Decrease in Horn Size and Increase in Age of Trophy Sheep in Alberta Over 37 Years

Abstract

Long-term data (1974–2011) from harvested bighorn rams (Ovis canadensis) in Alberta, Canada, suggested a reduction in horn size and in the proportion of trophy rams in the provincial population over time. Age at harvest increased over time, suggesting slower horn growth. Rams that experienced favorable environmental conditions early in life had rapid horn growth and were harvested at a younger age than rams with slower horn growth. Guided nonresident hunters did not harvest larger rams than residents, suggesting that few large rams were available. Resident hunter success declined in recent years. Despite an apparently stable population, successive cohorts produced a decreasing harvest of trophy rams. We suggest that unrestricted harvest based on a threshold horn size led to a decline in the availability of trophy rams. That decline is partly an inevitable consequence of selective hunting that removes larger rams. Although our analysis does not establish that evolution of smaller horns caused the observed decline in both horn size and harvest of trophy rams, we suggest that intensive trophy hunting may have artificially selected for a decrease in horn growth rate. © 2013 The Wildlife Society.

Full text

Management and Conservation
Decrease in Horn Size and Increase in Age of
Trophy Sheep in Alberta Over 37 Years
MARCO FESTA-BIANCHET,
1
De´partement de biologie and Centre d’E
´tudes Nordiques, Universite´de Sherbrooke, Sherbrooke, QC
Canada J1K 2R1
FANIE PELLETIER, De´
partement de biologie and Centre d’E
´tudes Nordiques, Universite´de Sherbrooke, Sherbrooke, QC Canada J1K 2R1
JON T. JORGENSON, Alberta Environment and Sustainable Resource Development, Suite 201, 800 Railway Avenue, Canmore,
Alberta Canada T1W 1P1
CHIARASTELLA FEDER, Alberta Environment and Sustainable Resource Development, Fish and Wildlife Division, 4919-51st Street,
Rocky Mountain House, AB Canada T4T 1B3
ANNE HUBBS, Alberta Environment and Sustainable Resource Development, Fish and Wildlife Division, 4919-51st Street, Rocky Mountain House,
AB Canada T4T 1B3
ABSTRACT Long-term data (1974–2011) from harvested bighorn rams (Ovis canadensis) in Alberta,
Canada, suggested a reduction in horn size and in the proportion of trophy rams in the provincial population
over time. Age at harvest increased over time, suggesting slower horn growth. Rams that experienced
favorable environmental conditions early in life had rapid horn growth and were harvested at a younger age
than rams with slower horn growth. Guided nonresident hunters did not harvest larger rams than residents,
suggesting that few large rams were available. Resident hunter success declined in recent years. Despite an
apparently stable population, successive cohorts produced a decreasing harvest of trophy rams. We suggest
that unrestricted harvest based on a threshold horn size led to a decline in the availability of trophy rams. That
decline is partly an inevitable consequence of selective hunting that removes larger rams. Although our
analysis does not establish that evolution of smaller horns caused the observed decline in both horn size and
harvest of trophy rams, we suggest that intensive trophy hunting may have artificially selected for a decrease in
horn growth rate. Ó2013 The Wildlife Society.
KEY WORDS artificial selection, bighorn sheep, horn size, sampling bias, time series, trophy hunting, ungulates.
Many studies have shown the ecological impacts of human
activities, particularly through habitat loss and fragmenta-
tion, overexploitation, and introduction of exotics (Estes
et al. 2011). Less attention has been paid to the possible
evolutionary impacts of human activities, despite recent
evidence that those activities can have substantial con-
sequences on fitness and lead to evolutionary change
(Allendorf and Hard 2009, Darimont et al. 2009). Human
harvests may affect evolution in fish (Edeline et al. 2009,
Hutchings 2009), mammals (Coltman 2008), and plants
(Law and Salick 2005), sometimes reducing population
growth (Swain et al. 2007). Trophy hunting, where males
with large horns, antlers, or tusks are selectively removed,
presents a particularly interesting case of possible artificial
selection because in many species (but not all; see Mainguy
et al. 2009, Rughetti and Festa-Bianchet 2010) harvest
pressure and sexual selection have opposing effects on these
sexually selected traits (Jachmann et al. 1995, Coltman
et al. 2003, Garel et al. 2007, Mysterud 2011).
