Marsupials are mammals that are defined by birthing their young prematurely and nurturing their offspring for the remainder of their developmental stage in a pouch covering the nipples (The Editors of Encyclopaedia Britannica, 2018). The most notable marsupials that come to mind may be the kangaroo, wallaby, wombat, or koala, all of which are endemic to Australia’s mainland and its neighboring islands, as are the majority of the world’s marsupials (The Editors of Encyclopaedia Britannica, 2018). While these animals may be unique and adorable to some, many species have become so abundant in regions of Australia that are heavily populated and urbanized by humans that they are widely considered a pest species. For instance, as of 2016, the kangaroo population in Australia had surpassed 44 million, a value double that of the human population (Beech et al., 2018).
Of all of the kangaroo species an Australian passerby might observe on a daily basis, the eastern grey kangaroo, or Macropus giganteus, is the most common (Figure 1).
Eastern grey kangaroos are endemic to the south and east coasts of Australia – the same regions most heavily populated by humans (Wilson & Coulson, 2016). Overpopulation of kangaroos in urban settings increases the risk of human-animal interactions resulting in injury to one or both parties. The primary concern is for motorists colliding with the animals on highways and roads, but many residents are apprehensive about encountering kangaroos during low-impact activities such as nightly walks through their urban and suburban neighborhoods (Beech et al., 2018). To add to the anxiety, kangaroos cohabitate with one another and roam in large, interactive groups called mobs. That being said, their group behavior increases the probability that human encounters will involve more than one animal. Additionally, because kangaroos are grazing animals, they pose the risk of outcompeting livestock animals and other wildlife, as well as having negative effects on native vegetation (Descovich et al., 2016).
Until recent years, wildlife managers have relied solely on lethal methods to reduce urban and suburban kangaroo populations to more sustainable levels. While many marsupials are considered protected species throughout Australia, wildlife managers are legally responsible for culling, or slaughtering, a certain quota of kangaroos each year based on the various factors in their regions, (i.e. potential conflicts with competitive species or livestock, local human population density, whether the target kangaroo population after culling will be sustainable, conservation of symbiotic plant and animal species, etc.) (Descovich et al., 2016). With each annual culling animal welfare activists oppose the yearly practice with more intensity, despite the enforcement of various laws to ensure that kangaroos are being killed humanely, such as the fact that kangaroos must only be shot in the head during a cull (BBC, 2017). According to the Australian Department of the Environment and Energy, 1,632,098 kangaroos were culled in 2015 in New South Wales, Queensland, South Australia, and Western Australia (BBC, 2017). The ethical pushback of kangaroo culling as a means to manage populations had led to the development of a nonlethal, ultra-humane method: a birth control implant for female kangaroos. Before elaborating on how the implants act as contraceptives, however, it is imperative to understand the reproductive cycle of kangaroos.
Kangaroo mating and subsequent reproduction is highly dependent on narrow windows of opportunity afforded by very specific environmental conditions. Because much of Australia is so arid, males will only produce sperm and females will only be equipped to receive the sperm during periods of high precipitation that ensure the growth of enough vegetation to survive on (Burnie and Wilson, 2001). Favorable environmental conditions act as a signal to the female reproductive system to stimulate the beginning of the estrus cycle, or a period of sexual receptivity in which the female kangaroo can copulate with a male kangaroo (Swinbourne, 2017). The follicle housing the oocyte grows and secretes estrogen. Estrogen causes the walls of the uterus to thicken as cells proliferate rapidly. As the uterine lining grows, the various components of the vagina enlarge and conform to prepare for copulation (Norris & Carr, 2013). Within a week of the onset of estrus, a spike in luteinizing hormone causes an oocyte to be released from an ovary, after which it will travel to the fallopian tubes (Swinbourne, 2017).
