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Friday 10th December 2010
In this article, I will look at what can be learned from genetic studies of lion populations, with some interesting data revealed about male lion dispersal distances and relationships among females in neighbouring prides. Also, I will discuss the results from a study using computer models to predict the “minimum” and “best” number of lion prides needed in a particular population to long-term prevent inbreeding. Very few lion populations remain in Africa that suit the “best” category, and for the much more numerous small populations, intensive genetic management will likely be required.
Genetic studies are fantastic
Genetics is the most fascinating field of study in biology, bar none. And I am not at all opinionated in this matter. For those who dislike genetics, this probably resulted from the ordeal of having to take introductory courses, which for some reason seem to be made intentionally boring. Mendel and his peas, Hardy-Weinberg equilibria, linkage disequilibria, binomial expansions, Pearson’s chi-square tests, Fisher’s exact test, inbreeding coefficients – it’s all statistics and formulas. I guess it could be termed a rite of passage?
But for the survivors an amazing diversity of research fields open up, and my favourites have always been evolutionary genetics and population genetics. Modern genetic techniques are incredibly powerful and can ferret out the smallest difference in genetic sequence among species, populations, and individuals. With such analytic techniques, the variety of results can give great insight into both the distant history and current events of populations.
Male lions don’t disperse far
With lions, some interesting genetic studies were done by Goran Spong, then at Uppsala University in Sweden. In the first study he used genetic data to estimate dispersal distance in male lions. Lions are particularly suited for such analyses as females generally stay with their natal prides, and even when prides split into smaller groups at times (pride fission), such females often settle in a different part of the original pride territory. Subadult males, however, always disperse from the pride into which they were born, either pressured by females, and/or convinced by aggression from new adult males that take over the pride. Dispersal is a rather slow process, and while young males take initial exploratory journeys, they generally return to the pride territory several times. During these early days of dispersal, loose coalitions lasting weeks to a few months can be formed with males of similar age from other prides. This is not an easy time for such young males, as moving into other lion territories entails risks of being wounded and killed, especially when they encounter resident adult males – there is a cost and a risk to dispersal. However, eventually the move is made, the young males relocate, and can then attempt to become pride males.
But they do not appear to move far according to Spong’s studies and other observations. Using genetic data (on an admittedly small sample), Spong was able to calculate that males only move an average of 1.3 home ranges away (range 0.3 to 3 ranges). In other words, males tend to take over prides practically next door from where they were born, and much more rarely move long distances. This is similar to data from radiocollared young males followed in Botswana – the prides they eventually took over were not far from home.
Pride females are close relatives
In a second study, Spong examined genetic relatedness among females in several prides. Given that females do not usually disperse, such relatedness was expectedly high, about what you would expect for half-siblings. Relatedness among females fell off with increasing distance, and when females from one pride were compared to others one or two pride territories away, it was unusual to find any close relatives. Spong did find groups of females in some neighbouring prides that genetically clustered closely together, and used the term “superprides” to describe these. He suggested that such close relationships among prides resulted either from pride fission in the past, and/or short dispersal distances among males as mentioned above.
Dispersal has long been recognized as a selected behaviour to prevent inbreeding. However, with the overall short dispersal distances recorded among male lions, and the possibility that females in neighbouring prides can exhibit substantial levels of relatedness to each other, overall genetic relationships among neighbouring lion prides could well be expected to show higher levels of kinship than among animals that disperse further. Much more needs to be learned about kinship among different prides of lions, and Spong’s studies in the Selous of Tanzania have laid a good foundation for studies in other areas.
In a third study of lion genetics Mats Bjorklund, also of Uppsala University, used computer models to predict levels of inbreeding among the increasingly isolated and smaller populations of lions found today in Africa. Inbreeding entails many negative consequences including reduced resistance to disease, increased probability of expression of deleterious genes, higher likelihood of morphological deformities, decreased fertility and increased infant mortality – in short, a practice to be avoided. Bjorklund used genetic data from Spong’s studies and known biological parameters of lions in the model, and found two factors of overriding importance: male dispersal rates and the total number of lions in the population.
Even a small decrease in male dispersal rates (from 100% to 95%) increased measures of inbreeding significantly. This can happen among wild lion populations, especially those in male depopulated areas, and/or in areas where immigration of new males happens infrequently. Inbreeding has occurred in the Ngorongoro Crater in Tanzania, for example, where a lion population was likely established by a few founders and subsequent immigration limited by difficulty of access. Even with dispersal inbreeding can result in cases where one group of males takes over two neighbouring prides and sires cubs in both. If their sons should then later take over one of these prides by moving to the next territory, they will be mating with their half-siblings, an event that probably occurs more frequently than suspected.
Minima and optima…
Bjorklund also stressed the importance of large population sizes to prevent inbreeding among isolated groups of lions. Since most lions in Africa now exist in populations cut off from each other by land used for human habitation and agriculture, Bjorklund estimated a minimum of 50 prides was needed in an isolated population to safeguard against future erosion of genetic variability, but 100 prides was preferable. The number of adult lions in a pride varies considerably with habitat and prey density, but an overall average of about eight adults per pride in eastern and southern Africa is a reasonable assumption. This means that only those populations with a minimum of 400 adults and a preferred 800 adults can be considered genetically stable in the long term. How many lion populations with those numbers exist in Africa?
… and few remaining lion populations qualify
Using total estimated lion numbers within 86 “Lion Conservation Units” identified during the technical sessions of the West and Central African Lion Conservation Workshop (2005) and at the Eastern and Southern African Lion Conservation Workshop (2006), there are rather few populations still in existence that meet Bjorklund’s criteria. Some of these Lion Conservation Units add populations across wide areas and are not realistic – for example Niokiolo-Guinee sums lions from Guinea, Guinea-Bissau, Mali, and Senegal together, which is nonsense. Nevertheless, even if we take the very conservative estimate that total numbers of lions (adults, subadults, cubs) in a pride is only double the number of adults, there are perhaps twelve populations that meet Bjorklund’s minimum requirement of 50 lion prides, and only five that have his more comfortable number of 100 prides.
This is not to say that the other 74 Lion Conservation Units with less than 50 prides are irrevocably doomed. What will be necessary for those populations, however, is careful genetic management, including translocation of males to mimic dispersal. But for some of those small populations it might already be too late as exemplified by disappearances of small lion populations that were until recently still thought to occur in Ghana and Nigeria. Overall, many more genetic analyses are needed especially among smaller populations to gauge their realistic chances of future survival.
Posted by Pieter Kat at 00:00
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