Long-term, individual-based monitoring of bighorn sheep
(Ovis canadensis) at Ram Mountain, Alberta, revealed that
rams with rapidly growing horns had reduced life expectancy
and reproductive success compared to rams with slow-
growing horns, because hunters selectively removed young,
large-horned rams (Coltman et al. 2003). In that population,
horn size declined substantially over 30 years partly through
density dependence (Jorgenson et al. 1998) and partly
through artificial selection (Coltman et al. 2003, Bonenfant
et al. 2009). However, whether environmental changes and
selective hunting affected ram horn phenotype over the entire
province was unknown. The selective effect of the trophy hunt
would increase with harvest intensity, yet harvest rates are
difficult to calculate because no reliable data exist on the
availability of harvestable rams. The number of licenses sold to
Alberta residents is unlimited, and the harvest is only limited
by the ability of hunters to find trophy rams. The provincial
bighorn sheep population likely increased in the late 1970s
and early 1980s, and has been stable or slightly increasing over
the past 25 years (Jorgenson 2008). Hunter success rate should
vary according to the availability of trophy rams.
Trophy hunts are by definition selective and may have an
evolutionary effect if they target traits with inheritable
components, as is the case for horn size in bighorn sheep
(Coltman et al. 2005). Selective effects are likely to be
Received: 21 February 2013; Accepted: 8 September 2013
Published: 5 December 2013
1
E-mail: m.festa@USherbrooke.ca
The Journal of Wildlife Management 78(1):133–141; 2014; DOI: 10.1002/jwmg.644
Festa-Bianchet et al. Trends in Horn Size and Age of Harvested Bighorns 133

strengthened if hunters cannot harvest small-horned rams,
and if young rams with rapidly growing horns become targets
several years before their large horns improve their
reproductive success (Coltman et al. 2002, 2003). The
definition of trophy sheep prevalent in most of Alberta over
the last 4 decades allows ram horns to reach the legal
minimum of 4/5 curl at 4–6 years of age (Pelletier et al.
2012), yet large horns improve the reproductive success of
rams beginning at 6–7 years of age (Coltman et al. 2002,
Festa-Bianchet et al. 2004). Rams with rapidly growing
horns therefore become vulnerable to hunting 1–3 years
before they experience the fitness benefits of large horns.
Little is known about possible differences in age and size of
trophy animals taken by resident versus nonresident hunters
in North America. Nonresident hunters must engage the
services of professional guides. They may harvest animals
with larger horns, because guides generally have better logistic
organization than resident hunters. In British Columbia,
however, no difference was found in horn size of rams of the
Rocky Mountain ecotype taken by resident and guided
nonresident hunters, whereas nonresidents killed rams of the
California ecotype with slightly smaller horns than those
taken by residents (Hengeveld and Festa-Bianchet 2011). For
roe deer (Capreolus capreolus) in Poland, nonresident hunters
took larger-antlered males than residents, mostly because
they hunted earlier and in areas known to produce deer with
large antlers (Mysterud et al. 2006).
