Why Manage Fertility?
Fertility control has been argued to be more ecologically sound, more humane, and more safe and cost effective than the use of mortality control. The shortcomings of mortality control for rodents with high breeding potential are particularly apparent in developing countries where highly toxic, often illegal, acute poisons are preferred, although such poisons have marginal impacts on reducing rat populations As described by Stenseth et al (2001), the use of mortality control results in increased reproductive compensation and survival within short-lived, fast breeding animals such as rodents; thus populations rapidly recover. Fertility control is ecologically superior because the adult animals occupy their ecological niche, limiting migration whilst being unable to expand populations in response to unusual increases in food supply. Fertility control is widely used in zoos for large mammals and has been growing in use for the management of large wild mammals such as deer (Europe), possums and pigs (New Zealand). It is also being increasingly used to manage feral pigeons, urban foxes and urban waterfowl. China has led the world in research on using fertility control for rodents due to the often severe problems Chinese farmers face with rodent outbreaks. And unlike many anticoagulant rodenticides or persistent synthetic oestrogen-like pollutants, the fertility limiting compounds used in China decompose very quickly. Many societies are increasingly intolerant of culling animals, even when those animals are considered pests or affecting the survival of wildlife. The use of fertility control is still new for many animal species, especially rodents, and the scientific advances made in this proposed project could have wide-spread implications for the control of many vertebrate pests around the world.
Fertility control of rodents would be a major technological advancement that could be used to manage rodent pest populations worldwide. Like locusts, armyworm and quelea birds, African rodent populations can undergo rapid population expansion. Although the drivers of such outbreaks are understood, preventing or managing such outbreaks often fails, or leads to large-scale environmental poisoning. The logic of fertility control becomes apparent in outbreak prevention because it slows down the exponential breeding potential of rodents over several generations but also keeps adult rodents alive, occupying their ecological niche rather than invading farm areas.
International Research on Fertility Control of Rodents
Researchers have been investigating the potential to manage rodent populations through different mechanisms that could limit reproduction for several decades. Although some research has looked at limiting male reproduction, more effective control can be achieved by limiting female reproduction or, ideally, limiting both sexes. Researchers have looked at different physiological processes as well as different delivery mechanisms. Some processes may cause permanent irreversible sterility whilst others may only have a temporary effect unless animals are repeatedly exposed.
In terms of delivery, there has been some research investigating the potential to genetically modify rodent species-specific pathogens to cause sterility. Although such work was successful in the lab through injecting the pathogen, the research was not able to work as effectively through natural transmission of the pathogen. And besides, such work raised considerable concerns about releasing genetically modified pathogens into the environment.
The most common form of delivery is by ingestion, where the fertility limiting agent is mixed into a food bait, much in the same way that poisons are usually delivered to kill rodents. However, there are some new promising ideas being trialled whereby rodents entering a bait station are sprayed with a compound that penetrates through their skin. Such a delivery system has some advantages in that it can deliver the exact dose required and that it could be selective in which species are sprayed and thus protect non-pest species.
Physiological processes to lower fertility or induce sterilityImmunocontraception works by introducing a virus or bacteria into an animal that contains specific antigens that bind to egg/sperm proteins or other organs/tissues involved in reproduction. This is similar to the principle of vaccination, but where the vaccine is targetting the reproductive system instead of a disease causing organism. Although such work has been promising in the lab, there remain considerable hurdles to releasing modified, attenuated organisms in to the wild where they may mix with other organisms.
Extracts from plants have led to some promising, naturally occurring compounds that can affect fertility. Some plants contain oestrogen-like compounds, and these can have fertility effects, particularly on males. Often the effect is not permanent and the animal recovers its normal level of fertility some time after eating the bait.
Synthetically derived chemosterilants have been shown to be very effective fertility limiting agents. These compounds can induce a kind of early menopause in animals by promoting a rapid depletion of eggs in the ovary. The results are permanent and can lead to total sterility of female and male rodents. Researchers continue to pursue their development.
The use of hormones is the physiological process being pursued in this project. Research on the use of hormones is well-researched in humans and other animals, and a variety of compounds are readily available. The advantage of using hormones is that there is a good understanding of their effects, their safety and environmental fate. The effect of hormones is reversible, and this could be perceived as an advantage or disadvantage depending on the context of useage. In the case of managing rodent population outbreaks, the delivery of hormones may be enough to regulate the population at key times to prevent outbreaks from occurring based on the timing and duration of baiting.