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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



= 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


Life History Trade-off:

Growth & Survival VS. Reproduction


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


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


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...


Life History Strategy

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


life history strategy for example

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


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


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

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


• 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


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...


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


how often to breed?

– Semelparity
– Iteroparity



reproduce once and die



reproduce repeatedly throughout life span


Any patterns in nature?

General pattern:


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)


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...


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


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


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


How many offspring to produce in each breeding event?

If iteroparous...
• General pattern: as more offspring are produced – the survival of each offspring
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...


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)


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


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)


– 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)



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



= gradual increase in mortality
and decline in fecundity with age


Life History Classifications

1. if resources are unlimited
2. if resources are limited

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