However, when the committee’s report was prepared, there were no data on resistance to immunocontraception, the heritability of such resistance, or the identity of specific genes that might affect responses to immunocontraceptives. National Park Service staff reported on Assateague Island that there were no indications that resistance was developing or that responses to immunocontraception were changing over time, after 19 years of herd management with PZP. Contraceptive effectiveness continues to be high (A. Turner, Assateague Island National Seashore, email communication, February 24, 2013). The immune response to immunocontraceptives depends on many nongenetic factors, such as nutritional status (Homsy et al., 1986; Chandra and Amorin, 1992; Chandra, 1996; Demas et al., 2003; Houston et al., 2007), and it was not possible for the committee to determine whether resistance to immunocontraception could develop. Similarly, it was not clear whether immunocontraception could inadvertently select for less immune-robust animals because they would not mount a strong response to PZP and would thus remain fertile. Presumably, any genetic background that would predispose animals to being immunocompromised would be under strong selection to be eliminated; even in a small population in which a deleterious mutation that compromised the immune system could become fixed, selection could act against individual animals that have the mutation, although the pressure of selection is smaller in small populations. In addition, Falconer’s (1965) equations apply to threshold or “all-or-none” characters whereas lifetime reproductive success—which contraception affects—is a continuous variable that is not subject to some threshold, so it is not clear whether the Falconer model applies, although other models might. Cooper and Larsen (2006) suggested that immunocontraception could be appropriate for management of species that have long generation times, like horses, because genetic changes (if any) due to immunocontraception would take decades to develop. That would also assume that large numbers of individual animals are contracepted indefinitely and never allowed to breed; this does not seem likely if populations are managed for genetic diversity. However, those concerns highlight the importance of monitoring genetic diversity in immunocontracepted populations (see Chapter 5).
At the population level, removing females even temporarily from the breeding pool is likely to reduce the effective population size (Ne) and genetic diversity of the population. As will be discussed in Chapter 5, reducing the number of breeders or increasing the variance in family size, which will occur as more females bear no young, will reduce Ne and increase the loss of genetic variability. (Tables 5-2 and 5-3 show that some populations display low levels of heterozygosity.)
Side Effects: Behavioral. There are two important considerations in evaluating the literature on contraceptive effects on particular aspects of behavior, particularly bonds between animals and stability of social groups. First, in no published study of immunocontraception have treatment and control groups been matched or balanced with respect to other variables that might affect behavior (such as age, dominance rank, tenure in the group, group size, social or reproductive history, and characteristics of other group members). Rather, investigators have had no control over those variables and thus only compared treated with untreated (or not currently treated) females. Studies in which those factors could be controlled or specifically have their effects measured would require large samples of animals of known history and would be virtually impossible to conduct in the field or even in captivity. Second, no study has been able to differentiate the behavioral effects of a contraceptive compound administered to an animal and the resulting absence of offspring. Thus, in no case can the committee conclude from the published research that the behavioral differences observed are due to a particular compound rather than to the fact that treated animals