An analysis of sport-harvested bighorn rams in British
Columbia revealed temporal changes consistent with hunter-
induced selection for small horns (Hengeveld and Festa-
Bianchet 2011), similar to results for European mouflon (O.
aries) in France (Garel et al. 2007) and for Iberian wild goat
(Capra iberica) and Barbary sheep (Ammotragus lervia)in
Spain (Pe
´rez et al. 2011). For Iberian wild goat, a decrease in
horn size over 18 years was accompanied by an increase of
about 4 years in the average age of harvested males,
suggesting that males took longer to become what hunters
considered a trophy. The average age of trophy-harvested
Barbary sheep males, however, decreased by about 6 months
over 9 years, possibly because high hunting pressure led to
males being shot as soon as they approached trophy size
(Pe
´rez et al. 2011). In British Columbia, no age trend
was found at harvest for bighorn sheep of the California
ecotype over 28 years. The average age of harvested Rocky
Mountain rams increased by about 0.7 years, yet their
average horn length did not change (Hengeveld and Festa-
Bianchet 2011). Together, temporal trends in age and horn
size of harvested animals may provide information on age-
specific horn growth, although when harvests are based upon
a minimum degree of horn curl, declining trends would be
underestimated by horn measurements of harvest animals
(Pelletier et al. 2012). Analyses of long-term data from
harvested trophy-hunted males, however, remain relatively
rare even though this information is often collected by
management agencies (Wishart 2012). We analyzed records
of more than 7,000 trophy-harvested bighorn rams in
Alberta over 37 years to test for temporal trends in age and
horn size, and compare the size and age of rams harvested by
Alberta residents and by guided nonresident hunters. We
sought to test the hypothesis that, because of high harvest
pressure and a phenotypically defined minimum horn size,
rams harvested in recent years would be older and have
smaller horns than rams harvested a few decades ago.
METHODS
Hunting Regulations and Data Collection
Over the period of data collection (1974–2011), nearly all
populations of bighorn sheep in Alberta outside protected
areas were hunted under a 4/5 minimum-curl regulation.
The season began in late August or early September and
usually closed at the end of October. Any Alberta resident
could purchase a license for trophy sheep and harvest 1 ram
with at least 1 horn where a straight line drawn from the most
anterior point of the base of the horn to the tip of the horn
extended beyond the anterior edge of the eye when viewed in
profile (picture in Pelletier et al. 2012). Rams fitting this
definition are referred to as legal rams. The 4/5-curl
definition of legal ram was adopted in 1968. A resident who
harvested a ram could not buy a trophy sheep license the
following year. In addition, about 80 licenses were available
yearly to nonresidents, who must engage the services of a
guide. Nonresidents were also limited to harvesting legal
rams and could only hunt in specific areas, generally north of
the Bow River, which flows between Banff and Calgary
(Fig. 1). The hunting season for nonresidents opened
Figure 1. Sheep Management Areas (SMAs) in Alberta. Boundary lines
inside each area refer to Wildlife Management Units.
134 The Journal of Wildlife Management 78(1)

approximately 1 week later and closed about 2 weeks earlier
than the season for residents.
Hunters were required to submit the head of harvested
rams for compulsory inspection and measurement. Alberta
Fish and Wildlife personnel estimated the age of the ram
based on horn annuli and measured total length along the
outside curvature and base circumference of both horns.
They also noted the Wildlife Management Unit (WMU)
where the hunter harvested the ram and the hunter’s
residency status.
By definition, legal rams are not a random sample of the
population, because they must have horns describing 4/5 of a
curl. Both size and shape of the horns determine whether or
not a ram can be legally harvested. A dataset of harvested
individuals would underestimate a possible negative trend in
horn size over time because small-horned rams cannot be
harvested (Pelletier et al. 2012). For Dall’s sheep (O. dalli)in
the Yukon and bighorn sheep in British Columbia, horn
growth rate is negatively correlated with harvest age; rams
shot at 4–5 years of age have horn growth in their first few
years of life about 30% greater than those shot at 10 years of
age or older (Loehr et al. 2006, Hengeveld and Festa-
Bianchet 2011). Unfortunately, horn increments were not
measured in Alberta. The bias in the hunter-killed sample
compared to the overall population likely decreases with ram
age, because although only exceptionally well-developed
rams can be legal at 4–5 years of age, most rams are legal by
the time they reach 8 years (see Results Section). Some rams,
however, never reach legal status. Accounting for these biases
in the interpretation of harvest-derived data is essential.
