Lion predation on elephants

I'm reading a bit about risk in large animal hunting, and I ran across an article by Dereck Joubert on elephant hunting by lions in Botswana.

Over the 4 years, we observed a total of 74 elephants killed by lions, including eleven elephants in 1993, seventeen in 1994, nineteen in 1995, and 27 in 1996, suggesting an increasing hunting success rate. All the elephants killed, with one exception, were from breeding herds (females and young). The exception was an adult bull, previously wounded by another bull, who remained alive for several days before eventually being killed by the lions. The great majority of the young elephants killed were males, and two-thirds of the kills were of elephants in the age range 4-15 years, with highest hunting success achieved for elephants aged 4-9 years (Table 1). The animals killed were commonly on the periphery of, or straggling behind, the breeding herds, with nearly half killed more than 50 m away from the main herd. Hunts were less commonly attempted on calves which were under the age of 4 years, which remained more closely associated with their mothers. Hunting success for elephants older than 4 years apparently doubled from 33% (n = 9) in 1993 to 62% (n = 61) in 1996. Many attempts to kill adults bulls were made in
1996, when we saw lions attacking elephant bulls almost nightly although only one hunt was successful. All except one of the kills were made at night, and hunts occurred more commonly on dark moon nights than when the moon was bright.

Well, hunting elephants ought to be pretty risky (otherwise, lions would do it all the time, right?). So how many lions got hurt during all these hunts?

There was a close resemblance between the methods that the lions used to hunt elephants and the technique commonly used to hunt buffalo. This tactic included first opportunistically detecting a straggler, or targeting a vulnerable member of the herd, then circling behind the selected prey. The lions then attacked by running in as a group. One or more lions leapt up onto the back or lower flanks and orientated along the spine of the prey. They then bit down on the backbone. The lion positioned highest up the spine would still be behind the ears of the elephant and just far enough back to be out of reach of the extended trunk. The elephant was then pulled down to its knees, not collapsed because of any fatal bite to the spine. Another approach involved a running hunt causing confusion and bunching of the elephant herds. This often resulted in one elephant falling or getting separated. In all cases a rear attack was employed, never a frontal attack. In one notable case, a single male lion ran at nearly full speed into the side of a 6-year-old male calf with sufficient force to collapse the elephant on its side. On only one occasion was a lion injured by an elephant in these hunts. In that case, the elephant collapsed on top of the lion. The resulting injury to the head was therefore recorded as accidental rather than as a result of a counterattack by the elephant.

OK, so the lions mostly limited their hunts to a class of most vulnerable elephants (subadults old enough to be isolated from their mothers, and inattentive to predators -- males amounted to 236 confirmed attempts versus 38 for females!). They adopted a special hunting style that they use for other dangerous large prey animals, attacking from the back by ambush. And during all these hunts (which totaled 74 kills out of 323 attempts) only one lion was confirmed injured. The paper doesn't say how serious the injury was, or if it
was eventually fatal, but elephant-falling-on-lion can't be a good situation.

Now, the relevant measure of risk in this instance is the injury rate (or even better, death rate) per successful kill. Unsuccessful attempts might fail for many reasons, including injury, but none of these unsuccessful attempts satisfy anybody's energetic requirements. So we have one serious injury per 74 kills. There may have been other injuries that weren't major enough to be observed or counted. Limiting to the one that was counted, we have a rate of serious injury of around 1.33 percent per kill; divided among the average number of lions that participated, which isn't specified.

From the elephant perspective, there appears to be a case for strategic indifference of adult males to predation on the younger males:

When these young elephants finished and called out to their families, the lions attacked. There was surprisingly little response from other nearby elephants. Older calves were attacked and killed within 50 m of the drinking bulls. The distress calls of the young elephant and lion growls seldom distracted them from drinking.

Tough to be a young male elephant.

References:

Joubert D. 2006. Hunting behaviour of lions (Panthera leo) on elephants (Loxodonta africana) in the Chobe National Park, Botswana. Afr J Ecol 44:279-281. DOI