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Guns, Sheep, and Genes: When and Why Trophy Hunting May Be a Selective Pressure

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Recreational Hunting, Conservation and Rural Livelihoods: Science and Practice, 1st edition.
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6
Guns, Sheep, and Genes:
When and Why Trophy Hunting
May be a Selective Pressure
Macro Festa-Bianchet1 and Ray Lee2
1Département de biologie, Université de Sherbrooke,
Québec, Canada
2Foundation for North American Wild Sheep, Wyoming, AZ, USA
Introduction
Recreational hunting is a lucrative economic activity that has been recognised
as one tool to foster the conservation of biodiversity by directing some of the
economic revenue it generates to conservation programs and by demonstrat-
ing to local populations the value of wildlife and habitat protection (Leader-
Williams et al., 2001). Recreational hunters are most interested in killing males
with large secondary sexual characteristics (or trophies) such as horns, antlers
or tusks. A hunter’s willingness to pay for the hunt is typically correlated with
the expectation of harvesting a male with large horns, antlers, or tusks. Because
hunters are willing to pay more to harvest males with larger trophies, trophy
hunting can produce a substantial amount of revenue, which can potentially be
used for conservation (Harris & Pletscher, 2002; Hofer, 2002). For example, in
North America many successful bighorn sheep Ovis canadensis reintroduction
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GUNS, SHEEP, AND GENES 95
programmes have been fi nanced entirely with funds generated through the
auction of trophy ram permits by the Foundation for North American Wild
Sheep. In these auctions, the highest bids are reserved for permits from areas
with a reputation for producing the largest rams.
Overall, there is little question that modern ungulate management, based
on recreational hunting, is sustainable from a demographic viewpoint. In
many countries where large predators have been eliminated, populations of
ungulates are increasing despite high levels of recreational hunting, sometimes
causing loss of biodiversity and economic damage (Côté et al., 2004; Milner
et al., 2006). Economically successful trophy hunting programmes exist in
many parts of the world (Leader-Williams et al., 2001). One area of particular
interest is Central Asia, where several mountain ungulates, but particularly
wild sheep, are the object of increasing interest from trophy hunters and where
both the ungulates and their habitat are threatened by poaching, overgrazing,
and disease transmission by domestic livestock.
However, recreational hunting does not just have a numerical impact on
exploited populations. Such hunting is typically selective, either in terms of
which sex–age classes are killed, or in terms of the specifi c morphological
attributes of the animals that are harvested. Sex- and age-specifi c mortality
caused by recreational hunting is typically very different from that seen in
non-hunted ungulate populations, and therefore it is likely to be different from
the mortality regimes prevalent during most species’ evolution. In particular,
in most hunted populations, the mortality of males and of prime-aged females
is much higher than in populations that are not hunted (Festa-Bianchet, 2003;
Bender et al., 2004; Milner et al., 2006). If hunting is the main cause of adult
mortality, as in many hunted populations of ungulates (Langvatn & Loison,
1999; Bender et al., 2004; Milner et al., 2006), one may expect recreational
hunting to become a selective pressure on life-history patterns. Life-history
evolution and sex- and age-specifi c survival probabilities are intimately linked,
so that a change in mortality patterns is likely to change the fi tness payoffs of
different reproductive strategies. For example, in heavily hunted populations
animals may benefi t by investing as many resources as possible in a reduced
number of breeding opportunities.
Recently, researchers have become increasingly aware that the selective effects
of recreational hunting in general, and of trophy hunting in particular, may
be important (Jachmann et al., 1995; Harris et al., 2002; Coltman et al., 2003;
Festa-Bianchet, 2003; Garel, 2006). In some circumstances, intense removal
of trophy males may select for genetically small horns, antlers or tusks. This
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has been suggested for African elephants Loxodonta africana (Jachmann
et al., 1995), one population of bighorn sheep (Coltman et al., 2003) and one of
European moufl on Ovis aries (Garel, 2006). But overall, very few studies have
examined the potential selective effects of trophy harvest, or of any other
management strategy. Trophy hunting may also have negative demographic
consequences, because males with large horns may also have other fi tness-
related genetic traits that could be selectively removed from a population.
While the potential evolutionary impacts of trophy hunting are worthy of
consideration, there is, again, currently not enough evidence to determine
when they should be seen as a signifi cant concern for conservation.
Any potential selective genetic effects of trophy harvest can be signifi cantly
decreased by lowering harvest pressure to ensure that some males with large
trophies participate in breeding. Good wildlife management programmes –
using reduced season lengths and limiting permits – are designed to regulate
hunting pressure to ensure adequate survival of older age animals. There are
a number of cases demonstrating that trophy size of bighorn sheep has not
diminished through time when populations are properly managed. In this
context, it is important to avoid human-made barriers to gene fl ow, such as
fences or major habitat disruptions, that could prevent movement between
hunted and protected areas. Gene fl ow from protected areas should reduce the
long-term genetic effects of selective removal of large-horned rams through
hunting.
