untitled

because its effect is specific to particular species and habitats. It must be examined

by way of carefully designed field experiments. Such experiments should follow

the effect of the treatment on the population’s dynamics rather than simply upon

reproduction, because compensatory changes to survival through density-dependent

effects may also occur and should always be expected. As mentioned above, two

rabbit populations in Australia were artificially sterilized to varying degrees. The reduced

recruitment of newborn was compensated by a density-dependent survival of the remain-

ing juveniles. Female sterility had to reach 80% before a decline in population was

observed (Twigg and Williams 1999; Twigg et al. 2000). Similar sterility experiments

on brush-tailed possums (Trichosurus vulpecula) in New Zealand showed population

declines with 50% and 80% sterile females provided immigration was prevented (Ramsey

2000).

Two additional methods of fertility control have been suggested: immunocontra-

ception and genetic engineering. These must await further research to demonstrate

their general applicability.

Immunocontraception

Antibodies can be raised in an individual against some protein or peptide involved

in reproduction, the antibodies hindering the reproductive process. Immunocon-

traception has been used to reduce reproductive rates, and hence mean densities, of

wild species that have become too numerous, such as elephants, horses, white-tailed

deer, fallow deer, and seals (Kirkpatrick and Rutberg 2001). One approach uses porcine

zona pellucida (PZP) protein antigens that raise antibodies to block sperm-binding

sites on the surface of the ovum in mammals (Kirkpatrick et al. 1997; McShea et al.

1997; Rudolph et al. 2000). This prevents fertilization. The protein must usually be

administered by injection or implant because most are broken down by digestion,

and the primary inoculation must be boosted 1–2 times in the following few

weeks. Generally the duration of these vaccines is less than 1 year, so animals must

362 Chapter 20

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

0 20 40 60 80 100

% sterilized

Mean litters / group

n = 5

n = 4

n = 3

n = 2

n = 1

Fig. 20.1Mean number
of litters produced
during a season of
births by a group of
females of size nsubject
to varying rates of
sterilization. One female
is dominant in each
group and the other
females subordinate to
her. Only the dominant
female breeds. She
relinquishes dominance
if sterilized, and the
subordinates are then
free to breed. (After
Caughley et al. 1992.)