In areas where there is perceived or actual conflict between humans and macaques, or macaques are considered to be ecologically damaging or invasive, various methods of population control are sometimes employed to either reduce or eliminate their presence. In captive settings, like rescue centers, it is important to prevent new births as space and opportunities are limited. Unless the species is part of a viable conservation breeding programme and there are genuine prospects for eventual release, population control is also necessary in zoos.
Before any measures are taken, it is vital to fully understand the nature of the problem. Conflict between humans and macaques can be very real, with serious negative effects for both species, and macaque populations can grow unsustainably large when they are fed (intentionally or not) by us. Yet conversations with multiple practitioners and researchers indicate that perceived “overpopulation” can be misleading, and that reports of “conflict” can range from the serious (attacks, destruction) to harmless macaque sightings in areas that they regularly inhabit. Reliable data on the size and ecology of the local population, the nature of the conflict, and local views on the matter must be obtained before population control is seriously considered.
Contraception (birth control) programmes usually involve sterilization, in which targeted individuals are rendered permanently infertile. Depending on the desired outcome, those running the programme may aim to sterilize all individuals (where the aim is to remove the entire population) or a percentage or specific subset of the population (where the aim is to reduce population size or growth). Where the aim is to control rather than eradicate a population, a successful programme must be ongoing, with new treatments carried out on a regular basis.
In certain contexts, impermanent methods may be preferred - for example in captive settings where the macaques have good prospects for release into sparsely populated, ecologically suitable areas.
Any form of contraception will have an effect on the target group’s social structure; birth, parenting and (particularly female) kinship are central components of macaque social life and eliminating these from the lives of individuals or the entire group could negatively affect their welfare. In captivity or in the presence of conflict, however, a contraception programme may be amongst the least harmful options and produce the kindest outcomes.
A 2018 review of wildlife population control methods lists the desirable attributes of a contraceptive programme for wildlife species (paraphrased here): No health risk for the individual; little impact on secondary sexual characteristics or behavior of the individual; little impact on the overall social group behavior; 100% reversibility; long-term anti-fertility effects; applicable in males or females; easy administration; no refresher or booster needed; economically feasible; does not enter the food chain; not a major pollutant.
The most common contraceptive measures for macaques are briefly described below.
Surgical methods are probably the most effective, but can be challenging to carry out on a large scale.
In male macaques, castration is thought to be the most effective permanent form of birth control, but disrupts hormonal and sexual function and social mechanisms, hinders development and may increase the frequency with which the castrated individual is a target of aggression from group mates. Increasingly, tubal ligation (vasectomy) is preferred. The procedure is more complex and invasive, and may be less reliable. Nevertheless, it is the preferred method for many population control initiatives because it is ultimately less disruptive than castration and veterinary advances mean that it is becoming simpler and safer.
Tubal ligation can be performed in females, or contraceptives (e.g. Implanon) can be surgically implanted. These can be effective in some species, but their efficacy can vary dramatically between macaque species. Like other hormonal contraceptives, implants may be associated with behavior changes, including increased aggression.
Chemical methods Some believe that immunocontraception may be the most appropriate method for controlling wildlife populations in general, but this has not been trialled widely in wild macaques and some earlier researchers expressed concerns about the ecological impacts of such methods. As contraceptive technology advances, immunocontraception, which can be used both males and females may be a technique worth exploring. A second trial is planned in Hong Kong as of 2020.
Chemical “castration” has been explored as a non-invasive option for male macaques. Some projects have reported success, but is thought by some not to be sufficiently effective.
In captive situations, oral contraceptives can be effective in some female primates, but this could vary dramatically between species. Requires daily administration, so not feasible for free-ranging primates.
Successful sterilization projects A team led by Paolo Martelli of Ocean Park Hong Kong has been carrying out an ongoing sterilization programme for over a decade, which initially focused on the sterilization of female macaques in the city. As veterinary technology has progressed,the focus has changed, and endoscopic vasectomies are performed on males. Health checks and rabies vaccinations are administered at the same time. In this case, the macaque population in the area is well-understood and constantly monitored, and individuals are known to those carrying out procedures.
Culling is another word for killing individuals as a means of controlling a “problematic” population. Culling is used as a management strategy for macaque populations in some areas, and can range from the unrestricted killing of random individuals to the planned, systematic eradication of entire social groups. There are often cultural, ethical or ecological barriers to the use of culling as a management strategy.
Even in cases where the local population regards macaques as “pests”, they may not want the monkeys to be killed. Macaques can be religiously, or otherwise culturally important, but beyond that, people sometimes just like them, or have an ethical aversion to killing.
When all is considered, it is difficult to carry out a humane cull of a highly social, group-living animal species. There is little published information available about the most commonly used culling methods for macaques, but shooting (accompanied by trapping, or not) is often reported. It is questionable whether shooting is a humane way to kill an animal, but beyond that, the random removal of individuals from an established social group is a welfare issue for those monkeys that survive.
Macaques may not be as abundant as they seem (see Public Perception). Up-to-date population data are unavailable for many of the species assumed to be abundant. This means that the “culling” of any of these species could be problematic from a conservation standpoint.
More philosophically, it may be difficult to ethically justify the killing individual beings (or the eradication of entire populations) because they cause us inconvenience. Explore animal ethics in greater details here.
Population control programmes can be involved, expensive, and can have negative welfare effects on the individual macaques involved. They often require long-term commitment to be effective. If possible, less manipulative solutions like feeding bans, buffer crops, monkey-proofing, and/or education and awareness should be trialled first, and these should always be run alongside any population control programme.