In this chapter we consider under what circumstances hunting may be a
selective pressure leading to changes in gene frequencies in hunted popula-
tions, examine current research needs and suggest management strategies to
reduce the genetic impact of trophy hunting.
The case of Ram Mountain
Ram Mountain is an isolated outcrop in Alberta, Canada, 30 kilometres from
the main range of the Rocky Mountains (Figure 6.1). Since 1971, it has been
the site of numerous wild sheep studies. Some of the most recent studies have
dealt with the potential for artifi cial selection, through trophy hunting, of
bighorn sheep. This potential was increased by a number of factors, of which
possibly the most important was that rapidly growing rams developed horns
large enough to be considered trophies before those horns gave them a repro-
ductive advantage (Hogg & Forbes, 1997; Coltman et al., 2002) (Figure 6.2).
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GUNS, SHEEP, AND GENES 97
Figure 6.1 Location of Ram Mountain, Alberta.
UNITED STATES
ALBERTA
CANADA
Edmonton
km
10000
Calgary
Ram
Mountain
Figure 6.2 Bighorn rams – most horn growth occurs before 5 years of age.
Ram age (years)
Horn length or base (cm)
97
Base
Length
53
10
30
50
70
90
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Rapidly growing horns put a ram at risk of being harvested at four years of
age, but do not give him a mating advantage unless he survived to seven to
eight years (Figure 6.3). Rams with slow-growing horns that did not meet the
legal defi nition of ‘trophy’, on the other hand, could expect to die of old age,
after some of them had reached a high dominance status and obtained a high
mating success. Coltman et al. (2002) also noted that younger rams, using
a variety of mating strategies, sired about 50 per cent of the lambs on Ram
Mountain.
Other characteristics of the bighorn sheep population made it especially
susceptible to artifi cial selection. Harvest by Alberta residents was not limited
by a quota, but only by a defi nition of which rams could be legally hunted in
terms of horn shape and size. In theory, all rams could be shot as soon as their
horns reached 4/5 of a curl (Figure 6.4). The population is small (it ranged
from 26 to 152 adult sheep during the 30-year study) and isolated, therefore it
is susceptible to genetic drift and cannot receive ‘unselected’ immigrants from
protected areas (Hogg et al., 2006). Genetic drift due to the small popula-
tion size, however, cannot currently be distinguished from the selective effect
attributed to hunting.
Figure 6.3 Mating success in relation to horn length and age.
2
15
Relative horn length (cm)
10
5
0
5
10
Mating success
1
2
4
6
8
10
Age
12
14
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GUNS, SHEEP, AND GENES 99
Of course, many factors have been shown to affect the size of secondary
sexual characteristics (horns and antlers). Hengeveld & Festa-Bianchet (2006)
and Plensky (2006) stated that their fi ndings suggest that trophy sheep man-
agement based on minimum horn curl criteria and unlimited entry hunts
may over time favour rams with slow growing horns. Habitat degrada-
tion in the study area was also mentioned by these authors. Indeed, in ear-
lier reports on Ram Mountain (Jorgenson et al., 1993, 1998; Festa-Bianchet
et al., 1997; LeBlanc et al., 2001) reductions in horn size were attributed to
density related decreases in forage availability. Frisina & Frisina (2004) note
that habitat conditions over ride genetic potential when it comes to horn
size. While genetics certainly plays a role in horn size, nutrition typically
dominates.
Reviewing some of the European literature, Geist (2004) noted that declines
in trophy quality of antlers could be reversed with time and careful manage-
ment. Like the authors mentioned above, Geist explains the apparent reduc-
tions in horn size primarily through nutrition. He felt that, contrary to the
fears of Coltman et al. (2003), the apparent decline in horn and body size they
observed was not permanent, and could be reversed.
Figure 6.4 A barely legal four-year-old ram from Alberta. Shot two days after leav-
ing a protected area. The plexiglass is used to determine if the tip of the horn extends
beyond an imaginary straight line drawn from the base of the horn to the tip of the
eye – that makes the ram legal to harvest.
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Research needs
The reason why trophy hunting may be a selective pressure is that hunters are
attracted by the same morphological character that may determine male mat-
ing success. Clearly, to understand the potential impacts of trophy hunting on
the distribution of male mating success, we need to know the relationships
between horn or antler size, mating success and interacting variables such as
age, body mass, and population density. Much of the controversy that fol-
lowed the publication of the paper by Coltman et al. (2003) is attributable to
a lack of understanding of these relationships. They are likely to vary substan-
tially across species, and possibly across populations as well. Sexual selection,
favouring males with large horns or antlers, will counter the potential artifi -
cial selection of trophy hunting. The outcome should depend on the specifi c
relationships between horn or antler size and mating success, the proportion
of males killed by hunters according to their age and horn or antler size, and
probably also on how changes in the age structure of the male population
affect male–male competition (Clutton-Brock et al., 1997).