When the conditions are right, male kangaroos will sniff the urine of receptive females for pheromones to detect whether the females are nearing ovulation, and the first male to make contact is typically the one to successfully conceive with the female (Ganslosser, 1995). Successful fertilization in the fallopian tubes leads to the implantation of the fertilized oocyte in the uterine lining (Hickford, 2010). The ruptured follicle that previously released the oocyte becomes the corpus luteum and, through progesterone secretion, initiates the luteal phase. During the luteal phase, substances are secreted by the lumen to provide nutrients to the growing embryo. This yolk sac is depleted just as quickly as the embryo develops and contributes to a hasty parturition.
Parturition, or birth, is stimulated by an increase in relaxin, a hormone that softens and relaxes the cervix, and a decrease in progesterone (Norris & Carr, 2013). Additionally, the prostaglandins mesotocin and oxytocin regulate the uterine contractions that deliver the premature fetus to the outside world (Renfree & Shaw, 1996). At just one month old, without any hind limbs or eyes, the fetus must venture to the mother’s pouch without any assistance and find a teat (Figure 2). The joey will finish developing in the pouch for the next eight to nine months, relying on two types of milk to advance its growth (Norris, 2013). Females nurturing a developing joey can immediately resume their estrus cycles and have the potential to be continuously pregnant. If environmental conditions become adverse and the mother must preserve her energy stores for the survival of herself and her developing joey, she can arrest gestation for up to 11 months by inhibiting the implantation of a growing embryo. This state of embryonic dormancy is known as diapause, and such conditions as low food availability, drought, or weaning a nearly full-term joey off of the mother’s milk act as signals to initiate diapause (Renfree, 1993).
The two contraceptive implants at the forefront of kangaroo fertility control are the deslorelin and levonorgestrel implants. Deslorelin is an agonist of gonadotropin-releasing hormone and inhibits the production and secretion of luteinizing hormone and follicle-stimulating hormone from the anterior pituitary gland. In this way, the follicle never develops to give way to the release of an oocyte (Herbert et al., 2010). The levonorgestrel implant prevents the surge of luteinizing hormone that typically stimulates ovulation (Herbert et al., 2010). A trial was conducted from 2003 to 2009, during which wildlife managers employed two separate measures to reduce fecundity in a population of eastern grey kangaroos that had free-range of a golf course in South East Queensland, Australia. In 2003, female kangaroos were tranquilized and implanted with the deslorelin contraceptive. In 2007, females were tranquilized and implanted with the deslorelin contraceptive and males were tranquilized and sterilized by vasectomy. The 2003 trial resulted in zero population growth, as did the 2007 trial that combined both female and male contraceptive treatment. The data represented in Figure 3 suggest that the deslorelin implant alone was able to reduce the population birth rate to nearly 0 from the time the trial was conducted until 2009, when the last census was taken (Tribe et al., 2014). This study supports the potential for the deslorelin implant to serve as a long-term contraceptive method for pest kangaroo populations in urban and suburban regions of Australia.
Levonorgestrel, the alternative contraceptive implant, has proven to effectively reduce fecundity in kangaroos for a longer period of time than deslorelin. Researchers captured 65 adult female kangaroos that had free-range over a golf course in Victoria, Australia and implanted a third of the group with levonorgestrel, a third with deslorelin, and the rest of the kangaroos served as the control group. Reproductive success was tracked over a period of eight years. The control group experienced the lowest reproductive failure rates, the group treated with deslorelin experienced reproductive failure rates nearly four times greater than the control group, and the group treated with levonorgestrel experienced reproductive failure rates 74 times greater than the control group. Furthermore, while the deslorelin appeared to lose its effectiveness in the population after only three years, the levonorgestrel treatment continued to be effective for five years after implantation (Wilson & Coulson, 2016).
Considering all contraceptive treatment trials that have been conducted to date, both deslorelin and levonorgestrel are effective, inexpensive, and easily distributed solutions to control the skyrocketing populations of kangaroos in Australia. That being said, kangaroos that are treated with the deslorelin implant will need persistent follow up more frequently than those treated with levonorgestrel. Either way, hormonal contraception offers a more humane and sustainable solution to the method that is currently employed, (i.e. culling over a million kangaroos annually).
