Man-eating lions and their decision-making

Should one dismiss the past as something of no significant relevance? Well, Justin Yeakel, of the University of California Santa Cruz, and collaborators (from Field Museum of Natural History in Chicago, University of Puget Sound, University of Cambridge, and University of Utah) seems to think otherwise as exemplified by their paper, ‘Cooperation and individuality among man-eating lions’, in the Proceedings of the National Academy of Sciences.

Historical Background
Let’s go back to March 1898 when the British were building a railway bridge across the Tsavo River in southern Kenya. Unfortunately, their progress was hampered by a partnership of two adult male lions, which launched nightly attacks on the rail workers’ camps. This killing spree lasted for nine months, until they were killed by Colonel John Patterson.

The final death toll has always been ambiguous. The first estimate was 28 victims, i.e. the 28 Indian workers who were killed by the lions. But in 1920, Patterson himself revised the death toll to 135, supposedly after factoring in the natives (estimates of which range from zero to 107).

Yeakel’s methodology
To confirm the final death toll, Yeakel used the premise that the Tsavo lions’ diet can be deciphered by analysing the isotopic ratios of nitrogen and carbon in their hair and bone samples (obtained from the Field Museum in Chicago), for it would reflect the isotopic ratios of their prey (i.e. grazing and browsing animals and humans, such as the Taita people who lived in the Tsavo area). More specifically, dietary inputs of the last 2-3 months of the Tsavo lions could be deduced by analysing the hair keratin from the rapidly regenerating tuft hairs of the tail, whilst the lifetime average could be deciphered by analysing the bone collagen. Once this was accomplished, Yeakel modelled the prey combinations which were most likely to produce these distinct isotope ratios.

So, how many?
The final modest estimate was that the lions ate around 35 people. It is likely that the number of humans killed might be greater than the number of humans eaten, for there might have been cases when the lions couldn’t escape with their prey or when the bodies were recovered before the lions could properly sink in its teeth. Perhaps some may have even succumbed to their injuries later.

The Lions’ platter
The results also revealed that, for most of their lives, these lions ate grazing animals- until March 1898. Although they hunted cooperatively (despite hunting humans not really requiring cooperative hunting for they are significantly less hassle than the larger ungulates), there was a disparity in their diets. One lion ate more grazers and some occasional humans (around 11, i.e. approximately 13% of its food intake), whilst the other ate both grazers and humans (around 24, i.e. approximately 30% of its diet). It is quite likely that the latter’s substantial preference for humans had to do with its severe dental problems and jaw injury which may have impeded its ability to hunt.

Why humans?
So why did these lions widen their dietary preferences to include humans?
One possibility was the existing scarcity of the habitual prey, which may have been the result of
i. the Tsavo region experiencing drought in 1898,
ii. unhindered hunting of the lions’ usual prey,
iii. the rinderpest virus (from Europe) which had killed most of the lions’ conventional prey.

Baldus (2006) reports a similar (and fairly recent) case from Tanzania. His previous study (2004) estimated that lions are responsible for one third of the 200-odd humans killed in Tanzania each year by animals (Packer et al states that, between 1990 and 2004, 563 humans were killed and 308 injured by man-eating lions). Nonetheless, from August 2002 to April 2004, a young adult male lion single-handedly killed 35 humans and injured at least 10 in Mkongo Ward, south west of Dar es Salaam. These took place on a thin stretch of agricultural land, along the southern bank of the Rufiji river and enclosed on its western and southern sides by the Selous Game Reserve. Significantly, a human population of approximately 13,000 was residing near the game reserve. But more importantly, the Mkongo lion shared a striking similarity with the dominant Tsavo man-eater: he had a broken upper left molar with a serious abscess. It is likely that, because of the permanent pain, he would have preferred the humans.

Yet, Baldus stresses that most man-eating lions in Tanzania are healthy, with no signs of infirmities. So what transforms the grazer and browser-preferring lion into a human-preferring lion? Fall in the habitual prey densities.

Carnivore-human conflict
Carnivores are forced into conflict with humans when their habitats and habitual prey densities decline (usually due to human related activities such as the creation/expansion/encroachment of agricultural and/or settlement lands, virulent ungulate poaching/hunting) (Hackel 1999; Schiess-Meier et al, 2007). This carnivore-human conflict has resulted in a decline in carnivore population. In 2005, there were only approximately 5,750 Lycaon pictus a.k.a African wild dogs (Swarner, 2004; Lindsey et al, 2005). Cheetah (Acinonyx jubatus) population has fallen from an estimated 30,000 in 1975, to less than 15,000 in the 1990s, the famed Serengeti National Park in Tanzania having just around 200-250 cheetahs (Kelly 2001).

Thus, very likely, it was the change in environmental conditions which changed the dietary specialisations of these lions. What does this bode for the rapidly expanding human civilisation in Africa and Asia? And more importantly, what does this bode for the wildlife population and their natural habitats?

Yeakel, J., Patterson, B., Fox-Dobbs, K., Okumura, M., Cerling, T., Moore, J., Koch, P., & Dominy, N. (2009). From the Cover: Cooperation and individuality among man-eating lions Proceedings of the National Academy of Sciences, 106 (45), 19040-19043 DOI: 10.1073/pnas.0905309106

Baldus, R. (2005). A man-eating lion (Panthera leo) from Tanzania with a toothache European Journal of Wildlife Research, 52 (1), 59-62 DOI: 10.1007/s10344-005-0008-0

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Hackel, J. (1999). Community Conservation and the Future of Africa's Wildlife Conservation Biology, 13 (4), 726-734 DOI: 10.1046/j.1523-1739.1999.98210.x

Kelly, M. (2001). Lineage Loss in Serengeti Cheetahs: Consequences of High Reproductive Variance and Heritability of Fitness on Effective Population Size Conservation Biology, 15 (1), 137-147 DOI: 10.1111/j.1523-1739.2001.99033.x

Baldus, R. 2004. Lion Conservation in Tanzania Leads to Serious Human-Lion Conflicts. With a Case Study of a Man-Eating Lion killing 35 People. Tanzania Wildlife Discussions Paper No. 41, GTZ Wildlife Programme in Tanzania, Wildlife Division, Dar Es Salaam.

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Packer, C., Ikanda, D., Kissui, B., & Kushnir, H. (2005). Conservation biology: Lion attacks on humans in Tanzania Nature, 436 (7053), 927-928 DOI: 10.1038/436927a

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