In the case of bighorn sheep, rams develop large horns a few years before
horns play an important role in both dominance status and mating success.
Importantly, while horn size and mass (both of which are age-related and
affect social rank) play an important role in affecting the mating success of
rams near the top of the dominance hierarchy, there is little evidence that they
affect the mating success of subordinate rams (Hogg & Forbes, 1997; Coltman
et al., 2002). The success of alternative mating strategies adopted by all rams
other than the top two to three in the social hierarchy appears independent of
horn size. Therefore, one cannot simply assume a linear relationship between
horn size and mating success. Factors such as age, mass and the characteristics
of competing rams must be taken into account. Similarly, in red deer Cervus
elaphus both age and antler size play a role in mating success: average antler
mass peaks at about ten years of age and remains high thereafter, but mating
success peaks at eight to eleven years of age, then declines (Kruuk et al., 2002).
The genetic impacts of harvesting a large-antlered male when he is seven or
when he is 12 are, therefore, very different.
Immigration from unselected areas, and the effects of barriers, corridors,
and habitat fragmentation are also worthy of investigation, because immigra-
tion from protected areas should lessen the selective effect of trophy hunting
(Hogg, 2000; Hogg et al., 2006). In species with a high level of male mating
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GUNS, SHEEP, AND GENES 101
skew such as fallow deer Dama dama, removal of a few highly successful males
may have a selective effect, but may also increase the effective population size
by allowing a larger number of males to mate, again depending on the particu-
lar mating system of each population (Apollonio et al., 1989). In other species,
such as white-tailed deer (Odocoileus virginianus), large-antlered males may
not be able to monopolise a large proportion of estrous females, and therefore
lower the potential for artifi cial selection through trophy hunting.
Finally, in some species, such as mountain goats Oreamnos americanus, the
size of the horns may affect harvest decisions by trophy hunters, but may not be
directly involved in mating success, for example if the latter was more depen-
dent on male body mass. Therefore, several interesting avenues of research
on the effects of selective harvests are open, with important implications for
wildlife management in addition to our fundamental interest in understand-
ing evolution in wild populations.
So far, unfortunately, very few studies of ungulates have quantifi ed male
reproductive success, and fewer still have been able to correlate individual
mating success with physical and population attributes (Hogg & Forbes, 1997;
McElligott et al., 2001; Coltman et al., 2002). Without information about
the relationships between horn or antler size and mating success, it is diffi -
cult to assess the potential selective impact of various management strate-
gies. Similarly, there has been little work on long-term changes in horn shape
and size in hunted and non-hunted populations (Coltman et al., 2003; Garel,
2006), and more is needed, particularly in situations where pedigrees are also
available. Long-term monitoring of marked individuals, whose physical char-
acteristics can be measured repeatedly, offers the most promising avenue to
learn more about artifi cial selection through recreational hunting.
Management
Some wildlife managers have long considered the potential impacts of over
utilisation of the renewable wildlife resources they are charged with managing.
When the recreational harvest of wild sheep in the southwestern United States
was fi rst reopened (1952 in Nevada, 1953 in Arizona), after up to a century of
closure, strict regulations were written – addressing season lengths, sex, age
and horn length – to limit the harvest (Lee, 1989a).
In Arizona, harvest protocols were established to provide: (1) ade-
quate ratios of young to old rams; (2) a desired ratio of rams to ewes; and
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(3) a restriction of harvest to no more than ten per cent of the total rams,
nor more than 25 per cent of the rams older than six years of age. Lee (2000)
reported that Arizona’s current sheep management plan calls for the harvest
of 6 per cent of the estimated rams, or 12 per cent of the estimated number of
older age (six and above) rams, whichever number is the lowest. The Boone
and Crockett scoring system measures both the mass and the symmetry of
the horns; it is a measurement of the total number of inches of total horn
length and horn circumference at the base and at each of the quarters, with
differences between the two horns deducted from the total score. Lee (1989b)
showed the marked increases through time in 162 plus and 170 plus point
rams in the harvest in Arizona. These increases were from 12 per cent of the
harvest in 1953–1958 to 46 per cent in 1983–1988 for 162 plus point rams,
and from two per cent of the harvest to 14 per cent for 170 plus point rams.
Lee (1991) illustrates this increase in size for the 1431 rams harvested in the
period 1953–1990. These results demonstrate the long-term sustainability of
these harvest regimes.