To examine how horn length and ram age affected the
probability of attaining legal status, we used data from 2
long-term studies in Alberta where legal status was assessed
yearly on marked rams by experienced observers. Data on age
and legality were available from Sheep River and Ram
Mountain, whereas data on yearly horn length were only
available from Ram Mountain (Jorgenson et al. 1998).
Data Analyses
The harvest dataset was first checked by Alberta Fish and
Wildlife biologists to remove entries with missing horn
measurements, ram age, or obvious errors, such as wrong
Wildlife Management Unit or harvest outside the hunting
season. We excluded illegally harvested rams (primarily
sheep that did not meet the legal definition) from analyses.
We also excluded rams taken by First Nations, as subsistence
harvest is not restricted by horn size nor based on licensing
requirements. That process removed 7% of entries, leaving
about 7,100 rams of known age and horn size. To account for
possible geographic variation in horn size, we considered 8
Sheep Management Areas (SMA; Fig. 1), delineated by Fish
and Wildlife biologists based on genetic differences and local
knowledge of barriers to movement (Alberta Fish and
Wildlife, unpublished data). Each SMA consisted of several
WMUs, which may vary in regulations such as season
opening date, definition of legal ram, permitted weapons,
and whether or not nonresidents were allowed to hunt.
Generally, rams are larger in the southern than in the
northern half of the province, with the exception of SMA
Cadomin (Fig. 1), which also produces large rams. The
registration database classifies hunters as Alberta residents or
nonresidents. We restricted analyses of effects of hunter
residency to WMUs where hunting by nonresidents was
allowed, during times when both types of hunters could hunt.
No areas are reserved for nonresident hunters. From 1996,
the definition of legal ram was changed from 4/5 to full curl
in 3 WMUs. We excluded from analyses rams harvested in
those units after regulations were changed. We also excluded
rams harvested during late season hunts based on a draw of a
small number of permits that were instituted in a few WMUs
in 2005 and later.
We used the monthly values of the Pacific Decadal
Oscillation averaged from April to September (summer
PDO) to assess the effects of environmental variability on
horn growth (Loehr et al. 2010). Most horn growth occurs
during the first 4 years of life (Jorgenson et al. 1998).
Therefore, to select the time period to consider for PDO
effects, we calculated mean PDO while harvested rams were
in different ranges of age between birth and 4 years (0–4, 1–4,
0–3, 1–3, 0–2, 1–2, and 2–3). We then used the Akaike’s
Information Criterion (AIC) to evaluate which period best
fit horn length and base circumference of rams. As both
measurements are influenced by age at harvest (Hengeveld
and Festa-Bianchet 2011), we compared a model including
PDO with a model including only age at death (Table S1,
available online at www.onlinelibrary.wiley.com). From this
model selection, we retained the average PDO when rams
were aged 1–4 for subsequent analyses.
We analyzed age at death, horn length, and base
circumference using linear mixed effect models (Pinheiro
and Bates 2000) including SMA as a random effect to
account for both regional differences in horn size and
changes in the distribution of the harvest over the years of the
study. To calculate cohort-specific harvest, we summed
harvested rams by year of birth. This analysis included only
cohorts from 1970 to 2000, that would have been completely
harvested over the period of monitoring. Rams born in 1970
would have been 4 years old at the start of the time series,
those born in 2000 would have been 11 years old in 2011.
Rams 12 years old made up only 2.4% of the harvest;
therefore, our analyses are unlikely to be biased by a few very
old rams from the 1999–2000 cohorts that may have been
harvested after 2011. All analyses were implemented in R
version 2.15 (R Development Core Team 2012).
RESULTS
Between 1974 and 2011, harvested rams showed a slight and
nonlinear decrease in horn length, smaller base circumfer-
ence, and an increase in age (Table 1). The temporal trend
in horn length was quadratic, reflecting a slight increase in
1974–1985 and a decrease from about 1986 (Fig. 2A), while
the decrease in base circumference appeared linear (Fig. 2B).
