MALE MOVEMENTS, TERRITORIES, AND LION MORTALITY IN THE OKAVANGO REGION, 1996 - 2001

Pieter W. Kat, PhD, Okavango Lion Research Project

Introduction

Over the past five years, five prides of lions have been studied with varying intensity in the southeastern part of the Moremi Game Reserve and neighbouring concession areas. Lions were located over 2,200 times during this period, and the study has focused on reproduction, disease, ecology, and genetics. This paper will focus on several aspects of lion ecology including males and male movements, fluctuations in territory size and occupation, and rates and causes of mortality among males, females and cubs.

Male lions

Groups of adult male lions are known as coalitions. Coalitions are either formed from a cohort of young males born into a pride, and/or from associations of unrelated young males formed during their nomadic phase. Average male coalition size in the study area is 2.3 (range 2-4, 10 coalitions present among the five study prides over the past five years), and the average length of tenure (the time coalition males spend with a pride of females) was 20.6 months (range 7-28mo; Fig. 1). Causes for loss of tenure include death of a coalition partner (in coalitions of two males), eviction of the coalition by incoming males, and pride abandonment. During the study, four coalitions have been reduced to a single male by the death of a coalition partner, two coalitions have been evicted by incoming males, and three coalitions have abandoned former pride territories to take over new prides of females; in every case such pride abandonment involved a relatively short-distance move to a neighbouring territory. In all three cases of pride abandonment, males that moved to new groups of females did so when all available reproductive females in the former prides had cubs ranging in ages between six and 18 months, and all males initially tried to include both the new and former prides within their scope of movements. On average, pride males began exploring neighbouring territories 14 months after becoming established, and opportunistically mated with neighbouring pride females and/or nomadic females throughout their tenure.

Figure 1: Tenure length in months of nine coalitions present among three prides in the study area over the past five years. The shortest tenure was not used to calculate average tenure length.

The Santawani pride, numbering between four and 10 reproductive females between 1996 and 2001, has had a particularly high rate of turnover and attempted establishment of male coalitions (Table 1). During the past five years, six male coalitions have entered the area, and have attempted to establish themselves with varying rates of success. Average tenure length for five of these coalitions (the sixth coalition is currently in residence) is 14.4mo (range 3-24mo). Such high rates of male turnover have significantly negative effects on cub survival: of 53 cubs emerging from dens (actual numbers born are higher but impossible to establish accurately), only 15 survive to date, of which 7 are aged less than six months (survival rate of 28%). In comparison, two neighbouring prides have had coalitions of males with average tenure lengths of 27mo and 25mo, and of 61 cubs emerging from dens in those two prides, 31 have survived for an average survival rate of 51% (51.4% Mogogelo pride, 50% Gomoti pride). As a general observation, cub survival is significantly higher among prides that have coalitions whose tenure exceeds 25 months; this is consistent with observations made in the Serengeti. Longer tenures also compensate for the consistent observation that females experience a long delay (average of 264 days for 11 females in 3 prides) before the birth of their next litter. High rates of male turnover and consequent low reproductive output are likely in areas where factors such as trophy hunting contribute to additive male mortality.

Table 1: Coalitions and tenure length of males in the Santawani area, 1996-2001.

Among the Santawani males that established tenure and have since been evicted, two coalitions of two males lost partners (one male in the first coalition was killed by other lions, one male in the second coalition was shot as a trophy), and one coalition of two males was evicted intact. Both surviving males of the two coalitions that lost males are alive today (18mo and 17mo after they lost their coalition partners), and continued initially to range through portions of their original pride territory (Fig. 2). Both these singleton males have mated with females of their original pride and females from neighbouring prides, and initially continued to range through parts of their original pride territory. Both males have now moved to neighbouring pride territories, and have tended to concentrate their activities in areas of high prey density. One of the two males has made associations with an adult male and a subadult male, but these associations did not result in the formation of a new coalition. The males in the evicted coalition have also remained in the area, but despite the absence of males
Figure 2: Representative GPS locations showing movements of a coalition of two males that entered the Santawani area in 1998. These males established themselves in the northeastern part of the Santawani territory, but the surviving male moved across three pride territories after his coalition partner was shot.

in the Mogogelo pride area, have not established themselves with these females (Fig. 3).

Figure 3: Movements as indicated by representative GPS locations of a coalition of two males that entered the Santawani territory in April 2000. The coalition was evicted in December 2000, and has moved across three pride territories.

In addition to their significant impact on rates of cub survival, male lions can also have important effects on patterns of pride territory occupation by females. Male influence on female movements falls into two categories: effects on females raising dependent cubs resulting from pregnancies from other males (including previous pride males), and more extensive effects on all females in the pride. The first category includes females with dependent cubs of all ages present when new males enter the pride territory. The presence of these new males might be temporary or result in the eventual takeover of the pride, and females with dependent cubs generally attempt to avoid encountering such new males by moving to peripheral areas of their pride territory (Fig. 4). As a general observation, dependent cubs less than 12 months old have a low probability of surviving pride takeovers by new males, but females with cubs older than 18mo have a greater probability of raising such cubs to age of dispersal. The second category of male influence on female territory occupation includes all females in the pride, regardless of the presence of dependent cubs. In this situation, new males do not necessarily attempt a pride takeover, but establish themselves at the edge of a pride territory. The net result of such new male presence is to significantly affect the movement patterns of females; in the case of the Santawani pride, all adult females stayed largely within their wet season pride range (Fig. 5). Prey availiability in this part of the territory is low during the dry season, and cub survival was low. It is interesting to note that a similar response in terms of shifts in pride territory usage has been noted for females of other carnivore species. In the case of bears, for example, female reproductive output suffered as a direct consequence of being relegated to habitats with low food availability by immigrant males (Wielgus & Bunnell, 2000; Swenson et al., 1997).