In New Mexico’s main populations of Rocky Mountain bighorn sheep on
Wheeler Peak and in the Pecos Wilderness, where the season fi rst opened in
1990, ram harvests were limited to 5 per cent and 8 per cent of the estimated
ram numbers, respectively. Mean ages and scores increased with this level of
harvest. Data from the Pecos Wilderness reports that for the 112 rams har-
vested since 1990, the mean age for the four top scoring rams increased from
8.4 to 9.6 years of age for the periods 1990–1997 and 1998–2006, respectively.
Mean score also increased from 171.1 to 173.9 Boone and Crockett points.
Winkler (1987) listed the methodologies for determining harvest quotas for
six US states and Mexico. These ranged from 8 per cent of the ram population
in New Mexico, to 12 per cent of the rams observed in Utah, to 15 per cent
of the mature rams observed in California. These numbers contrast with the
harvest percentage at Ram Mountain of about 40 per cent of the rams legally
(four-fi fths of a curl or more) available for harvest each year (Coltman et al.,
2003). Of the 57 rams harvested during the study period, 45 were harvested
before reaching eight years of age, and nine were shot as early as four years of
age. This does not represent sustainable trophy hunting.
Organisations such as the Boone and Crockett Club have kept long-term
records of trophies. Frisina & Frisina (2004) reviewed record books to deter-
mine the top 100 bighorn sheep entries. Of these entries, 47 per cent were taken
in the ten decades from 1880 to 1970, while 53 per cent were taken in just the
past two decades 1980s and 1990s. Due to the conservative hunt management
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GUNS, SHEEP, AND GENES 103
strategies, the number of trophy animals is on the rise. While there are many
factors that weigh upon this simple analysis, it is apparent that there have
been no long-term, adverse impacts due to trophy harvest.
While it may theoretically be possible to produce genetic changes due
to selective and intense levels of harvest, in well-managed populations, as
described above, this is not a management issue of signifi cant concern.
Management options
When faced with unclear results and complex relationships between variables,
scientists will typically attempt to increase their sample size. Managers and
conservationists, faced with immediate problems, cannot afford to wait for
more data. They must take decisions based on available evidence. Currently,
the evidence suggesting that trophy hunting has a deleterious effect is very
limited. Nevertheless, while artifi cial selection is not the most pressing threat
facing hunted populations of wild ungulates, that threat should not be ignored,
especially when there are management options readily available that should
prevent it. In this section we discuss some of those options in the light of the
available biological evidence.
We know what needs to be avoided: a situation where males with large horns
or antlers are shot before they can obtain the high breeding success that may
derive from those large weapons. Consequently, harvest should be directed
to the older age classes. In the case of bighorn sheep, that would mean har-
vesting rams aged at least seven to eight years. One practical problem is that
the ability of hunters to assess age in the fi eld varies from species to species:
in ibex Capra ibex, a six-year-old and a ten-year-old look very different even
from a distance, but in bighorn sheep they may not, and in chamois Rupicapra
rupicapra or tahr Hemitragus jemlahicus they do not. On the other hand,
long-term monitoring of the average age of harvested animals is an important
tool for managers. For bighorn sheep, we suggest that a situation where the
average age of harvested rams is less than six years should be a concern. If
the average age is nine years or older, there is much less reason to worry.
The potential selective pressure caused by trophy hunting is likely
to increase with the proportion of trophy males that are harvested. An
unlimited-entry scheme based on a morphological defi nition of what can or
cannot be shot may create a selective pressure favouring rams with slower
growing horns. Therefore, a quota system based on estimates of available
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MARCO FESTA-BIANCHET AND RAY LEE
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trophy males should be used to ensure that an adequate number of large males
survive to an advanced age and hence to decrease the potential selective effects
of hunting. In addition, some agencies are combining quotas with ‘any ram’ as
the legal animal for harvest – this further reduces the pressure on the older-
age-class males.
Research has shown the importance of connectivity among populations for
the exchange of genetic material (Hogg, 2000; Epps et al., 2005). Large pro-
tected areas such as national parks (there are large populations of wild sheep
in Glacier, Grand Canyon, Rocky Mountain and Yellowstone national parks
in the US, the Pinacate Reserve in Mexico, and Banff and Jasper in Canada),
or remote areas with very low hunting pressure, provide potential sources of
unselected immigrants that could limit any selective effects of trophy hunting.
It is therefore important to maintain those protected areas and to ensure that
movement routes are not blocked by barriers such as transportation corridors,
housing developments or fences. The potential selective effects of hunting
are likely to be greater in populations that are small and isolated, and where
the selective pressure is applied over the entire range of occurrence. On the
other hand, park managers should be concerned about the possible effects
that high hunting pressure outside the protected area may have on the genetic
structure of protected populations. Unidirectional gene fl ow from the pro-
tected area to outside may decrease the effective population size in the pro-
tected area (Hogg, 2000).
