Trophy hunting has long been used to control predator populations (e.g. cougars and wolves in the US). In most species, sport hunting can only threaten the future of a population when loss of males is so high that females can no longer find mates. However, in species where males provide essential paternal care, the removal of even a few individuals can have a severe impact on the entire population. In lions, replacement males systematically kill the cubs of any resident males that have been shot by sport hunters so excessive trophy hunting can have serious consequences for lion populations.
We used a highly detailed computer model, SimSimba, to measure the consequences of lion trophy hunting in a virtual lion population. Our simulations showed that trophy hunting could be sustained indefinitely if hunting were restricted to males over six years of age. This restriction maximizes the quantity and the quality of the long-term harvest. If trophy hunters kill too many young males (three to four years old), our simulations show that populations may be eliminated entirely, so it is critically important to restrict harvests to older animals. In further simulations, we found imposing and enforcing a minimum age of six years could also accommodate environmental disturbances and errors in age assessments.
In order for hunters to abide by an age minimum, they must be able to estimate a lion’s age. We have found that age can be estimated by the color of a lion’s nose. The triangular tip is pink in young lions, but becomes increasingly speckled with black pigment as lions age; we also published a lion-hunter’s guidebook that shows how to estimate age based on mane growth, tooth wear and coat coloration.
Most African countries now prohibit killing lions under six years old, although this has not been strictly enforced. After finding numerous examples of two-to-three-year-old males still being harvested throughout Africa, we analyzed data from various sources to determine if lions have been overhunted.
Looking at national statistics, we found that trophy hunting policies in Zimbabwe, Botswana and Namibia in the 1980s and 1990s caused significant declines in populations that limited subsequent trophy harvests. Countries with more conservative policies, such as Mozambique, did not experience similar declines. Botswana has since banned lion trophy hunting to conserve its lions, and Zambia and Zimbabwe imposed brief hunting bans in the early 2000s. We also found clear signs of overhunting at the ecosystem level and within individual hunting blocks in Tanzania. Our analysis suggested that lion harvests should not exceed one male per 1000 km2 in the Selous Game Reserve and one male per 2000 km2 in the rest of the country—assuming that trophy hunters continue to shoot males under six years of age. In response to our analysis, the Tanzanian government recently passed legislation limiting hunting to males over six years of age.