When looking for a biological explanation for the Tsavo lion attacks, some questioned whether the unique group was actually a different species of big cat that lived during the Pleistocene epoch. For instance, the males lack signature manes and often grow larger than average lions. Another theory attributed the Tsavo lions' aggression to elevated levels of testosterone [source: Newbart].
But so far, these conjectures have come up empty. The Tsavo lion's maneless state is likely an adaption to the hotter weather since fuller, darker manes usually correlate to cooler, wetter climates [source: West and Packer]. This makes sense since the Tsavo region receives little rainfall and high average temperatures. As for their relative size, the lions in the area aren't abnormally large when taken as a whole, and research hasn't confirmed the possibility of increased testosterone [source: Newbart].
So where does the attraction to humans as prey come into play for the "man-eaters of Africa" and the other groups of lions that have ravaged human settlements in modern times? Instead of biological instinct, experts often pinpoint external conditions that influence this predatory behavior.
Overall, the population of lions in Africa is on the decline, hovering somewhere between 16,000 and 47,000 as of 2006 [source: Frank et al]. At the same time, the human population has consistently been on the rise -- along with lion attacks. Tanzania in particular has experienced a harsh upswing in lion confrontations, with at least 563 related deaths since 1990 [source: Kvinta].
Lions will assault humans for a number of reasons. Altered habitats can impact the wildlife, putting the lions in a pinch for feeding themselves. When that happens, lions must sometimes practice prey switching, or changing up their diets to accommodate for the loss of usual food sources [source: Patterson]. This temporary change happens naturally throughout lions' lives because of the migration patterns of their prey. However, when that usual prey runs out, the lions may switch to another food source, like humans.
To get an idea of the factors behind such a substitution, let's go back to the 1898 railroad episode. Much of the larger wildlife such as buffalo and zebra herds had been decimated due to an infestation of rinderpest disease. That eliminated a hunk of the Tsavo lions' food sources. When the railroad project moved into the region, so did piles of bodies of workers who died from exhaustion or poor working conditions. These bodies weren't properly buried, probably attracting lions to the easy meal that made up for the lack of game. After acquiring the taste for human flesh, and without additional prey, the lions began going after live humans.
Many times, humans may serve as easier dinners to acquire than other wildlife. Old or injured lions may capitalize on the ability to sneak up on people and dine on softer flesh (compared to what they would normally chew through). In 2004, for example, a man-eating lion was responsible for slaughtering 35 people over the course of 20 months in Tanzania [source: Dickinson]. The big cat's autopsy revealed a severe abscess in one of its teeth, which experts attribute to why it began feeding off of people.
Another intriguing facet of the man-eating habit is the potential for lions to pass it down to their cubs. One study of the Tsavo lions by scientists at Chicago's Field Museum discovered that generations of the same pride exhibited similar human-eating tendencies [source: Holden].
Today, lions have less space to roam because farmers have converted land into arable plots. Along with agriculture, people also raise livestock, which lions will target, drawing them closer to homes. This type of development has also negatively impacted the amount of available prey for lions, building up to the perfect storm in eastern African countries like Ethiopia, Tanzania and Mozambique [source: Frank et al]. While it isn't a natural practice, survival instinct can set in, and lions can become man-eaters out of necessity.
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