Finally, we caution against the facile remedy of resorting to artifi cial trans-
locations to counter real or perceived ‘genetic problems. While in a few spe-
cifi c cases of very small and endangered populations genetic rescue is a valid
option (Hogg et al., 2006), systematic supplementation for trophy hunting
purposes is not sustainable and may have deleterious consequences (Tallmon
et al., 2004). Transplanted individuals may break up local adaptations or
introduce new pathogens or parasites. Attempts to artifi cially increase horn
or antler size by introducing animals from populations with larger trophies
are unjustifi ed both from a conservation and an ethical viewpoint.
Conclusions
The conservation of biodiversity cannot ignore social, economic and cultural
aspects, and trophy hunting offers a substantial potential for conservation
benefi ts. Although the potential for artifi cial selection by trophy hunting
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GUNS, SHEEP, AND GENES 105
may be a concern, it pales in comparison to the threats presented by habi-
tat destruction, poaching and diseases. Research on artifi cial selection in har-
vested wildlife population is only beginning to develop. Meanwhile, managers
should continue to consider the potential genetic consequences of different
selective harvesting schemes. Hunter groups should be encouraged to place
more emphasis on the conservation values of hunting and less on the size
of secondary sexual characteristics of harvested animals. More importantly,
however, the promise of trophy hunting as a conservation tool must be more
fully realised, so that a greater share of the proceeds from recreational hunting
in general, and trophy hunting in particular, fi nds its way directly into conser-
vation activities.
As an example of the possibilities of the trophy hunting as a conservation tool,
bighorn sheep numbers have more than tripled in North America during the
past 30 years. This is due primarily to the funds raised by hunter/conserva-
tionists and spent on the conservation of wild sheep.
Recreational hunters have driven one of the most successful conservation
programs in history.
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... Harvest data were analyzed for 12 Wildlife Management Districts (WMD;1,2,3,4,5,6,7,8,9,10,11,and 19) in a 45,793 km 2 area, roughly the northern half of Maine ( Fig. 1). This area contains a high proportion of suitable moose habitat in the form of active commercial forestlands, has had relatively consistent harvest over the study period (1980-2009L. Kantar, pers. ...
... Specifically, there has been no measurable decline in the proportion of harvested bulls ≥6.5 years that would indicate an overall younger age structure due to selective harvest of larger, trophy males (Fig. 2). Maine's current moose population estimate is >70,000 moose, and mean annual harvest has increased from 816 in 1980-1987to 2239in 2005-2009(MDIFW 2011, 2012). Current harvest is only about 3% of the current population estimate, but will probably increase as hunting interest and moose conflicts increase. ...
... Trophy hunting of bighorn sheep is a major socio-economic activity (Festa-Bianchet & Lee, 2009). ...
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Isolation of small populations is expected to reduce fitness through inbreeding and loss of genetic variation, impeding population growth and compromising population persistence. Species with long generation time are the least likely to be rescued by evolution alone. Management interventions that maintain or restore genetic variation to assure population viability are consequently of significant importance. We investigated, over 27 years, the genetic and demographic consequences of a demographic bottleneck followed by artificial supplementation in an isolated population of bighorn sheep (Ovis canadensis). Based on a long‐term pedigree and individual monitoring, we documented the genetic decline, restoration and rescue of the population. Microsatellite analyses revealed that the demographic bottleneck reduced expected heterozygosity and allelic diversity by 6.2 and 11.3%, respectively, over two generations. Following supplementation, first‐generation admixed lambs were 6.4% heavier at weaning and had 28.3% higher survival to 1 year compared to lambs of endemic ancestry. Expected heterozygosity and allelic diversity increased by 4.6 and 14.3% after two generations through new alleles contributed by translocated individuals. We found no evidence for outbreeding depression and did not see immediate evidence of swamping of local genes. Rapid intervention following the demographic bottleneck allowed the genetic restoration and rescue of this bighorn sheep population, likely preventing further losses at both the genetic and demographic levels. Our results provide further empirical evidence that translocation can be used to reduce inbreeding depression in nature and has the potential to mitigate the effect of human‐driven environmental changes on wild population. This article is protected by copyright. All rights reserved.
