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Keywords:

  • conservation;
  • evolution;
  • evolutionary enlightened management;
  • hunting;
  • large mammals;
  • management;
  • sustainability

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. The mechanisms of harvest selectivity
  5. Limited empirical evidence of directional selection
  6. Harvesting intensity is itself important
  7. Conclusion
  8. Acknowledgements
  9. References

1. Harvesting of large mammals is usually not random, and directional selection has been identified as the main cause of rapid evolution. However, selective harvesting in meat and recreational hunting cultures does not automatically imply directional selection for trait size.

2. Harvesting selectivity is more than a matter of hunter preference. Selection is influenced by management regulations, hunting methods, animal trait variance, behaviour and abundance. Most studies of hunter selection only report age- or sex-specific selection, or differences in trait size selection among hunting methods or groups of hunters, rather than trait size relative to the age-specific means required for directional selection.

3.Synthesis and applications. Managers aiming to avoid rapid evolution should not only consider directional selection and trophy hunting but also mitigate other important evolutionary forces such as harvesting intensity per se, and sexual selection processes that are affected by skewed sex ratios and age structures.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. The mechanisms of harvest selectivity
  5. Limited empirical evidence of directional selection
  6. Harvesting intensity is itself important
  7. Conclusion
  8. Acknowledgements
  9. References

There is increasing concern about the possible long-term evolutionary consequences of heavy human harvesting (Harris, Wall & Allendorf 2002; Festa-Bianchet 2003; Allendorf et al. 2008; Allendorf & Hard 2009; Darimot et al. 2009). Such effects have been linked to strong directional selection for specific phenotypic traits, such as against large fish because of mesh sizes of closing nets (Jørgensen et al. 2007) or against large trophy males because of hunter preference (Coltman et al. 2003; Garel et al. 2007). Directional selection effects of trophy hunting on size are well documented for bighorn sheep Ovis canadensis Shaw in Canada (Coltman et al. 2003). Trophy hunting is widespread (Courchamp et al. 2006; Johnson et al. 2010), so these results should be taken seriously.

However, most harvesting of large mammals is not a result of trophy hunting. Moreover, management regulations often restrict large mammal hunters from following their preferences. When comparing red deer Cervus elaphus L. harvest statistics across 11 European countries (Milner et al. 2006), the proportion of calves in the harvest varied from 10% to 40%, while males typically accounted for 40–60% of the remainder, i.e. male and female harvests were of similar magnitude. Trophy bulls usually make up a very small proportion of the harvest. Directional selection are sometimes reduced by counter selection pressure on small, young males (Mysterud & Bischof 2010), and trophy males are often shot at the age of trophy culmination (Apollonio, Andersen & Putman 2010). The degree of size selection may strongly differ between the age classes that are targeted in both males (Mysterud & Bischof 2010) and females (Proaktor, Coulson & Milner-Gulland 2007). Different selection pressures arise from harvesting aimed at meat provisioning, subsistence, recreation or population control. It therefore cannot be taken for granted that harvesting always induces strong directional selection as a result of hunter preferences for large sized individuals.

Identifying the level and pattern of selection is crucial for predicting expected rates of evolutionary responses to large mammal hunting. Here, it is argued that: (i) there are currently few studies documenting directional selection for body or trophy size despite claims on the contrary (Tenhumberg et al. 2004; Allendorf & Hard 2009) and (ii) that in many cultures, large mammal harvesting is not expected to induce strong directional selection in trait size. Harvest selectivity in mammals is complex because of highly variable environments, management culture and regulations. We also need to broaden our focus beyond trophy harvesting when considering evolutionary effects.

