By Melissa Wray In Kruger National ParkToday, there are 35 elephants roaming Thornybush's 11,500ha. Thornybush is located to the west of Timbavati Private Nature Reserve, in greater Kruger. This August, 22 elephant cows of reproductive age were darted with their final shot of a vaccine that will induce temporary sterility without affecting their sex hormones.The drug, known as PZP (porcine zona pellucida), contains proteins produced by female pigs. The vaccine successfully stops fertilisation of eggs produced by elephant cows by preventing sperm from binding to and penetrating the egg. The elephant creates an antibody to the pig proteins, and this antibody is what stops sperm from penetrating the egg.Thornybush Nature Reserve started the immuno-contraception programme this year as part of its elephant management plan. According to studies carried out by provincial authorities and the ecologists that have been monitoring the veld conditions in the reserve, the reserve can support about 26 elephants. As there are several lodges in the reserve, veld condition is optimised for eco-tourism.Thornybush's elephant population was projected to reach 57 animals by 2010, making immuno-contraception an attractive option. Although having recently purchased more land, the reserve is unable to expand further. Warden of central and northern Thornybush, Mike Pieterse, contacted Audrey Delsink for advice.Delsink has been heading up Makalali Game Reserve's immuno-contraception programme for the last five years, the longest running project of its kind. Together they identified 22 cows to receive the PZP vaccine, and the cows were all darted from a helicopter. Each cow has to initially receive three doses of the drug, administered about a month apart.The darting process initially took an hour, but by the third injection flying time was reduced to 32 minutes. A booster shot is required every year to ensure the contraception is effective. Pregnant animals are not affected by the contraceptive, and so cows that are currently pregnant will give birth in due course. In order to check the correct animals are darted, elephant identikits are drawn up.With the contraception programme now underway, Thornybush has hired Melodie Bates to do behavioural studies on the elephants. For the next year at least, she will spend time with the elephants, performing intensive observations to determine all the cow-calf relationships and social hierarchy. In two years, the herd will have a zero growth rate as all pregnant females will have given birth, and the remaining females will be contracepted.At this point, Thornybush will reassess the herd in light of the behavioural monitoring, and split the population in the least socially disruptive manner. This will also give them a chance to find a new home for the excess animals. Pieterse says, "The process is expensive. It works out at between R1,000-R1,500 per elephant per session." This includes helicopter time, veterinarian fees, the dart and the vaccine, and the time of experts such as Delsink and Dr Henk Bertschinger, who creates the vaccine.The salary, accommodation and transport for a full-time elephant watcher are further expenses. Small reserves where individual elephants can be identified and located for three months in a row are better suited to immunocontraception as an elephant management tool.