Male Biased Dispersal in Cougars

Over the past two years, I have been studying cougar ecology and prey relationships in northeast Oregon. During this time, one of the most fascinating aspects of cougar ecology that I witnessed was their ability (and propensity) for long distance dispersal. The long distance dispersal ability of cougars has recently garnered national media attention. Despite the absence of established cougar populations east of the Mississippi River, two cougars have recently been documented (unfortunately due to their deaths). One cougar was shot in a Chicago suburb in 2008 and another cougar was recently run over in Connecticut. Genetic evidence confirmed both of these cougars originated in cougar population residing in the Black Hills of South Dakota – indicating these individuals dispersed at least 1,600 and 2,500 km, respectively. Long distance dispersal is not an uncommon for cougars, with dispersals of over 1,000 km previously observed (Thompson and Jenks 2005, Stoner et al. 2006). While dispersal, including long distance dispersal, is a common event for cougars, there is a very strong sexual bias in dispersal patterns. Upon reaching independence, female cougars tend to be philopatric (i.e., establish a territory within or adjacent to their natal range) and rarely make long distance dispersal events. In contrast, sub-adult, male cougars appear to always disperse a substantial distance from their natal territory (Sweanor et al. 2000). In fact, as of 2001, no study had documented a single instance of a sub-adult, male cougar establishing a territory that overlaps its natal range (Logan and Sweanor 2001).

For dispersal to be advantageous, it should increase the fitness of the dispersing individual (Morris 1982). However, survival rates of dispersing cougars are very low (Logan and Sweanor 2001), suggesting individuals would be better served to stay within their natal territory. This raises the question, ‘Why do male cougars disperse and female cougars typically stay close to their natal ranges?’. Several hypotheses have been proposed to explain male biased dispersal in polygamous, large mammals: 1) competition for mates, 2) competition for resources, 3) avoidance of inbreeding (Sinclair 1992), 4) higher cost of female dispersal (e.g., lower survival),and 5) increased breeding opportunities for dispersing males (Pusey and Packer 1987). Below, each of these potential hypotheses will be addressed based on existing knowledge of cougar ecology.

Increased breeding opportunity hypothesis:

The argument for this hypothesis is that the fitness of polygamous males is limited by the number of females that they can impregnate. As a result, males would increase their fitness by moving to areas with higher densities of potential mates. In some social species (such as lions) this benefit of searching for additional mates is offset by the potential for other males to come in and kill your existing offspring when you are away (Pusey and Packer 1987). For this hypothesis to hold true for cougars, it would be expected that males would exhibit transient behavior rather than territorial behavior. Adult, male cougars are highly territorial and will often fight to defend their territory (Logan and Sweanor 2010). Transient behavior is typically only observed in sub-adult males that have not yet established a territory. Established males will usually only abandon a territory if they die or if they lose a fight. In the case that they lose a fight, they typically will not disperse, but rather shift their territory or reduce the size of their territory (Logan and Sweanor 2001, 2010). Adult, male cougars do not exhibit transient behavior in an effort to increase breeding opportunities. Based on this evidence it appears that this hypothesis does not explain male biased dispersal in cougars.

High cost of female dispersal hypothesis:

The basis for this hypothesis is that females must place a great deal of energy into successfully rearing offspring to maximize their fitness. Females will be better able to maximize reproductive success by raising young in environments where they have knowledge about local food resources and denning sites. Since females have existing knowledge of their natal areas it is best for them to be philopatric. However, the same argument could be made for male cougars, which would not explain why male cougars are prone to dispersal. The other argument for this hypothesis is that females have lower survival when dispersing. Some of the potential threats that cougars face while dispersing include, crossing unsuitable habitat, encountering humans, crossing roads, and intraspecific strife. In general, female cougars experience very little intraspecific strife, whereas male cougars (particularly dispersing sub-adults) experience high levels of intraspecific mortality (Logan and Sweanor 2001, Quiqley and Hornocker 2010). This would suggest male cougars face lower survival rates while dispersing and further refute this hypothesis for cougars.

Competition for mates hypothesis:

For this hypothesis to hold true, it is expected that subordinate members of the sex that competes for mates will disperse (Dobson 1982). Cougars have a polygamous mating system. Males will breed with as many females as they can gain access to and females have little trouble finding available mates (Logan and Sweanor 2010). Males are highly territorial and defend their territories to protect potential breeding opportunities. Consequently, it appears that cougar ecology fits the pattern of this hypothesis. Young males will disperse until they find an area where they can establish their own territory. If they stay in their natal area they will likely be killed by a resident male. As a result, it is in their best interest to disperse until they locate a territory with potential mates where there is little competition from other males. However, two issues arise with this hypothesis based on cougar ecology. First, if competition is truly the driving factor, you would expect to see males that lose a battle with another cougar would disperse out of the area. On the contrary, the loser will just shift his territory enough to avoid the winner (Logan and Sweanor 2001). Second, male cougar territories are constantly fluctuating and openings frequently occurring. Given the risky nature of dispersal, you would expect dispersing sub-adults to only disperse until they find an open territory. However, dispersing males often travel substantial distances (much further than expected) while other males are colonizing their natal areas at the same time (Logan and Sweanor 2001). It appears the support for this hypothesis is equivocal.

