Fayrer-Hosken et al. reply

This number of subadult cows cannot be moved without disrupting the social order within their herds. Keeping them in their herds would mean that (assuming a mean herd size of 12.4, as shown in our study, and an average of 3 subadult females per herd) Kruger National Park would have to move 1,033 elephants — an unrealistic and expensive proposition. Hence the only practical way to remove 250 subadult females would be to cull them, which Pimm and van Aarde agree is an unacceptable solution.

We have shown that immunocontraception using porcine zona pellucida (pZP) works in the African elephant, although its long-term effectiveness in controlling populations is still being evaluated in South Africa. The cost and speed of field delivery have not been assessed for vaccinating large groups of elephants. However, contrary to the calculations of population modellers1,2, immunocontraception has worked in herds of wild horses and white-tailed deer3.

Preserving these magnificent creatures and their genetic contribution for the future is a common goal. On the basis of a single administration of a multiple-release pZP vaccine that is being developed for use in horses (I. K. M. Liu, personal communication), it should be possible to reduce the first three vaccinations used in our original study to a single dose and so minimize the stress, cost and labour of elephant immunocontraception.

We therefore question Pimm and van Aarde's criticism regarding the practicality of field immunocontraception for Kruger Park's elephant herds. It is our judgement that preserving these animals through immunocontraception is a realistic strategy that would save elephants without having to kill them.