The mechanisms of harvest selectivity

  1. Top of page
  2. Summary
  3. Introduction
  4. The mechanisms of harvest selectivity
  5. Limited empirical evidence of directional selection
  6. Harvesting intensity is itself important
  7. Conclusion
  8. Acknowledgements
  9. References

The factors affecting patterns of harvest selectivity in terrestrial ecosystems can be broadly organized into: (i) hunter preferences and (ii) opportunities to be selective via (a) management regulations (quotas; economic costs etc.), (b) hunting methods (stalking vs. drives etc.), (c) animal trait variation (strength of hunter cues, appearance), (d) animal behaviour, (e) animal abundance, (f) population structure (sex ratio and age structure) and g) habitat openness. Table 1 gives an overview of common traits targeted by hunters in terrestrial ecosystems and the cues the hunters use to separate individuals at the within-species level most relevant for directional selection. The hunters’ preferences are likely to differ depending on hunter motivation (i.e. meat, recreation or trophy), level of knowledge and skill (use of guides), cultural background, religion (taboos), individual ethics and animal trait variation. For example, trophy hunters using guides shot larger moose Alces alces L. in Alaska, because guides took client hunters to areas with lower population densities and therefore larger moose (Schmidt, Ver Hoef & Bowyer 2007). More importantly, strong directional selection for size is often unlikely because of limited (or redirected) opportunities for hunter selection because of both direct and intentional factors such as quotas or economic costs of high pricing and also time limitations, cost of lost opportunity, and indirect and non-intentional factors through animal behaviour and abundance (Table 2).

Table 1.   Traits used for direct hunter preference or selection
Trait targetedHunter cueExamplesReferences
Age: juvenile vs. yearling/subad.Juvenile traits (short jaw), small body size, overall appearance (fur colour etc.)Cervids 
Age: yearling/subad. vs. adultBody size, antler or horn sizeCervids 
Sex: male vs. femaleSexual traits (presence of penis)Cervids 
Secondary sexual traits (presence, size or form of horns, antlers, tusks; colour of mane)Elephants, lions Panthera leo L., cervidsKurt, Hartl & Tiedemann 1995; Whitman et al. 2004
Sexual body size dimorphismChamois 
Females: reproductive statusOffspring vs. no offspring at heelMooseEricsson 2001
Brown bearBischof et al. 2009
ChamoisRughetti & Festa-Bianchet 2011
Female sizeBody sizeCervids 
Male sizeTrophy or body sizeCervids, bovidsColtman et al. 2003
Special trophies –‘oddities’Parück vs. normalRoe deer Capreolus capreolus L. 
Colour morphsBlack morphs vs. normalSpringbok Antidorcas marsupialis Sundevall, roe deer 
White morphs vs. normalSpringbok 
Silver morphs vs. normalFox Vulpes v. fulvus DesmarestHaldane 1942
Table 2.   Mechanisms affecting the level of hunter selection beyond hunter preferences
FactorEffect on selectivityMechanismType of selection
Management
 Quota: sizeMore selective if small quotaLaw enforcement and time limitationDirect
 Quota: specificityLess selective the more specific the quotaLaw enforcementDirect
 Quota: scaleLess selective if quota for a region or team rather than for individualCompetition among huntersDirect
 Duration of hunting seasonMore selective the longer the hunting seasonTime limitation, but depletion may reduce selectivityOpportunity for direct
 Size of hunting estateLess selective on smaller estatesFewer to choose fromOpportunity for direct
 PriceLess selection the more costly to shoot the larger oneNot all hunters have endless amount of moneyDirect
Hunting implementation
 Hunting methodStalking more selective than drive huntMore time to assess differences when animals are calmIndirect, and opportunity for direct
 Use of guidesUse of guides increase selectivityGuides assess size better; know where largest animals are livingDirect, opportunity for direct
 Use of dogsUse of dogs may lower selectivityPreference of dogs might differ from preference of hunterIndirect, and opportunity for direct
 TrappingUse of traps may change selectivityTraps differ in specificity due to variation in catchabilityIndirect
Animal
 Trait variation (Table 1)More opportunities to select if animals differ in traitsHunters ability to select differ, and differences can affect chances of being observedIndirect, and opportunity for direct
 Animal population density (and skewed sex ratio)More selective the more to choose fromTime limitation to find animals at low densityOpportunity for direct
 Grouping behaviourMore opportunities to select if animals in herdsEasier to assess differences in size when individuals are closeIndirect, and opportunity for direct
 Mother-offspring bondMore selective if strong bondIf offspring do not follow mothers closely, more difficult to separate mothers from non-mothersIndirect, and opportunity for direct
 Sexual segregationMore (or less) selection if sex groups spatially segregateSpatial search of hunter increase likelihood of shooting a given sex (but may decrease selection in some cases)Indirect, and opportunity for direct
 Home range sizeMore selection for animals with large home rangesMore exposed if large home range sizeIndirect
 Activity levelsMore selection for animals that are more activeMore active more exposedIndirect
 Habitat useMore selective harvest in open habitat (or farmland)More vulnerable if using open areas (or farmland)Indirect, and opportunity for direct
 Individual personalityMore selection for animals that expose themselves moreAnimals may differ in their propensity to take riskIndirect
Landscape factors
 HabitatMore open habitat increase selectionEasier to see what is availableOpportunity for direct

