9643

• UCREFRP Catalog Number: 9643
• Author: Seener, J. W.
• Title: Inbreeding Depression and the Survival of Zoo Populations
• USFW Year: n.d.
• USFW - Doc Type: 209-217
• Copyright Material: YES

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FIGURE 1. Essential features
ot a model of captivity-bred
populations.

SENNER/CHAPTER 12
INBREEDING DEPRESSION AND THE SURVIVAL OF ZOO POPULATIONS

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GENERATION

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deaths from genetic load occur before the animals are put into the pool of
potential breeders. The next generation of parents are selected without
regard to ancestry and the sex ratio is made as even as possible. The sex
ratio of newborns, however, is represented by a binomial distribution de-
pendent on the expected proportion of males, m, in a sample of newborns.
4. Numbers of offspring refers to offspring surviving to adulthood
which are fit for breeding. Animals dying before reproductive age-for
any reason-are not counted in this model. Animals which show any de-
tectable genetic abnormality are not counted as breeding adults although
they may be of value for other uses. (In actual practice there may be
some problem in deciding whether a phenotype is a rare but nonnal vari-
ant of the species or is a harmful defect.)
5. Both initial and maintenance population size are meant to be effec-
tive population sizes (Kimura and Ohta, 1971), particularly the inbreed-
ing effective population size.

the population size (line N) can no longer be maintained at its limit. The
proportion of male offspring (line m) increases (in mammals) because fe-
males are more susceptible to inbreeding depression. The probability of .....
survival (line S) is high for about 15 generations but drops sharply and
approaches zero by the 25th generation. Extinction occurs when there is
less than one surviving offspring of each sex in a generation. The
probability of extinction is therefore a function of the number of oft'~
spring in a generation (a function of the number of breeding females and
their fecundity), the survival of the offspring (a function of inbreeding)i
and the sex ratio of the surviving offspring (also a function ofinbreeding)~
Any model is a mix of simplifying assumptions and overly detailed.
analysis of parameters which reflect the particular biases and interesfs 0
the model's builder. Important simplifying assumptions made here are:
1. Nonoverlapping generations. Building a model with overlappin
generations is possible, but would require additional, though not enligb
ening, complications.
2. Simplified scheme of population size. Each population is found
with 1 animals which then multiply to beooQ1e M animals in the
generation. The population then stays at this maximum limit, M, U
inbreeding depression is so strong that replacement rates are not rea
and the population size drops.
3. Fecundity follows a random distribution. The number of offsp
produced by each female follows the Poisson distribution, as expec
the ideal case that all females are equally fecund (Cavalli-Sforza .
Bodmer, 1971-page 311), with the mean of the distribution equal ~
expected fecundity of a female. Inbreeding depression is translated ln
. lowered value for expected fecundity, equivalent to specifying that

The following cases illustrate the sensitivity of the model to each one
of the nine variables.

Fecundity

When reproductive rates are high there is an excess of offspring, thus
.. pennitting selection for vigor or other traits. When reproductive rates are
low every animal is needed for the next generation of breeders and no
. such selection is possible. Figure 2 shows the effect of varying the number
of viable offspring per female. The lines show probabilities of population
survival where female fecundity is three, five and seven offspring. The
model is quite sensitive to fecundity, suggesting that good husbandry and
environment conducive to breeding can have a dramatic impact on
'val of zoo populations. It is perhaps no coincidence that the most
SUccessful examples of captive propagation have been two species closely
related to common domestic animals: Przewalski's horse (Equus przewal-
~kii) and the wisent (Bison bonasus).

iability Depression

The number of viable offspring declines as the inbreeding coefficient
Creases. This decline, known as inbreeding depression, can be divided
..~ three separate effects: viability depression, fecundity depression and
ratio depression. The first of these, viability depression, is the failure

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