Influence of drought on predation of elephant (Loxodonta africana) calves by lions (Panthera leo) in an African wooded savannah

1. What they did

Adult elephants are too big and too well defended for most predators, so lions rarely kill them. The researchers wanted to know whether drought changes this in Hwange National Park, Zimbabwe, where the elephant population is very large and many elephants crowd around water points in the dry season. They expected that in dry years, hunger and thirst would make calves more likely to get lost or fall behind their family groups, and so become easier prey.

From 1998 to 2004 they recorded every lion kill they came across while studying 18 prides and 14 male coalitions over about 7000 km². They did not search for kills systematically. Instead they followed up all signs of lion activity: vultures gathering, lions, hyaenas or other scavengers, visible carcasses, or the smell of a carcass. A kill was counted when the hunt was seen (by the team or by reliable observers such as park staff, safari guides or researchers), or when signs on the carcass and around it (spoor, kill signs, droppings) showed that lions had made the kill.

For elephants, they also:

• Separated true lion kills from carcasses that lions had only scavenged, using the pattern of bite and claw marks and the signs of a struggle.
• Collected the lower jaw of every elephant killed by lions and aged it from the teeth (the foramen mentale method).
• Recorded the date, place and, where possible, the lions present.
• Compared the number of elephant kills each year with the rainfall of the wet season before.

To check for bias, they compared the kills they saw happen with the kills they found later.

2. What they found

Lion diet

• 203 lion kills were recorded, 50 of them seen as hunts. The team also saw 83 hunts that failed.
• Buffalo were the most common prey (32% of kills), then elephants (22%), wildebeest (11%), giraffe (9%) and zebra (7%). These five made up 83% of kills.
• In the kills that were seen, buffalo gave 49% of the meat (biomass), elephants 16% and wildebeest 15%.
• The mix of species did not differ statistically between seen and found kills. Even so, large prey (over 400 kg) were more common among found kills, and wildebeest, warthog and impala among seen kills, so the authors note a possible bias.

Elephant kills

• 44 elephants were killed by lions, 8 of them seen. All were dependent young that could not survive without their family group.
• Their average age was 2.4 years (from newborn to 7.5 years). The most common age was under 1 year, and 88% were under 4 years old.
• Seven of the eight kills that were seen were of lone calves that had lost contact with their family. Only one was killed with adults nearby.
• Most elephant kills were in the dry season, mainly in October at the end of it. None were found from December to March, the main rains.
• More elephants were killed in years after a wet season with below-average rainfall, and few or none after good rains. After two poor rainy seasons in a row, the 2003 dry season gave 23 elephant kills, over half the total. No elephant kills were found in 2001 or 2004.
• Kills clustered where elephants crowded at water in the dry season: Nehimba Seeps (15), Shumba Pans (8) and Somavundla Pans (6).

How lions killed elephant calves

• Single lions (always males) killed elephants three times; the oldest calf killed by a single lion was 3 years old.
• Groups of females at elephant kills averaged 7 adults, larger than the park’s usual pride of 2.5 adult females.
• Lion kills had bite marks on the throat, trunk, ears and back and deep claw marks on the back, flanks and ears. Lions feeding on a dead elephant left marks mainly on soft parts (groin, belly, face, trunk tip, around the anus). Real kills usually also had flattened vegetation and drag marks nearby.
• Twice lions were known to attack a family group to take young. Once, the team pieced together what happened from tracks in soft, wet ground the morning after the hunt: seven lions ambushed a small family, a young elephant escaped after dragging a male lion about 10 m, and a calf under a month old was killed. In another hunt a lioness jumped on a 5 to 6 year old that had lagged behind, but two adult females came back and defended it, and it escaped.
• In October 2003 the team found 30 fresh elephant carcasses within 500 m of Nehimba Seeps. Lions had killed some, but many seemed to have died of thirst and hunger.

3. What they concluded

• Lions in Hwange mostly eat large and medium-sized prey, like lions in many other studies.
• It is unusual for elephants to be a regular part of lion diet. In Hwange it happens because of drought.
• In dry years, elephants crowd around a few water points, eat out the food nearby, and must walk long distances between water and food. At the seeps, large bulls dominate the wells the elephants dig, and fights are common, so calves and young elephants often cannot drink before the family moves on, or are left behind waiting. Calves that are separated or cannot keep up are very vulnerable to lions.
• Without watching elephant herds, the authors could not tell whether lone calves had been abandoned or simply got lost.
• Lion prides at elephant kills may be large because calves are easy to find in one area during dry years, or because a calf takes several lions to kill (lions cannot suffocate an elephant by holding its nose or throat).
• High levels of calf predation in dry years may be a sign that the Hwange elephant population is close to the limit its habitat can support (density dependence).

4. How tracking was or was not used

Tracks and signs were used to find kills and to decide what killed the animal. The team followed up signs of lion activity and decided whether lions had made a kill from signs on the carcass and nearby, such as spoor, droppings and kill signs. For elephants they told kills from scavenged or starved carcasses by the pattern of bites and claw marks and by signs of a struggle such as flattened vegetation and drag marks. In one case they reconstructed a whole hunt from tracks in soft, wet ground the morning after. The team was radio tracking that group of lions when they found those tracks.

5. Tracker or observer certification

Not reported. Who read the signs is not described, apart from the authors themselves, and no tracking qualification is given. Kill identification also relied on “reliable observers” (park staff, safari guides and other researchers), whose skills are not described. Many of the found kills, including most elephant kills, depend on this sign interpretation.

6. Other technologies, skills or innovations

• Ageing elephants from the lower jaw using the foramen mentale method, which relates the wear of the molar teeth to a small hole on the side of the jaw.
• Radio tracking of lion groups.
• Biomass estimates from published adult weights, and from a growth curve for young elephants.

7. Implications for conservation (from the paper)

• Very high elephant numbers and dependence on water lead to crowding near water points in Hwange and other parks, using up food nearby and causing elephant deaths in dry years.
• Artificial water points let water-dependent animals use areas that would otherwise be too dry, and elephants crowd around them in dry years.
• High predation on young elephants in dry years may show that the population is close to density dependence.