The Evolution of Sex-Biased Dispersal in Lions

Abstract

Most female lions remain in their natal pride for their entire lives, but about a third emigrate before they reach four yrs of age. Most emigrating females leave either when they are evicted by an incoming male coalition or when the adult females of their pride give birth to new cubs. One cohort of females left because they avoided mating with males of their father's coalition. Cohorts of dispersing females leave together and form a new pride whose range almost always includes at least part of their natal range. Females that leave their natal pride suffer reduced fitness: dispersing females in the Serengeti first breed at a later age than non-dispersing females and dispersing Ngorongoro females suffer higher mortality. Dispersal patterns of large and small cohorts are such that pride size rarely exceeds or goes below the range of sizes that confers the maximum reproductive success per female. Pride fissions have no consistent effect on the average levels of genetic relatedness within prides. All males leave their natal pride. Most leave at a male takeover, but one male cohort was evicted by their fathers whereas several others left voluntarily during their fathers' tenure. A greater number of large male cohorts leave their natal pride than expected from their competitive ability and males rarely show an interest in mating until they enter a new pride. Larger male coalitions are more likely than small ones to gain residence in a pride adjacent to their natal pride, and also gain their first pride at a younger age. However, not all large cohorts are so successful because most cohorts of 6-8 males permanently split up soon after emigrating from their natal pride. About half of the secondary dispersal shown by resident adult coalitions is voluntary. Such voluntary secondary movements occur in three contexts: the coalition (1) annexes an additional pride thereby gaining access to more females (2) abandons one of several simultaneously held prides to spend more time with prides in which their cubs are younger and hence more vulnerable to infanticide (3) abandons one pride for another pride. In the third case males only abandon small cubs when going to much larger numbers of females, but they may abandon more females for fewer when their daughters are reaching maturity in the former pride. Males rarely reside in prides once their daughters have matured, and rarely return to breed in their natal pride. Compared to the large Serengeti population, the small isolated Ngorongoro population has lower levels of dispersal, higher levels of homozygosity and some deleterious effects of inbreeding. Most female lions remain in their natal pride for their entire lives, but about a third emigrate before they reach four yrs of age. Most emigrating females leave either when they are evicted by an incoming male coalition or when the adult females of their pride give birth to new cubs. One cohort of females left because they avoided mating with males of their father's coalition. Cohorts of dispersing females leave together and form a new pride whose range almost always includes at least part of their natal range. Females that leave their natal pride suffer reduced fitness: dispersing females in the Serengeti first breed at a later age than non-dispersing females and dispersing Ngorongoro females suffer higher mortality. Dispersal patterns of large and small cohorts are such that pride size rarely exceeds or goes below the range of sizes that confers the maximum reproductive success per female. Pride fissions have no consistent effect on the average levels of genetic relatedness within prides. All males leave their natal pride. Most leave at a male takeover, but one male cohort was evicted by their fathers whereas several others left voluntarily during their fathers' tenure. A greater number of large male cohorts leave their natal pride than expected from their competitive ability and males rarely show an interest in mating until they enter a new pride. Larger male coalitions are more likely than small ones to gain residence in a pride adjacent to their natal pride, and also gain their first pride at a younger age. However, not all large cohorts are so successful because most cohorts of 6-8 males permanently split up soon after emigrating from their natal pride. About half of the secondary dispersal shown by resident adult coalitions is voluntary. Such voluntary secondary movements occur in three contexts: the coalition (1) annexes an additional pride thereby gaining access to more females (2) abandons one of several simultaneously held prides to spend more time with prides in which their cubs are younger and hence more vulnerable to infanticide (3) abandons one pride for another pride. In the third case males only abandon small cubs when going to much larger numbers of females, but they may abandon more females for fewer when their daughters are reaching maturity in the former pride. Males rarely reside in prides once their daughters have matured, and rarely return to breed in their natal pride. Compared to the large Serengeti population, the small isolated Ngorongoro population has lower levels of dispersal, higher levels of homozygosity and some deleterious effects of inbreeding.