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1

Life History• Resources are often limited...

– organisms must decide how to allocate time/energy to different activities
throughout lifespan
– allocation decisions will affect an individual’s fitness – by influencing growth,
reproduction and survival
• Therefore, organisms will develop allocation strategies over evolutionary time
to adapt to abiotic and biotic conditions in their environment

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Trade-off

= increased allocation of time/energy to some activities results in a decreased allocation to other activities
– increase in one thing means a decrease in something else

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Life History Trade-off:

Growth & Survival VS. Reproduction

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Mainly concerned with...
Reproductive effort

the proportion of available resources that an individual allocates to
reproduction throughout its lifespan
= current + future reproductive output

future reproductive output = survival + fecundity in the future
(fecundity = number of offspring produced/event)
-- Lifetime Reproductive Success

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lifetime reproductive success

the number of offspring produced throughout the lifespan of an individual
– ~ similar to fitness... but not does not involve the offspring surviving to reproductive age

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Organisms may adopt different strategies over evolutionary time...

Life History Strategy
The optimal strategy will be a compromise between the allocation of time/energy to growth/survival versus reproduction...

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Life History Strategy

= set of choices and decisions resulting in an individual’s allocation to reproductive effort through its lifespan

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life history strategy for example

-invest heavily in current reproduction
-invest heavily in current growth/survival (delay reproduction)

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Invest heavily in current reproduction

– may drain a parent’s energy reserves
– reduce ability to grow → lower probability of survival – If survive, may produce fewer offspring in the future

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– Invest heavily in current growth/survival (delay reproduction):

• faster growth → higher probability of survival
– larger size – more resources available for future reproduction

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• Life history traits include:

– Body size / growth
– Age at sexual maturity
– Number of reproductive events
– Number of offspring produced per event – Offspring size
– Amount of parental care
– Senescence, programmed death

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A life history strategy integrates all of these traits in a way that

maximizes fitness Because the strategy influences an individual’s fitness... the strategy will be molded by natural selection over evolutionary time
Which strategy evolves depends on environmental conditions...

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Most life history strategies can be described by asking 3 questions:

– How often to breed?
– When to begin producing offspring?
– How many offspring to produce in each breeding event?

answers express each species trade-off between reproduction and adult growth/survival

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how often to breed?

– Semelparity
– Iteroparity

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Semelparity

reproduce once and die

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Iteroparity

reproduce repeatedly throughout life span

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Any patterns in nature?

General pattern:

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General pattern:

semelparity occurs more for organisms living under variable environmental conditions
• Therefore, semelparity is favoured when:
– Adult survival is low (lifespan < 1-2 years)
– Or adult survival is high – but long intervals between years
with conditions suitable for high offspring survival
» Organisms store resources and reproduce when
conditions are favourable and most offspring are likely to survive (Carpe diem! Seize the day)

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Agaves (the “century plant”)

– inhabit climates with erratic rainfall
– plants store nutrients and grow for several years (average life span ~ 25 years)
– Semelparous
• Reproduce during an unusua lly wet year
• Seeds have a higher chance of establishment/survival
• Parent plant diets after flowering (reproduction)
Semelparous not only under variable environmental conditions, but when parents are not likely to survive breeding...

20

mayflies (also fish flies)

- adult life stage is short lived lasting hours to a couple of days
- do not consume food as an adult
-sole purpose is reproduction
- after copulation the female will go off to lay her eggs and die, the male just goes off to die

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Eg. Salmon

– Fish grow rapidly at sea for several years
– Huge effort to migrate up rivers to reach spawning grounds (>150 km!) – Semelparous
• during spawning migration - females convert a large portion of body tissue into eggs
• Reproduce and die shortly after spawning

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preying mantis

- male mantids are often semelparous
- females mantids can be iteroparous
- this is largely due to the tendency of the female to consume the male after copulation
- eating the male provides more nutrients, and this might lead to more eggs or stronger eggs and it is in their genes

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How many offspring to produce in each breeding event?

If iteroparous...
• General pattern: as more offspring are produced – the survival of each offspring
decreases
Fewer offspring → allocate more resources per offspring More offspring → allocate less resources per offspring

Produce more offspring over lifespan if produce a lower than maximum number of offspring in a given year...

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Great Tits in England (~chickadee)

– Most frequent = 9 eggs/clutch
– More offspring survive from a 12 egg/clutch
Why would parents produce less young than they are capable of doing? Large clutch may drain a parent’s resources:
• reduce adult survival → fewer total offspring production over lifespan (LRS)

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When to begin reproducing?

• General pattern: Age of maturity increases as adult lifespan or annual survival rate increases
Remember... General pattern: fecundity ^ with body size Growth is important!
-Long lifespan
-Short lifespan

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Long lifespan (high annual survival rate):

• current fecundity should not jeopardize future growth, survival and reproduction

– favour growth in 1st few years to increase fecundity
– favour reproduction over lifespan (~ breed older)

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– Short lifespan (low annual survival rate):

• current fecundity at the expense of adult survival (especially if semelparous) – do not favour growth – may die before breeding
– favour current reproduction (~ breed younger)

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Birds

• Low annual survival – breed younger • High annual survival – breed older
...but do not delay too long!
... most organisms experience senescence

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senescence

= gradual increase in mortality
and decline in fecundity with age

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Life History Classifications

Resource-based:
1. if resources are unlimited
2. if resources are limited

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