Fitness consequences of dispersal decisions for male mountain gorillas (Gorilla beringei beringei)

  • Robbins A
  • Robbins M
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The fitness consequences of dispersal decisions are difficult to quantify, especially for long-lived species with complex social systems. To calculate those conse-quences for male mountain gorillas, from the perspective of both subordinate and dominant males, we used behav-ioral and demographic data obtained over 30 years from the Virunga Volcano population to develop an agent-based model that simulates the life history events, social struc-ture, and population dynamics of the species. The model included variables for birth rates, mortality rates, disper-sal patterns, and reproductive skew. The model predicted an average lifetime reproductive success (LRS) of 3.2 for philopatric males (followers) and 1.6 for emigrants. The benefits of philopatry were most sensitive to opportunities for social queuing and to female transfer preferences, but philopatry remained the best strategy over a wide range of group conditions and hypothetical simulations. The av-erage LRS for dominant males was 4.5 when a subordi-nate stayed and 4.6 when the subordinate emigrated. The dispersal decision of the subordinate male had little im-pact on the fitness of the dominant male because it came relatively late in the dominant male's reproductive life span, and it changed his group composition only incre-mentally. The fitness consequences for the dominant male were most sensitive to the degree of reproductive skew. Since subordinates suffer a fitness loss when they leave a group, they should accept whatever reproductive restraint is needed to avoid eviction, and the dominant male does not need to offer concessions for them to stay. The domi-nant male may offer reproductive concessions for other rea-sons, such as peace incentives or to confuse paternity, or he may not have complete control of reproduction within his group.

Author-supplied keywords

  • Agent-based model
  • Dispersal
  • Lifetime reproductive success
  • Mountain gorillas
  • Reproductive skew

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  • Andrew M. Robbins

  • Martha M. Robbins

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