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Do predators select only weak or crippled prey?

1:39 pm August 2nd, 2012

HOOK AND FUR
By Bob Brown
Of all the myths surrounding predators and predation, none is more persistent than the notion predators deliberately cull weak, crippled, or inferior animals from prey populations. However, professional hunters and trappers almost unanimously reject that contention. In fact, many believe predators select the healthiest animals.

A research article, “Differential Prey Used by Male and Female Cougars in Washington” (Journal of Wildlife Management (May 5, 2011) seems to indicate cougars are selective when it comes to prey.

Kevin R. White, Gary M. Koehler, Benjamin T. Maletzke and Robert B. Wielgus, authors of the above article, conducted a research study on elk/deer predation by cougars in the Cle Elum area from 2003 to 2008. It is often assumed male and female cougars have the same impact on prey, but because of differences in body size and behavior, male and female cougars may prey upon different species, sexes and ages of their prey, which could have important implications for wildlife conservation and management. The objective of the research study was to elucidate that assumption.

For the study, nine female and nine male cougars were fitted with GPS radio collars and 436 predation sites were investigated. Of those, prey remains were found at 345 sites. Of the 345 sites, 184 had remains of mule deer, 142 elk, and 17 with remains of four other species; three mountain goats, four beaver, five coyote, and one domestic dog.

The researchers believe male and female cougars use prey differently and may have dissimilar effects on prey population growth. Female cougars kill more mule deer and male cougars kill more elk. Proportionately, male cougars kill four times more adult elk than female cougars and females kill two times more adult mule deer than males, suggesting males may have a greater effect on population growth of elk and females a greater effect on mule deer population growth. The researchers also support the differential prey use hypothesis suggested by Murice G. Hornocker’s (Wildlife Monographs 21) analysis of mountain lion predation upon mule deer and elk in Idaho’s primitive area.

Hornocker said the differences in the size of male and female cougars may permit males to kill larger and older prey (i.e. elk) more frequently than females. With and without offspring females kill very few elk, suggesting females may avoid the risk of injury associated with larger, stronger prey. Female cougars may seek smaller prey such as mule deer or elk claves, which provide less biomass, but a more constant food source.

Based on evidence, the research concluded female cougars kill mule deer more frequently than elk, regardless of season. However, during summer, females kill juvenile elk more frequently than juvenile mule deer. Female cougars may select juvenile elk during summer due their smaller size compared to adult elk and juvenile’s inexperience in predation avoidance. In addition, female cougars may have a greater ability to detect larger juvenile elk calves and deer fawns or may simply prefer the larger prey biomass.

What are the management implications? The researchers believe their research has demonstrated different sexes of cougars could have different effects on prey populations and suggests the sex of predators should be incorporated into ungulate management and conservation plans.

Alternating the sex composition of cougar population through harvest may offset predation on one species or age class, but could exacerbate predation on others. Also, managers should consider the effects of predator harvest on numbers of males and females, not predators in general as males and females appear to exhibit different effects on prey. While the above suggestion may have merit, in my opinion the chances of its implication in the near future are slim and possibly never.

 

Outdoors writer Bob Brown lives in Roy. He can be reached at robertb1285 @fairpoint.net

One Response to Do predators select only weak or crippled prey?

  1. Bob Reply

    August 20, 2012 at 6:36 am

    I found this article interesting, and am familiar with other works by three of the four cited authors–all of whom are knowledgeable scientists concerning apex carnivores.

    The “persistent myth” of predators selecting inferior animals may not be a myth. Wolves are coursing predators that test a herd before selecting a meal. While no reference to the science is listed, the third and fourth paragraphs of this article are telling.

    Cougars, as stalk and pounce predators, one might argue, are less likely to cull the infirm, but a recent (2010) research report indicates that cougars (at least in Colorado’s Front Range, where they are called “mountain lions”) select deer with chronic wasting disease.

    Predators face injury, and even death, whenever they try to eat something that doesn’t want to be a meal. Hundreds of thousands of years of evolutionary refinement has equipped these top-tier carnivores with the skill or senses to pick the safest choice for a meal, while ensuring the future of their food supply. Even the strategy of taking the young makes sense, as it leaves the parents to produce more young. Also, since cervids (deer, elk, etc.) generally exhibit “compensatory mortality,” that is, deaths from natural causes decrease to compensate for deaths from predation, taking young, or infirm, leaves healthy breeding stock to ensure a future supply. Unfortunately, cougars themselves exhibit “additive mortality,” and do not adjust birth or death rates to compensate for losses by human predation. Recent science illustrates the problems of removing predators from a system, as well as the damage it causes to our own food web.

    Cougar effects on deer-dominated systems.
    Food chain effects of predators.

    Finally, for the hard-core reader who wants to know the most current science, the book Trophic Cascades: Predators, Prey, and the Changing Dynamics of Nature edited by Terborgh and Estes (2010, Island Press)expresses what we have learned about predators in marine, aquatic, and terrestrial systems.

    There’s no reason to rely on anecdotal evidence about predators, when our knowledge of predators has significantly increased thanks to scientific observations in the past half-century, including the major contributions by scientists noted in the article.

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