For the entire dataset, horn base circumference and length
were weakly correlated (r¼0.35; P<0.0001). Between 1980
and 2010, horn length for 6-year-old rams decreased by
approximately 3 cm, or 3.5%. Average age increased from
Festa-Bianchet et al. Trends in Horn Size and Age of Harvested Bighorns 135

6.8 years to 7.5 years (Fig. 3A; b¼0.017 0.003, adjusted
R
2
¼0.49, P<0.001), mostly because of a decline in the
proportion of males aged 4 or 5 years, from about 25% in the
1980s to less than 10% in recent years (Fig. 3B; b¼
0.0004 0.001, adjusted R
2
¼0.48, P<0.001). The
database contained only 6 3-year-olds (less than 0.05% of
the total). The summer PDO averaged over years 1–4 had a
positive effect on horn base circumference, a strong negative
effect on age at harvest, and no effect on horn length
(Table 1). Although age at harvest had the expected positive
and quadratic effect on horn length, it had a surprising
negative effect on base circumference (Table 1; Fig. S1).
Repeated measurements of live rams suggest that both horn
length and base increase non-linearly with age, growing at a
slower rate in older rams (Jorgenson et al. 1998).
For residents, the number of licenses sold and total harvest
increased from 1974 to 1984 (Fig. 4). In 1987, the fee
doubled from $20 to $40 and license sales declined. Harvest
of rams by resident hunters did not vary much between 1980
and 1992 and then declined (Fig. 4). Harvest increased with
the number of licenses sold, but success rate declined
(Table 2). Success rate averaged 7.2% and ranged yearly from
5.2% to 11.4%. Both harvest and success rate, however,
declined over the last 15–20 years (Table 2). For example, for
every 2,000 licenses sold in 1974, 124 rams would have been
shot with a success rate of 6.8% (Table 2). For every 2,000
licenses sold in 2010, harvest would have been 107 rams (a
14% decline), for a success rate of 5.5%. Success of
nonresidents averaged 48% (range 26–65%) and showed
no temporal trend (data not shown). Most (mean 79%, yearly
range 68–89%) of the harvest was by residents.
The number of rams harvested from each cohort first
increased for cohorts born through the 1970s, declined for
cohorts born in the 1980s, and remained low for cohorts born
in the 1990s (b¼634 295, P¼0.040, b
2
¼0.160 
0.074, P¼0.039, adjusted R
2
¼0.27). Rams harvested
increased slightly for the most recent cohorts (Fig. 5).
Assuming a constant provincial population of 6,000 bighorn
sheep outside National Parks (Jorgenson 2008), production
of harvested rams dropped by 35%, from 1 ram shot/year per
24 sheep for cohorts born in 1975–1982 to 1 ram per 37
sheep for cohorts born after 1990.
Residents harvested rams with larger bases and longer
horns, but slightly younger than those taken by nonresidents
(Table 3). As reported for the overall analysis (Table 1), the
average summer PDO when rams were aged 1–4 years was
associated with decreasing age at harvest and increasing base
circumference.
The probability to fit the definition of legal ram increased
with both horn length and base circumference but was most
closely associated with length (Fig. 6). Horn length and
circumference explained respectively 42% and 19% of the
deviance (horn length: 0.25 0.03, Z¼7.593, P<0.001,
circumference: 0.41 0.07, Z¼6.151, P<0.001). At Ram
Mountain, a third of rams 8 years old were not legal,
whereas only 5% of Sheep River 8-year-old rams were not
Table 1. Temporal trends in horn length (cm), horn base circumference
(cm), and age at death (years) for bighorn rams shot in Alberta, 1974–2011.
Estimates are from linear mixed effect models accounting for Sheep
Management Area as a random effect. To assess the effects of
environmental variability, we calculated the average summer Pacific
Decadal Oscillation (PDO) while rams were aged between 1 and 4 years.