Figure 4: Movements as indicated by representative GPS locations of two female lions (green and red) and a pride male (yellow) The female represented by the green diamonds was raising cubs not belonging to the new coalition, and clearly avoided areas frequented by the new pride males as well as pride females without cubs at the time of takeover.

Figure 5: Movements of pride females away from areas occupied by incoming males. In 1997, Santawani females largely ranged over the northwestern part of their territory in the dry season, but by 1999, territory occupancy had shifted to the southeast in response to the presence of a new coalition of males (red triangles).

Pride territories

Previous studies on lion territories have indicated that, with minor exceptions, patterns of territory occupation and patterns of territory use by resident lions is stable. In our study area, we have observed a variety of different patterns, ranging from stable occupation over the past five years, to changes in patterns of territory usage as discussed above, to complete replacement of resident prides by incoming prides. Stable occupation of a pride territory has been exhibited by the Mogogelo pride, that has occupied an area around the Mogogelo river during the dry season and a larger area including aprts of the Santawani and Gomoti prides in the wet season (Fig. 6). Seasonal movements of lions in this pride have been predictable and relatively constant over the years, and survivorship of pride females and cubs has been high. In contrast, the Gomoti pride was completely ousted from its pride territory in August 2000 by the River pride (Fig. 7). The River pride, comprising two adult males, six adult females, and seven subadult offspring moved in from the Chitabe area in response to the sudden high prey density along the Gomoti River, which started flowing following high rainfall during the 1999/2000 wet season. There were almost no aggressive interactions with the Gomoti pride, as the females were still largely within their wet season range close to the Veterinary Cordon fence, and the Gomoti pride males had by this time abandoned their pride to take over the Santawani pride. The net result of this takeover of a pride territory has been that the Gomoti females

Figure 6: Stable occupancy of pride territory exhibited by the Mogogelo pride during wet and dry seasons, 1996-2001.

have been forced through the Cordon fence, with the result that several females were poisoned by livestock owners. Because both radiocollared females in the Gomoti pride have been killed, it is uncertain at this point whether any females remain alive.

Such changes in territory occupation patterns can be expected in the Okavango area because of the highly variable nature of environmental parameters associated with unpredictable changes in quantities of rain and extent of floods in the Delta.

Figure 7: Complete takeover of the Gomoti pride territory by the River pride: Yellow squares indicate representative GPS positions of the Gomoti pride between 1996 and 1999/2000, and red squares show present range of the River pride.

Lion mortality

Rates and causes of mortality among lions in three study prides have shown that mortality is highest among adult males (7 currently alive of 21 known males in the study area, mortality rate = 67%, annual mortality = 13%), followed by cubs (38 reached 2 yrs old, 8 cubs currently < 6mo of 114 cubs emerging from dens, mortality rate = 60%), followed by adult females (14 currently alive of 27 total = 48%, annual mortality = 10%). Causes of mortality among adult males and females also differ; fights with other males are the most important contributory factor to known mortality of males, and problem animal control and disease contributing most significantly to mortality among females (Tables 2 and 3). Most cubs died of unknown causes, although the greatest known contributing factor to cub mortality (38%) is associated with attempted or actual pride takeovers by incoming males. As a general observation, primiparous mothers have very low rates of cub survival especially if these cubs are born out of synchrony with other cubs. Cub mortality rates differed among the study prides: while the Santawani pride had more cubs emerge from dens than other prides, a greater percentage of these cubs subsequently died, and the pride had the lowest cub survival rate of the three prides compared (Fig. 8). As discussed, such mortality was directly attributable to high rates of male coalition turnover in the Santawani pride.

Figure 8: Differential cub survival among three study prides.

Table 2: Sources of male lion mortality.

21 TOTAL, 7 ALIVE IN 2001 = 67%

Table 3: Sources of female lion mortality.

27 TOTAL, 14 ALIVE IN 2001 = 48%

Rates of female mortality also differed among the prides. From 1996 to 2000, all three prides showed an overall trend of increase in the number of reproductive females. This was largely attributable to recruitment of subadult females into the pride as these females reached reproductive age, and this recruitment exceeded mortality among adult females. In 2001, however, this trend changed drastically with high rates of mortality among adult females attributable to illegal problem animal control as females in the Gomoti pride were forced out of their territory by River pride lions, and Santawani pride lions moved through the Veterinary Cordon fence as an overall southward trend in territory occupation influenced by movements of new males into their former core territory. Currently, only the Mogogelo pride has shown an overall increase in the number of adult females since the study began (Fig. 9).

Figure 9: Differential female survival in three prides, 1996-2001.

Conclusions and management implications

References

Swenson, J.E., et al., 1997. Infanticide caused by hunting of male bears. Nature 386, 450-451.

Wielgus, R.B. & Bunnell, F.L., 2000. Possible negative effects of adult male mortality on female grizzly bear reproduction. Biol. Cons. 93, 145-154.