If there is little opportunity for choice, for instance because of a low population density (Tenhumberg et al. 2004), a skewed sex ratio leading to low density of one sex (Nilsen & Solberg 2006), a high quota relative to population size (Solberg et al. 2000), a short duration of hunting season, or small estate size, selectivity will be reduced. For example, moose hunters did not select for male age (older being larger), in a situation where a female-biased sex ratio and a young male age structure limited the opportunities to select (Nilsen & Solberg 2006). High competition among hunters is likely to produce the same effect. For example, for large carnivores in Scandinavia, quotas are given for large regions rather than to an individual hunter. Low selectivity was found for brown bears Ursus arctos L. under such management regulations (Bischof et al. 2009). A given hunter might prefer to shoot a very large male, but might not risk passing up a small bear because the quota for the area might be filled before the hunter encountered a large bear, and furthermore, the hunter has nothing to lose by shooting the small bear as there is no individual quota. Similar effects result as a consequence of team hunting on the same estate.

Clearly, a lack of appropriate cues may in many cases limits the hunters’ ability to select. Selection may decrease when there is low sexual body size dimorphism or lack of visual secondary sexual characters. By contrast, habitat openness promotes gregariousness, which can increase the likelihood of selection. Climate affects movement such as the timing of migration and can also affect opportunities for selection. We currently have rather limited knowledge of how much animal behaviour affects harvest-related selection in mammals, but it is likely to be an important factor. For example, young birds were more prone to being shot than adult birds because of difference in behaviour (Bunnefeld et al. 2009). Furthermore, selection on bold personality with fast growth has been found in fisheries (Biro & Post 2008).

In Fennoscandia, hunting of cervids is often carried out with the aid of dogs (either on a leash or barking), which is known to increase moose harvesting success by up to 56% (Ruusila & Pesonen 2004). Drive hunting in Europe is carried out both with and without the aid of dogs (Apollonio, Andersen & Putman 2010). In Nicaragua, dogs sometimes selected non-target prey species (Koster 2008), and it is possible dogs can be selective of scent from, e.g. rutting males, affecting selectivity. The spatial hunting behaviour of humans may also influence the selective pressures exerted (Schmidt, Ver Hoef & Bowyer 2007).

Limited empirical evidence of directional selection

  1. Top of page
  2. Summary
  3. Introduction
  4. The mechanisms of harvest selectivity
  5. Limited empirical evidence of directional selection
  6. Harvesting intensity is itself important
  7. Conclusion
  8. Acknowledgements
  9. References

In a recent review of terrestrial ecosystems, Allendorf & Hard (2009) pointed out that selection is important for trait evolution. This conclusion was based on theoretical modelling which indicated that size-selective harvesting can cause shifts in trait values. However, the few empirical cases that were listed consisted of different kinds of hunters shooting different kinds of animals without any evidence that the total harvest differed in terms of trait mean from what was available in the population. There was thus no clear link from theory (directional selection[RIGHTWARDS ARROW]evolution) to data (non-random harvest). A broader review undertaken here (Table 3) reveals that there are no clear-cut examples of directional selection apart from the case study of bighorn sheep arising from trophy hunting. The most common documentation of selective harvesting comes from comparing different groups of hunters or using different hunting strategies or methods, or comparisons of age or sex classes rather than size directly (Table 3). That hunters select adults over calves is not evidence of directional selection acting on size, this would require comparison with age-specific mean size within a population (the unit for evolution). The population mean or availability in the population is known only rarely, and these studies therefore cannot say with confidence that selection is directional.