... While numerous studies of fishes report evidence of evolution induced by intense harvest (reviewed in Hutchings and Fraser 2008), evidence for evolution through selective harvest in terrestrial species remains scarce and controversial (Coltman et al. 2003; Garel et al. 2007; Mysterud 2011; Traill et al. 2014), partly because the statistical techniques used to quantify evolutionary changes using pedigrees in earlier studies have been questioned (Postma 2006; Hadfield et al. 2010). Trophy hunting can be an important component of many conservation programs (Leader-Williams et al. 2001), and its economic revenues are partly driven by expectation of large trophy size (Bianchet and Lee 2009; Crosmary et al. 2013 ). In most of Canada, sport harvest of mountain sheep (Ovis canadensis and O. dalli) rams is based on a phenotypic definition of minimum horn curl that establishes whether or not a ram can be shot, with an unlimited number of permits available to resident hunters (Festa-Bianchet et al. 2014). ...
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The potential for selective harvests to induce rapid evolutionary change is an important question for conservation and evolutionary biology, with numerous biological, social and economic implications. We analyze 39 years of phenotypic data on horn size in bighorn sheep (Ovis canadensis) subject to intense trophy hunting for 23 years, after which harvests nearly ceased. Our analyses revealed a significant decline in genetic value for horn length of rams, consistent with an evolutionary response to artificial selection on this trait. The probability that the observed change in male horn length was due solely to drift is 9.9%. Female horn length and male horn base, traits genetically correlated to the trait under selection, showed weak declining trends. There was no temporal trend in genetic value for female horn base circumference, a trait not directly targeted by selective hunting and not genetically correlated with male horn length. The decline in genetic value for male horn length stopped, but was not reversed, when hunting pressure was drastically reduced. Our analysis provides support for the contention that selective hunting led to a reduction in horn length through evolutionary change. It also confirms that after artificial selection stops, recovery through natural selection is slow. This article is protected by copyright. All rights reserved.
... Moreover, our study provides additional information about interactions between extrinsic and intrinsic factors in large mammalian herbivores and its effects on antler size [25, 26, 29, 44, 67, 76, 77]. Our research is crucial for understanding the role of environmental factors in studies on body size and size of horn-like structures in relation to harvest of males [78, 79]. In addition to considering effects of harvest on the size of horn-like structures, our research demonstrates that density-dependent influences and cohort effects need to be taken into account. ...
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For long-lived species, environmental factors experienced early in life can have lasting effects persisting into adulthood. Large herbivores can be susceptible to cohort-wide declines in fitness as a result of decreases in forage availability, because of extrinsic factors, including extreme climate or high population densities. To examine effects of cohort-specific extrinsic factors on size of adults, we performed a retrospective analysis on harvest data of 450 male black-tailed deer ( Odocoileus hemionus columbianus ) over 19 years in central California, USA. We determined that population density of females had a more dominant effect than did precipitation on body size of males. Harvest of female deer resulted in increases in the overall size of males, even though a 6-year drought occurred during that treatment period. Body size was most influenced by female population density early in life, while antler size was highly affected by both weather early in life and the year directly before harvest. This study provides insights that improve our understanding of the role of cohort effects in body and antler size by cervids; and, in particular, that reduction in female population density can have a profound effect on the body and antler size of male deer.
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Secondary sexual traits (e.g., horns and antlers) have ecological and evolutionary importance and are of management interest for game species. Yet, how these traits respond to emerging threats like infectious disease remains underexplored. Infectious pneumonia threatens bighorn sheep (Ovis canadensis) populations across North America and we hypothesized it may also reduce horn growth in male sheep. We assess the effect of pneumonia on horn size in male bighorn sheep using 12 herd datasets from across the western United States that had horn growth and disease data. Disease resulted in 12–35% reduction in increment (yearly) length and 3–13% reduction in total horn length in exposed individuals. The disease effect was prolonged when pathogens continued to circulate in sheep populations. Further, disease likely delays the age at which horns reach ¾‐curl and prevents achievement of full‐curl. This is further evidenced with 6 of the 12 herds experiencing an increase in average age at harvest following die‐off events. Management of bighorn sheep for horn size and for population maintenance has focused on factors including nutrition, environmental conditions, and genetic diversity. We demonstrate that disease plays an important role in horn size: pneumonia disease outbreak events significantly reduced horn growth in male bighorn sheep, and continued horn stunting occurred when chronically infected individuals remained present in the population.