Competition for resources hypothesis:

Under this hypothesis, individuals would be expected to disperse because food resources are inadequate to support additional cougars. This hypothesis appears to have little support based on cougar ecology. Sub-adult male cougars have been observed dispersing out of areas where the cougar population is growing (Logan and Sweanor 2001), which suggests resources were not limited. Furthermore, at the same time some male cougars are dispersing out of the population new males are coming in from adjacent populations. If resource limitation was truly the mechanism driving dispersal, it would be expected that female cougars would disperse at the same rate as male cougars. Dispersal of female cougars is a relatively uncommon occurrence, suggesting there are sufficient resources to support additional cougars. However, Logan and Sweanor (2001) suggested limitation of food resources may be the mechanism that prompts female cougars, but not males, to disperse. For female cougars, philopatry is preferred, because dispersal equates to decreased survival and reproduction. However, if local densities of cougars are high, there may not be adequate resources available and the costs of philopatry may outweigh the costs of dispersal, prompting individual females to disperse.

Reduction of inbreeding hypothesis:

This hypothesis suggests that male cougars disperse to avoid inbreeding. Inbreeding reduces heterozygosity and increases the chance of deleterious, recessive alleles being expressed. As a result, fitness is decreased. Florida panthers have experienced high levels of inbreeding (Culver et al. 2000). This is likely the result of male cougars being unable to disperse from their natal grounds (i.e., there is no suitable habitat for them in other areas and dispersal corridors are missing). As a result, males were forced to settle in areas where they are likely related to the resident females. In most cougar populations, the exception being heavily exploited populations, female matrilines are formed on the landscape (i.e., all females in an area are related; Logan and Sweanor 2001). If sub-adult males were philopatric, there would be very high probabilities of intensive inbreeding. Males would be able to breed with mothers, sisters, daughters, and granddaughters – this would result in inbreeding depression. As a result, it was advantageous for males to disperse, prevent inbreeding, which would increase the fitness of their offspring. Logan and Sweanor (2001) suggest that avoidance of inbreeding is the driving force behind long distance dispersal of males. Not only do males need to avoid inbreeding with their philopatric relatives, they must also avoid their female relatives that dispersed short distances from their natal territories. Dispersal may also promote outbreeding. Dispersal into and out of adjacent populations can increase fitness by introducing beneficial alleles into populations. The fitness benefits of inbreeding avoidance and outbreeding may have been large enough to fix male biased dispersal as a trait in cougar populations – however, this hypothesis has not been directly tested.

While it appears that avoidance of inbreeding is the evolutionary pathway that best explains obligate dispersal of male cougars, the other hypothesis presented above likely play a partial role in explaining dispersal patterns. Sub-adult males will likely continue to disperse until they are mature enough to compete for mates or until they find an area with little competition for mates. Dispersal will also likely continue until they find an area with sufficient resources to survive. Ultimately, avoidance of inbreeding drives male biased dispersal and other factors (competition for mates and resources) determine where a dispersing male will eventually settle.

Citations:

Culver, M., et al. 2000. Genomic ancestry of the American puma. The Journal of Heredity 91:186-197.

Dobson, F. S. 1982. Competition for mates and predominant juvenile male dispersal in mammals. Animal Behavior 30:1183-1192.

Logan, K. A., and L. L. Sweanor. 2001. Desert Puma – Evolutionary Ecology and Conservation of an Enduring Carnivore. Island Press. Washington DC, USA.

Logan, K. A., and L. L. Sweanor. 2010. Behavior and Social Organization of a Solitary Carnivore. In Cougar: ecology and conservation, ed. M. Hornocker and S. Negri, p. 105-117. University of Chicago Press. Chicago, USA.

Morris, D. W. 1982. Age-specific dispersal strategies in iteroparous species: who leaves first? Evolutionary Theory 6:53-65.

Pusey, A. E., and C. Packer. 1987. The evolution of sex-biased dispersal in lions. Behavior 101:275-310.

Quiqley, H., and M. Hornocker. 2010. Cougar Population Dynamics. In Cougar: ecology and conservation, ed. M. Hornocker and S. Negri, p. 59-75. University of Chicago Press. Chicago, USA.

Sinclair, A. R. E. 1992. Do large mammals disperse like small mammals? In Animal dispersal: small mammals as a model, ed. N. C. Stenseth and W. Z. Lidicker, Jr., p. 229-242. Chapman Hall. London, UK.

Stoner, D. C., et al. 2008. Long-distance dispersal of a female cougar in a basin and range landscape. Journal of Wildlife Management 72:933-939.

Sweanor, L. L., et al. 2000. Cougar dispersal patterns, metapopulation dynamics, and conservation. Conservation Biology 14:798-808.

Thompson, D. J., and J. A. Jenks. 2005. Long-distance dispersal by a subadult male cougar from the Black Hills, South Dakota. Journal of Wildlife Management 69:818-820.