Table 3.   Studies of harvesting selection of mammals in terrestrial ecosystems. Dir. Sel. = evidence of direct selection on trait size (or character)
SpeciesTraitAssumed mechanismSelectivity comparisonPopulation average or availability knownDir. sel.Reference
Ungulates
 Bighorn sheepMale trophy sizeHunter preferenceRams of different sizesYesYesColtman et al. 2003
 ChamoisFemale reproductive statusManagementPopulations with different managementNoYes?Rughetti & Festa-Bianchet 2011
Female horn sizeHunter preferenceNo selection foundNoNoRughetti & Festa-Bianchet 2011
 Red deerAge and sexManagementMortality of marked individualsYesNoLangvatn & Loison 1999
Male body massHunter methodMonteria vs. trophy-stalking vs. management catch vs. bycatchNoNoMartínez et al. 2005
Male body and trophy sizeHunter methodCommercial vs. selective monteriaNoNoTorres-Porras, Carranza & Pérez-González 2009
 Roe deerMale body massManagementLocal vs. client hunters; early vs. late season; habitat opennessNoNoMysterud, Tryjanowski & Panek 2006
Age and sexHunter vs. lynx Lynx lynx L.NoNoAndersen et al. 2007
 MooseFemale reproductive statusManagementSurvival with or without reproduction (marginally sign.)YesYes?Ericsson 2001
Age and sexHunter preference (females, none found for males)Age groups; within season declineNoNoNilsen & Solberg 2006
Age and sexManagementSex-specific age groups; years with low and high quota relative to population sizeNoNoSolberg et al. 2000
Male trophy sizeHunter preference; implementationWith or without aid of guidesNoNoSchmidt, Ver Hoef & Bowyer 2007
Calf sizeNone foundMale vs. female calvesNoNoMoe et al. 2009
 Elk Cervus elaphus L.Age and sexHunter preferenceHunters vs. wolves Canis lupus L.NoNoWright et al. 2006
 White-tailed deer Odocoileus virginianus ZimmermannAge and sexTrapabilityTrapability of marked individualsYesNoHiller et al. 2010
Age and sexHunter methodArchery vs. firearmNoNoMattson & Moritz 2008
Male ageManagementMortality of marked individualsYesNoWebb, Hewitt & Hellickson 2007
AgeHunter preference(?)Mortality of marked individualsYesNoPac & White 2007
Disease prevalence (CWD=Chronic Wasting Disease)None found (animal behaviour hypothesized)Periods of different harvesting methodsNoNoGrear et al. 2006
 Mule deer Odocoileus hemionus RafinesqueAge and conditionAssumed noneHunter vs. mountain lions Puma concolor L.NoNoKrumm et al. 2010
 Wild boarAge and sexHunter methodEspera vs. Monteria huntNoNoBraga et al. 2010
AgeHunter preference(?)Hunters vs. wolves vs. estimated populationYesNoNores, Llaneza & Alvarez 2008
Age and sexHunter preference(?)Mortality of marked individualsYesNoToïgo et al. 2008
Age and sexNone foundMortality of marked individualsYesNoKeuling et al. 2010
Large carnivores
 Brown bearBody massHunter methodMoose vs. bear specialist huntersNoNoBischof et al. 2008
Age and sexAnimal behaviourMortality of marked individualsYesNoBischof et al. 2009
Mountain lionAge and sexHunter preference(?)Mortality of marked individualsYesNoCooley et al. 2009

There is little doubt trophy hunting is directionally selective, but the level of directional selection for hunters targeting meat, subsistence, recreation or population control rather than trophies is not well documented. We do know that foreign trophy stalkers select differently than local hunters (Martínez et al. 2005; Mysterud, Tryjanowski & Panek 2006) and in some cases, selection is based on size. A lower level of selection will strongly affect the expected rate of evolutionary response. Harvesting selection will always work against forces of natural selection (Ratner & Lande 2001). It is not known when selection pressure from harvesting is strong enough to alter the fitness landscape. The level of trait heritability is clearly also critical, but not the focus here.