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Horns, antlers, and other horn-like structures are products of sexual selection, confer reproductive advantages, and are heritable and honest indicators of individual quality. In addition to serving key biological functions, horns and antlers garner societal interest that, when combined with the powerful motivation to acquire trophy animals, likely has spawned a growing hornographic culture fixated on males with exceptional horn-like structures. The concern that harvest of large, fast-growing males may cause evolutionary change to the very trait being sought has been the source of controversy in the popular and scientific literature over the past 2 decades. Mountain sheep (i.e., bighorn and thinhorn; Ovis spp.), possibly the only large ungulates in North America managed almost exclusively as trophy species throughout their ranges, embody this controversy, which has led to polarizing views among scientists and stakeholders as to how mountain sheep should be managed. Our goal in this commentary was to discuss the relative contributions of the key ecological and intrinsic factors that influence horn growth, how those factors might interact with harvest strategies, and identify what determinants of horn size are most amenable to management and most effective in achieving desired outcomes. Despite repeated results demonstrating that age or nutrition frequently override genetic contributions to size of horns, attention has been given to the role of genetics and its relationship to harvest of mountain sheep. Given the hyperbole surrounding trophy management and big horns, we suggest the importance of females in the management of mountain sheep has been largely forgotten. Maternal condition can instigate life-long effects on size and growth of males (via maternal effects), and abundance of females, in turn, affects nutritional limitation within populations through density-dependent feedbacks. If production of males with large horns is an objective, we contend that management programs should, integrate monitoring of nutritional status of populations, and where evidence indicates nutritional limitation through density dependence, seek to regulate abundance and per capita nutrition via harvest of females. We propose that extrinsic regulation (i.e., removal by harvest or translocation) is the most effective way to manage per capita availability of forage resources and, thus, nutritional limitation on growth of males. Not only can female harvest improve growth in body size and horns of males through enhanced nutrition of growing males and their mothers, such management also 1) may yield a nutritional buffer against environmental stochasticity and erratic population fluctuations, 2) be employed in areas where other management alternatives such as habitat manipulation may not be feasible, 3) may reduce frequency or magnitude of epizootic die-offs, and 4) will increase hunter opportunity and involvement in management. Ultimately, we call for greater recognition of the pervasive role of the ewe, and other female ungulates, in the production of trophy males, and that accordingly, females be better integrated into harvest and management programs. © 2017 The Wildlife Society.
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Selective harvest may lead to rapid evolutionary change. For large herbivores, trophy hunting removes males with large horns. That artificial selection, operating in opposition to sexual selection, can lead to undesirable consequences for management and conservation. There have been no comparisons of long-term changes in trophy size under contrasting harvest pressures. We analyzed horn measurements of Stone's rams (Ovis dalli stonei) harvested over 37 years in two large regions of British Columbia, Canada, with marked differences in hunting pressure to identify when selective hunting may cause a long-term decrease in horn growth. Under strong selective harvest, horn growth early in life and the number of males harvested declined by 12% and 45%, respectively, over the study period. Horn shape also changed over time: horn length became shorter for a given base circumference, likely because horn base is not a direct target of hunter selection. In contrast, under relatively lower hunting pressure, there were no detectable temporal trends in early horn growth, number of males harvested, or horn length relative to base circumference. Trophy hunting is an important recreational activity and can generate substantial revenues for conservation. By providing a reproductive advantage to males with smaller horns and reducing the availability of desirable trophies, however, excessive harvest may have the undesirable long-term consequences of reducing both the harvest and the horn size of rams. These consequences can be avoided by limiting offtake.
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To assess the influence of trophy hunting on Mongolian Argali Ovis ammon, we compared the ages of trophy rams (n=64) taken through Mongolia's legal hunting programme with those of mature rams that died of natural causes (n=l 16). A two-sample Kolomogorov-Smirnov test indicated that the distributions of the two groups were different (P=0.001). A two-sample t-test indicated the distributional differences were due, at least in part, to differences in the mean ages between the natural deaths and hunter harvested samples (P=0.001); the distributions were not centered at the same value or the means in the two populations differ. Application of the Central Limit Theorem affirms that the distribution of the sample mean ages for natural death and hunter harvested populations will be approximately normal, making the t-test applicable. The mean age for the natural death sample was 8.7 years (range: 7.0-13.0) compared to 9.4 (range: 7.0-13) for the hunter harvested sample. At the 95% confidence level, the true difference in ages between the trophy kills and natural deaths is between 3 months and 1 year. Since on an average, rams killed by hunters are older than the average age of natural death for mature rams, the legal hunting programme is having little effect on ram age structure. Thirteen years may represent the upper limit of lifespan for wild Mongolian Argali, which is slightly shorter than the life span of North American wild sheep, namely Thinhorn Sheep Ovis dalli and Bighorn Sheep Ovis canadensis.
