Explaining patterns of age-specific performance
Author(s)
Rebke, Maren
Rebke, Maren
Rebke, Maren
Type
Thesis
Abstract
Individual life histories are frequently studied to gain insight into the mechanisms of
ageing. However, various challenges complicate the accurate quantification of age-specific variation in fitness. In this thesis I develop and apply methods to accurately
characterise patterns of ageing, and to explain why such patterns arise.
All mammals and birds have an upper bound on litter size, and for many species
this limit is quite low. In addition, in many species, not all individuals breed at
every possible opportunity. Reproduction should consequently be considered as
two processes: whether an individual breeds or not and the number of offspring
produced. These processes mean that reproduction in many species does not follow
a Poisson process as is often assumed in analyses of breeding performance. A more
appropriate model for a repeated ordinal response like annual reproductive success
is a proportional odds model with a random intercept for individuals. Such a model
has not previously been used in ecology or evolutionary biology. I apply this model
to analyse age and temporal variation in the number of
fledglings produced annually
by male and female common terns (Sterna hirundo). I use data collected from this
intensively studied, long-lived species, repeatedly throughout the thesis.
The proportional odds analysis reveal that reproductive performance in females
initially increased with age, before declining as individuals began to senesce. But
why does this pattern arise? Is it purely an effect of getting physiologically older or
are other processes involved? I estimate the effect of the length of time spent with
the current partner using the common tern data. Despite the quality of the data, it is
not always obvious if unmarked partners are new or not. I use a hierarchical Bayesian
model of the steps that lead to the number of
fledglings. Modelling this complicated
process requires a complex model, but results show that no substantial amount
of observed age-related patterns in reproductive performance can be attributed to
length of pair bond.
While the proportional odds and Bayesian analyses account for repeated measures
on individuals they do not account for compositional change. Such a change in the composition of the population caused by heterogeneity between individuals can
mask true rates of individual change. I develop a novel retrospective decomposition
method related to the Price equation to address this issue. The equation gives
the exact contributions of selective disappearance and average change in individual
performance among survivors to the aggregate change at the level of the population.
This equation can be extended by including a term for the compositional change
due to selective appearance of individuals in the study population.
I apply this decomposition to the common tern dataset to disentangle whether
apparent increases and decreases in reproductive performance with age reflect genuine
changes within individuals or are an artefact of compositional change in a
heterogeneous population. I show an improvement in average reproductive performance
of individuals over most of adult life and give support for reproductive
senescence at old ages. I show that the contribution of compositional change is of
minor importance, suggesting that population-level averages accurately capture the
individual-level ageing process well.
Can the decomposition method I develop be applied to other systems? Does
it lead to similar conclusions? I apply it to two different datasets dealing with
functioning at old age in humans: the ability to live independently in the Danish
1905-cohort, and cognitive functioning for people aged 80 and older participating
in the Chinese Longitudinal Health and Longevity Survey. In both studies I reveal
that average individual functioning declines at old ages. Although the decline is also
apparent at the population level it is less strong due to the tendency of individuals
with lower functioning to drop out earlier. Finally, I illustrate the general use of
the decomposition by applying it to epidemiological and economic studies in the
appendix.
Overall, I find that reproductive performance improves over many age classes
before senescence begins. Numerous processes can influence rates of age-related
change, with results apparently specific to the trait and population under study.
ageing. However, various challenges complicate the accurate quantification of age-specific variation in fitness. In this thesis I develop and apply methods to accurately
characterise patterns of ageing, and to explain why such patterns arise.
All mammals and birds have an upper bound on litter size, and for many species
this limit is quite low. In addition, in many species, not all individuals breed at
every possible opportunity. Reproduction should consequently be considered as
two processes: whether an individual breeds or not and the number of offspring
produced. These processes mean that reproduction in many species does not follow
a Poisson process as is often assumed in analyses of breeding performance. A more
appropriate model for a repeated ordinal response like annual reproductive success
is a proportional odds model with a random intercept for individuals. Such a model
has not previously been used in ecology or evolutionary biology. I apply this model
to analyse age and temporal variation in the number of
fledglings produced annually
by male and female common terns (Sterna hirundo). I use data collected from this
intensively studied, long-lived species, repeatedly throughout the thesis.
The proportional odds analysis reveal that reproductive performance in females
initially increased with age, before declining as individuals began to senesce. But
why does this pattern arise? Is it purely an effect of getting physiologically older or
are other processes involved? I estimate the effect of the length of time spent with
the current partner using the common tern data. Despite the quality of the data, it is
not always obvious if unmarked partners are new or not. I use a hierarchical Bayesian
model of the steps that lead to the number of
fledglings. Modelling this complicated
process requires a complex model, but results show that no substantial amount
of observed age-related patterns in reproductive performance can be attributed to
length of pair bond.
While the proportional odds and Bayesian analyses account for repeated measures
on individuals they do not account for compositional change. Such a change in the composition of the population caused by heterogeneity between individuals can
mask true rates of individual change. I develop a novel retrospective decomposition
method related to the Price equation to address this issue. The equation gives
the exact contributions of selective disappearance and average change in individual
performance among survivors to the aggregate change at the level of the population.
This equation can be extended by including a term for the compositional change
due to selective appearance of individuals in the study population.
I apply this decomposition to the common tern dataset to disentangle whether
apparent increases and decreases in reproductive performance with age reflect genuine
changes within individuals or are an artefact of compositional change in a
heterogeneous population. I show an improvement in average reproductive performance
of individuals over most of adult life and give support for reproductive
senescence at old ages. I show that the contribution of compositional change is of
minor importance, suggesting that population-level averages accurately capture the
individual-level ageing process well.
Can the decomposition method I develop be applied to other systems? Does
it lead to similar conclusions? I apply it to two different datasets dealing with
functioning at old age in humans: the ability to live independently in the Danish
1905-cohort, and cognitive functioning for people aged 80 and older participating
in the Chinese Longitudinal Health and Longevity Survey. In both studies I reveal
that average individual functioning declines at old ages. Although the decline is also
apparent at the population level it is less strong due to the tendency of individuals
with lower functioning to drop out earlier. Finally, I illustrate the general use of
the decomposition by applying it to epidemiological and economic studies in the
appendix.
Overall, I find that reproductive performance improves over many age classes
before senescence begins. Numerous processes can influence rates of age-related
change, with results apparently specific to the trait and population under study.
Date Issued
2011
Date Awarded
2012-02
Advisor
Coulson, Tim
Vaupel, Jim
Sponsor
Max Planck Institute for Demographic Research (MPIDR)
Creator
Rebke, Maren
Publisher Department
Biology
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)