Some of the most recent work done by Packer's team has just been highlighted in a pretty nice write up by Mattt Walker in the BBC, representing a paper just coming out. The most interesting finding: Male lions kill (or attempt to kill) females from neighboring prides in order that their own pride obtains numerical superiority in pursuit of territorial competition.
In the study, authored by Anna Mosser and Craig Packer and coming out in Animal Behavior, the authors analyzed 38 years of data collected across 46 prides of lions in the Serengeti National Park, Tanzania. They looked for factors affecting female fitness. They found that competition between prides for territories directly affected female fitness. The largest prides obtained and held the best territories, and having neighboring prides negatively affected female fertility and individual female survival.
(a) Per capita adult female reproductive success versus total number of adult neighbours and (b) average adult female monthly mortality and wounding rates versus number of male neighbours (N = 115, non-edge prides only, numbers at the base of each bar denote sample sizes). [Figure 2 from the paper]
Interestingly, when prides had neighboring prides, females were found alone very rarely, suggesting a reaction to increased risk of inter-territorial fighting.
The level of inter-pride territorial behavior was inversely correlated with pride age. In other words, prides that had recently split into two, and were thus genetically more related through recent kinship, fought less.
An important aspect of territorial behavior that is parallel to that found in chimpanzees was also noted, regarding the role of males:
Overall, males were more important in group-territorial competition than expected, and female mortality and wounding rates were significantly associated with male neighbours, suggesting that males may use lethal aggression to tip the balance of power in favour of their prides.
Lions are odd because of all the living cats, only they are socially gregarious. The research in this paper suggests that simple numerical advantage aids in territorial competition and has shaped the evolution of lion social behavior.
I would add the following conjectures: It is unlikely to have more than one large territorial cat species common in a given region, and herds (of prey) make group living possible for lions (but not tigers).
This paper is a richly done project. As stated, years of research are involved, and the paper itself is a detailed statistical analysis of these data. It will become a classic for those reading in mammalian behavioral biology. My only complaint with it is that it was not published in an Open Access journal.
Mosser, A., & Packer, C. (2009). Group territoriality and the benefits of sociality in the African lion, Panthera leo Animal Behaviour DOI: 10.1016/j.anbehav.2009.04.024