Genetic insights into dispersal distance and disperser fitness of African lions (Panthera leo) from the latitudinal extremes of the Kruger National Park, South Africa

1. What they did

Female lions usually settle close to where they were born, while males can travel more than 200 km to find a new pride. Earlier studies in bush-like areas such as Kruger National Park and Selous in Tanzania suggested much shorter journeys, of 20 to 30 km. How far lions move, and how well the movers fare, matters for the spread of diseases such as bovine tuberculosis (BTB) and feline immunodeficiency virus (FIV), both present in Kruger lions.

The researchers used the lions’ DNA to look at dispersal. Between 1998 and 2004, 109 lions were captured at call-up stations, sedated and blood sampled: 97 lions from 21 prides, and 12 singleton lions. The samples had been collected for a BTB study, so they came from two areas at opposite ends of the park, more than 200 km apart: the far north, then free of BTB, and the south, where BTB was common. The north had fewer lions per area, larger female territories (about 300 km² versus 70 km²) and lions in better body condition.

They read 11 genetic markers inherited from both parents in all 109 lions, and a stretch of DNA passed down only from mothers (mitochondrial DNA) in 28 lions. From these they worked out how related lions were within and between prides, which lions were probably born locally and which were probably newcomers, and how this changed with sex, age and body condition. They tested four ideas: that dispersal distances in bush-like areas are short, that few males disperse, that dispersal peaks at around 40 months of age, and that dispersers are less fit.

2. What they found

Two different populations

• Lions from the north and the south were genetically quite distinct, suggesting only limited movement between the two areas (an estimated 0.62 migrants per generation).
• The DNA passed down from mothers formed two very different groups, one for northern and one for southern females. No female carried a haplotype from the other end of the park.

How far lions moved

• Within prides, females were related to each other at about the level of first cousins, but males and females in the same pride were not related, showing that males came in from other prides.
• Females from different prides were still related up to about 20 km apart, so most females did not move further than this from their birth pride.
• Males showed no such pattern within 75 km, so they moved further, generally more than 25 km.
• Two lions showed long-distance male gene flow of 75 to 200 km: an eight-year-old male from a northern genetic group, sampled in the south in a nomadic group, and a seven-year-old female in the north whose father was probably a southern male.

Who dispersed

• In the north, about 24% of lions were probable dispersers and 51% probable residents. In the south it was the other way round: 58% dispersers and 25% residents.
• Across both areas, the fraction of dispersers was twice as high in males as in territorial females (0.52 versus 0.26).
• In the south, non-territorial females dispersed as often as males (0.64). Seven of the 12 southern prides sampled were non-territorial, against one of nine in the north.
• In the north, males in the same pride were often related, even into adulthood, suggesting male coalitions stayed together. In the south, males five years and older were not related to their pride mates, suggesting coalitions broke up before adulthood.

Age and fitness

• In the south, few lions aged 1 to 3 years were dispersers. As many as 90% of lions aged 3.5 to 7 years were dispersers, but only 29% of the younger and older age classes together.
• In the north, dispersers were less likely to be in top body condition.
• Singleton lions were often old: half the singletons were over eight years, against 10% of pride lions.

3. What they concluded

• The authors rejected the idea of short dispersal distances in Kruger: male lions can travel large distances and breed successfully, even in bush-like ecosystems, though the park’s long narrow shape may play a part.
• Most females stayed within about 20 km of their birth pride. The mitochondrial DNA was so different between north and south that the authors describe two distinct female-line subpopulations.
• Male long-distance dispersers seem to be few or to breed little.
• Dispersers appear to have low fitness: lower body condition, and fewer of them at older ages, which may reflect deaths or old males being evicted to live as singletons.
• In the south, the high female dispersal rate, unstable male coalitions and many non-territorial prides point to unstable territories and high pride turnover in 1998 to 2004. Low body condition and high lion density may both play a part; a link with BTB is possible, though another study found no link between BTB and body condition.

6. Other technologies, skills or innovations

• Population genetics from blood samples: microsatellite markers inherited from both parents, and mitochondrial DNA inherited only from mothers, which separates female movement from male movement.
• Bayesian cluster analysis (the program Structure) to sort individual lions into probable residents, probable dispersers and lions of mixed ancestry.
• Lions were aged from tooth wear, scored for body condition, and microchipped so they could be recognized if caught again.

7. Implications for conservation (from the paper)

• High local dispersal rates in both sexes, and long-distance movement by males, mean infectious diseases could spread quickly through the Kruger lion population.
• Dispersers tend to be in poorer body condition, which makes them more vulnerable to FIV, BTB and probably other infections, adding to the disease risk.
• Infection and the inability to keep a place in a pride may feed into each other.
• The large genetic differences between north and south could be taken into account in management policies.