In the bighorn sheep case (Coltman et al. 2003), the smaller males became the more successful breeders, which is a quite extreme example of harvest-driven directional selection. However, for elephants Loxodonta africana L. in Tarangire National Park, Tanzania, the larger males retained a higher mating success even under poaching pressure (Ishengoma et al. 2008), suggesting that the fitness landscape did not change qualitatively. In Alpine chamois Rupicapra rupicapra L., horn length appears to have a limited role in male reproductive success, and hunter selection was regarded as unlikely to yield an evolutionary response in males (Rughetti & Festa-Bianchet 2010) or females (Rughetti & Festa-Bianchet 2011). Harvesting effects are expected to be stronger in small populations (Hard, Mills & Peek 2007; Steenkamp, Ferriera & Bester 2007), and effective population size might become an issue (Sæther, Engen & Solberg 2009). For large populations, there is less likely to be uniform harvest pressure, and regions with limited harvesting might buffer selective effects of harvesting through migration (Tenhumberg et al. 2004).

Harvesting intensity is itself important

  1. Top of page
  2. Summary
  3. Introduction
  4. The mechanisms of harvest selectivity
  5. Limited empirical evidence of directional selection
  6. Harvesting intensity is itself important
  7. Conclusion
  8. Acknowledgements
  9. References

A mild preference for large quarry size, for example in cultures where animals are hunted for meat, does not imply an absence of evolutionary effects but we need to consider other mechanisms. Even non-selective harvesting may theoretically affect trait evolution (Bischof, Mysterud & Swenson 2008).

The intensity of harvesting per se and the timing of the harvest relative to the age of first reproduction may be important in this context because life expectancy is a crucial fitness component in large mammals. The same harvest pressure is thus more important for males than females, because of lower life expectancy of males (Toïgo & Gaillard 2003). Under heavy harvest pressure, individuals that begin reproduction at a young age and at a light weight have a greater chance of reproducing at least once compared with those that begin reproduction at heavier weights, later in life (Proaktor, Coulson & Milner-Gulland 2007). However, we do not know how strong harvest pressures need to be to outweigh the (high) cost of early reproduction. Population differences in harvesting pressure have been shown to correlate with the proportion of juveniles reproducing in wild boar Sus scrofa L. (Servanty et al. 2009) but no trend towards earlier maturation was found for red deer in populations where a high proportion of non-breeding juveniles are harvested (Mysterud, Yoccoz & Langvatn 2009). Hunter preference for non-reproducing females is common (Table 1).

Furthermore, skewed sex ratios and age structure in harvested populations may lead to a relaxation of sexual selection processes. Limited intra-male competition for mates in harvested populations with skewed population structure is suggested by observations of younger males rutting more in synchrony with older males (Mysterud et al. 2008) and a reversal towards female-biased dispersal (Pérez-González & Carranza 2009) in red deer. Lower levels of sexual selection might favour development of lower male body- and trophy sizes, as suggested by growth patterns of moose (Mysterud, Solberg & Yoccoz 2005; Tiilikainen et al. 2010).

Conclusion

  1. Top of page
  2. Summary
  3. Introduction
  4. The mechanisms of harvest selectivity
  5. Limited empirical evidence of directional selection
  6. Harvesting intensity is itself important
  7. Conclusion
  8. Acknowledgements
  9. References

Understanding the mechanisms by which harvesting might affect trait evolution is crucial for management to select efficient mitigative efforts. It is emphasized here that harvesting, although selective, is not always expected to be strongly directional as a result of hunter preferences for large-sized individuals. In meat and recreational harvesting cultures, harvesting pressure, skews in population structure and timing of harvest relative to age at maturity are potentially more important drivers. Although the empirical basis for advice is currently weak, managers aiming to avoid artificial selection should also maintain a ‘natural’ population structure and target a high proportion of individuals that have not reached the age of maturity.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. The mechanisms of harvest selectivity
  5. Limited empirical evidence of directional selection
  6. Harvesting intensity is itself important
  7. Conclusion
  8. Acknowledgements
  9. References

I am grateful to Vidar Holthe, Richard Bischof, Jean-Michel Gaillard, Dave Koons, James Speed, Asbjørn Vøllestad and one anonymous referee for helpful comments.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. The mechanisms of harvest selectivity
  5. Limited empirical evidence of directional selection
  6. Harvesting intensity is itself important
  7. Conclusion
  8. Acknowledgements
  9. References
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