Sample sizes differ as not all measurements were available for all rams.
Variables Coeff. SE PN
Horn length 7,037
Age 4.501 0.220 <0.001
Age
2
0.149 0.014 <0.001
Harvest year 21.802 3.934 <0.001
Harvest year
2
0.005 0.001 <0.001
Horn base 7,030
Age 0.047 0.013 <0.001
Harvest year 0.007 0.002 <0.001
PDO 0.228 0.039 <0.001
Age 7,107
Harvest year 8.860 1.208 <0.001
Harvest year
2
0.002 0.0003 <0.001
PDO 0.559 0.053 <0.001
Figure 2. Mean (SE) horn length (A) and horn base circumference (B) of 6-year-old bighorn sheep rams harvested in Alberta between 1974 and 2011.
Results were similar for other age classes, 6 years was the modal age at harvest.
136 The Journal of Wildlife Management 78(1)

legal (Fig. S2). These trends are biased because once rams
reach legal status, some are harvested and exit the sample.
For that reason, we do not provide a statistical analysis.
Given that rams with only exceptionally developed horns
could be legal at a young age, variability in horn length of
harvested rams should increase with increasing age at
harvest. As expected, coefficients of variation increased with
age for horn length, but we found no clear effect of age on
base circumference (Fig. S3).
DISCUSSION
Our results are consistent with the hypothesis that selective
hunting contributed to a decrease in both horn size and
availability of trophy rams (Coltman et al. 2003, Bonenfant
et al. 2009, Festa-Bianchet and Lee 2009). The size and
number of trophy rams harvested increased during the late
1970s and early 1980s, then declined. The initial increase in
horn size between 1974 and 1980 was partly explained by an
increase in PDO, which had very low values in 1970–1975.
Better access to previously remote areas because the road
network expanded, may also have allowed hunters to harvest
some large rams. Resident hunter success rate also declined
over recent years. Although the entire provincial bighorn
sheep population was not regularly censused, partial censuses
and expert opinion by wildlife biologists suggest that it likely
increased between 1974 and 1985, possibly accounting for
the initial increase in harvest. The provincial bighorn
population has been stable over the last 25 years
(Jorgenson 2008), but trophy ram harvest declined over
that period. Multiple lines of evidence from our analyses
suggest that the decline in harvest is caused by a decrease in
the number of rams reaching legal status, as a result of a
decline in horn growth rate.
The decrease over time in the proportion of harvested rams
aged 4 or 5 years implies that horn growth slowed, so that
rams take longer to become legal. An increase in age over
time was reported for trophy-harvested Iberian wild goats
(Pe
´rez et al. 2011) in Spain and Rocky Mountain bighorn
rams in British Columbia (Hengeveld and Festa-
Bianchet 2011). In contrast, reduced postwar harvest of
red deer (Cervus elaphus) in Hungary led to an increase in
antler length and number of tines of the very largest
harvested stags (Rivrud et al. 2013). Because the antlers of
only the largest stags were measured, however, it is unclear
Figure 3. (A) Average age (SE) of bighorn rams harvested in Alberta, 1974–2011. (B) Proportion of rams (SE) aged 4 or 5 years in the harvest each year.
Figure 4. Number of trophy sheep licenses sold and harvest of bighorn rams
in Alberta, 1974–2011, resident hunters only.
Table 2. Yearly harvest of bighorn rams and success rate by Alberta
residents as a function of the number of permits sold and year, 1974 to
2011.
Variables Coeff. SE P
Harvest
Licenses sold 0.032 0.007 <0.001
Harvest year 0.059 0.012 <0.001
Harvest year
2
0.147 0.032 <0.001
Success rate
Licenses sold 0.002 0.0003 <0.001
Harvest year 24.90 6.439 <0.001
Harvest year
2
0.006 0.002 <0.001
Festa-Bianchet et al. Trends in Horn Size and Age of Harvested Bighorns 137

how different harvest strategies affected antler size at the
population level (Pelletier et al. 2012).