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We investigated management of wildlife, habitat and the hunting programme in Aksai County, Gansu Province, People's Republic of China, during 1997-2000. Argali Ovis amnion is the focal species both for conservation and hunting. The hunting programme is intended to produce incentives to conserve wildlife and habitat. Poaching, a serious concern throughout western China, has been reduced in recent years in Aksai. Wildlife population trends are unknown because standardized surveys were begun only in 2000. Threats to argali in Aksai include livestock grazing, placer gold mining, and development of a dam, reservoir and aqueduct. The number of hunters participating in the programme (c. 3 per year) could provide considerable funding (c. $60,000 per year), but the allocation of these funds within China has provided too little for conservation at the local level, thus undermining the intended incentive system. Because local wildlife protection officials have been denied both funding and authority to deal with t
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We explore some of the consequences of harvest on population growth rate, age and sex structure in a Norwegian population of red deer Cervus elaphus, using age-structured demographic models. Survival rates were estimated from individuals marked and monitored annually during 1977-1995, and information about reproduction patterns were obtained from hunting material in the same region. The population had an actual doubling time of 14 years, corresponding to a multiplication rate of 1.051. Harvesting led to a reduction of about 10% of the potential multiplication rate that equalled 1.166. Including stochasticity had only a small effect on the population multiplication rate. Due to a high hunting pressure, males had less than a 10% chance of reaching 4.5 years of age and the male-biased harvest strongly biased the sex ratio. Assuming that when the number of females per male increases above a given threshold some females would not manage to mate, we investigated at which level male harvesting could be maintained without having demographic consequences on the population growth rate. We concluded that the hunting pressure on males could probably be increased further but indirect consequences of a strongly biased sex ratio (e.g. on population genetic structure) remain to be studied. Variation in the multiplication rate mainly resulted from the variation in winter calf survival. In its present form the harvesting regime reduces the growth rate and biases the sex and age structure, but does not seem to threaten the population's viability and productivity.
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Trophy hunting is a management goal for many populations of ungulates and has important implications for conservation because of the economic value of trophy males. To determine whether population density affected horn growth of males, a marked population of bighorn sheep (ovis canadensis) in Alberta, Canada, was studied for 27 years. For the first 9 years, population density was kept stable by removing adult females; afterwards, the numbers of ewes and yearlings tripled before beginning to decline. Horns were measured during repeated captures of marked rams. As the number of adult ewes and yearlings increased, ram horns were shorter and thinner because of decreased horn growth before 4 years of age. Some compensatory horn growth may have occurred at 5 years of age. The effects of population density on horn growth ceased when rams left the nursery groups to join all-male groups. Doubling of male numbers had no detectable effect on net annual horn growth of males greater than or equal to 4 years old. Sp
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Sexual dimorphism is an important characteristic of many mammals, but little is known about how environ- mental variables may affect its phenotypic expression. The relationships between population size, body mass, seasonal mass changes, and sexual mass dimorphism were investigated using 22 years of data on individually marked bighorn sheep (Ovis canadensis) on Ram Mountain, Alberta. The number of adult ewes was artificially maintained low from 1972 to 1981 and then allowed to increase. The body mass of males from 0 to 7 years of age was negatively affected by population density. Female body mass was negatively affected by population density up to 2 years of age. As the number of ewes increased, sexual mass dimorphism of sheep aged 2-7 years declined. Population density had a nega- tive effect on seasonal mass changes of young males and females. Density also had a weak but significant positive ef- fect on yearly mass gain of 2-year-old females, suggesting compensatory growth. Females appear to compensate for resource shortages early in life, while males show a lifelong negative effect. We suggest that these sexual differences are due to the greater flexibility of resource allocation to growth or reproduction by females than by males. 1670
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Conservation science is often interested in predicting the future state of populations living in variable environments. Behavioral ecology is often interested in understanding individual behavior in terms of adaptive strategies for dealing with variable ecological, social, or phenotypic states. To the extent that individual behavior is strategic, predictable in different environments, and has population-level consequences, the study of behavior can advance the predictive goals of conservation. Mating system studies, via data on variation in individual reproductive success, have most frequently been applied to conservation in the estimation of effective population size and prediction of genetic trajectories of single populations. However, mating strategy may also influence dispersal, gene flow, and genetic trajectories at spatial scales involving many populations. Mating competition within populations and dispersal will often have opposing effects on effective population size. Future studies of mating systems might aid conservation (i) in evaluating the genetic effects of mating strategy at different spatial scales, and (ii) by providing, in place of point estimates, descriptions of the way in which variation in lifetime reproductive success and population dispersal pattern change in reaction to environmental variation. Such information may allow conservation science to provide better guidance regarding the appropriate spatial scale of habitat protection and restoration and effective strategies for reserve design and re-design.
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Removal of 5 of the 8 most successful males in a fallow deer lek between breeding seasons led to an increase in fighting rate of males and a decrease in mating rate of both sexes. The increase in fighting was likely due to disruption of the social hierarchy. The lower mating rate of females was not due to disruption of copulations by territorial males, nor did it appear to result from disturbance from fights. We suggest that difficulties in mate choice related to an unstable male hierarchy resulted in females either increasing their length of stay in the lek, visiting the lek more than once before mating, or leaving the lek without mating.