# BIOL 107 Lecture Notes - Parasitoid, Life Time Fitness, Ovipositor

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Published on 25 Jan 2013
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How Many Offspring Should an Individual Produce in a Given Year?
The more offspring a parent (or pair of parents) attempts to raise at once, the less time and
energy the parent can devote to caring for each one
Clutch Size in Birds
Selection will favour the clutch size that produces the most surviving offspring
Any individual offspring will survive decreases with increasing clutch size
The ability of the parents to feed any individual offspring declines as the number of offspring
increases
In fig 13.19a, we assume that the decline in offspring survival is a linear function of clutch size,
but the model depends only on survival being a decreasing function
o The number of surviving offspring reaches a maximum at an intermediate clutch size
In an experiment, the mean clutch size was 8.53
o The average number of surviving offspring from clutches of each size was also
determined
This number was highest for clutches of 12 eggs
When researchers added 3 eggs to each of a large number of clutches, the most
productive clutch size was still 12
In other words, birds that produced smaller clutches apparently could have
increased their reproductive success for the year by laying 12 eggs
This indicates that natural selection favours larger clutches than the
birds in the population actually produce
o Because the average clutch size was less than the most
productive clutch size, these results are not consistent with
Lack’s hypothesis
Lack’s hypothesis: selection will favour the clutch size
that produces the most surviving offspring
Another experiment showed that: The majority of studies have shown that birds lay smaller
clutches than predicted
Lack’s hypothesis assumes that there is no trade-off between a parent’s reproductive effort in
one year and its survival or reproductive performance in future years
o When reproduction is costly and selection favours withholding some reproductive effort
for the future, the optimal clutch size may be less than the most productive clutch size
Second, Lack’s hypothesis assumes that the only effect of clutch size on offspring is in
determining whether the offspring survive
o Being part of a large clutch may, impose other costs on individual offspring than just
reducing their probability of survival
Clutch size affects not only offspring survival, but also offspring reproductive performance
o This suggest that there is a trade-off between the quality and quantity of offspring
produced
o When larger clutches entail lower offspring reproductive success, the optimal clutch size
will be smaller than the most numerically productive clutch size
Thrid, the discrepancy between Lack’s hypothesis and the behaviour of individual birds may
sometimes be more apparent than real
o In fig. 13.17, the birds that laid fewer than 12 eggs did so because they had lower
reproductive capacities-and that each bird was producing a clutch size that would
optimize its own reproductive success
Lack’s hypothesis predicts that parents will attempt to rear that number of young that
maximizes the number of surviving offspring. Data indicate that parents often rear fewer
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