong function of birthweight) and interbirth intervals mainly determine reproductive success -> expect strong size-number tradeoff

(2) Trade-offs in pregnancy are primarily between maternal maintenance and fetal growth, and they involve maternal-fetal conflicts

(3) Trade-offs after parturition involve investment in current vs future offspring, and they are mediated by:

• nursing and mother’s condition via lactational amenorrhea (affecting prolactin levels), and by timing of introduction of supplementary foods;influence timing of start of next menstrual cycles

(b) fecundity during waiting time to next conception - poor energetic condition(and low ovarian steroid levels) are associated with reduced ovulation frequency - might expect male-female conflict, and mother-offspring conflict, over conception, with mechanisms that may become dysregulated

Mothers may also benefit from a faster reproductive rate (and shorter interbirth intervals) even when it involves higher offspring mortality

Evidence for trade-off between offspring number and quality

Hormones and trade-offs in females over a female’s reproductive period

Three stages, for indices of fecundity:

(1) Rise over the first decade after menarche (slow climb to full fertility) -> may be due to selection against high potential survivorship costs of child-bearing for younger females

(2) Fairly stable from mid-20s until mid-30s

(3) Decline to menopause at about 50 - proximately due to depletion of follicles, number fixed at birth;disorders of this process are premature ovarian failure and polycysticovary syndrome- ultimately may be due to some combination of (a) higher risks of child-bearing with increased age, (b) higher risk of death from other causes with increased age, for self and mate, (c) benefits from enhancing the reproduction of one’s grandchildren

(a) higher risks of child-bearing with increased age, (b) higher risk of death from other causes with increased age, for self and mate, (c) benefits from enhancing the reproduction of one’s grandchildren (lower infant mortality of grandchildren, better condition of daughter)

->trade-off between existing and future children

->trade-off between further reproduction and survival

Trade-offs Underlying the Evolution of Menopause, a trait unique to humans (among primates)

Nature 2004

PNAS 2007

Trade-offs andCONFLICTS

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PRSLB 2007 Nope

The data on effects of grandfather presence

Some medical implications of tradeoffs in human life history

(1) Declines in testosterone with age lead to to increased risk of obesity, insulin resistance, and cardiovascular disease; testosterone supplementation may increase morbidity and mortality in individuals under energy limitation

(2) Exposure to reproductive hormones (including contraceptives), and older age at first birth in women, can increase the risk of some reproductive cancers (breast, ovarian, testicular, prostate, uterine, cervical)

(3) Hormone physiology and tradeoffs may differ between human populations historically exposed to different selective pressures affecting life history (eg pygmies)

(4) Body size and life history are seldom taken into account when developing and using animal models of diseases relatedto physiology

(5) It may be essentially impossible to maximize health with regard to competing functions that trade off, such as maintenance vs reproduction