In Alberta, the alternative that hunters stopped harvesting
legal young rams seems unlikely given the very low success
rate. However, data on the proportion of hunters that choose
not to harvest a legal ram that they may encounter would be
informative. Slower horn growth rate also may result from
high population density, as reported at Ram Mountain in the
1980s and early 1990s (Jorgenson et al. 1998). That
explanation appears unlikely, because the effect on horn
growth at Ram Mountain required a doubling of population
size, whereas in general the provincial population appears
stable. Changes in sex ratio through intense removals of
mature males may also affect the level of male–male
competition, reducing sexual selection (Mysterud et al.
2008), but we have no reliable data on temporal trends in sex
ratio for the provincial population.
An increase in age at which rams become legal would
reduce the availability of legal rams, because more would be
lost to natural mortality. Monitoring of marked rams in 2
populations in Alberta suggests that 19–27% of 4-year-olds
would die of natural causes before reaching age 6 (Loison
et al. 1999). After accounting for age, our analyses also
suggest a decline in horn length and base circumference over
the last 2 decades. Compared to 20–30 years ago, a greater
proportion of rams may possibly now die without their horns
ever attaining legal status; however, we currently cannot test
this hypothesis. We underscore that rams that do not reach
legal status cannot enter our sample, because harvesting them
is illegal. We previously showed (Pelletier et al. 2012) that
horn measurements of harvested rams would underestimate a
decrease in horn size by 10–15%. Therefore, the actual
decrease in horn size of bighorn rams in Alberta in recent
years was likely greater than suggested by our results.
The negative temporal trends in horn size remained after
accounting for age and summer PDO; the latter had a
positive effect on horn base circumference of young rams.
We were puzzled to find no effect of PDO on horn length,
considering that a positive effect was reported for Dall’s
sheep in the Yukon (Loehr et al. 2010). Because the
definition of legal rams is affected mostly by horn length, and
a much greater proportion of asymptotic size is reached by
age 4–5 for circumference than for length (Jorgenson
et al. 1998), in the harvest sample the effects of PDO are
more easily detectable on base circumference than on horn
length. The effects of PDO are likely not detected on horn
length because more rams from cohorts that experienced
favorable environmental conditions become legal at an early
age (Table 1). Rams that develop rapidly are harvested at a
younger age (Loehr et al. 2006, Hengeveld and Festa-
Bianchet 2011). At Ram Mountain, rams born at low
population density had rapid horn growth and many were
harvested when aged 4 or 5 years (Jorgenson et al. 1998).
Rams from cohorts that experienced favorable environmental
conditions (high average PDO) during their first 4 years were
harvested at a younger age than those that developed more
slowly. Rams from cohorts experiencing rapid early growth
that survive to be harvested at ages 6 or older would be
smaller than the average for their cohort and would appear to
have average horn length for these ages. Harvest-induced
survival bias may be weaker in cohorts that developed under
poor environmental conditions, because few rams from those
cohorts are legal at 4 and 5 years of age. Regardless of
environmental conditions during early development, only
rams with exceptionally large horns are legal at 4 or 5 years
(Fig. S2). That contention is supported by the positive
relationship between coefficient of variation in horn length
and age at harvest (Fig. S3); among young rams (aged 4–6
years), those with average-sized horns cannot enter the
Figure 5. The number of rams harvested in Alberta in 1975–2011 from
cohorts born from 1970 to 2000.
Table 3. Effect of hunter residency on age, horn base circumference, and
horn length of a subset of bighorn rams harvested in Alberta, 1974 to 2011.
Estimates are from linear mixed effects models including as random effects
the 5 Sheep Management Areas where nonresident hunters could hunt.
Analyses are restricted to 4,392 rams taken when both groups of hunters
were allowed to hunt. To assess the effects of environmental variability, we
calculated the average summer Pacific Decadal Oscillation (PDO) while
rams were aged between 1 and 4 years.
Variables Coeff. SE P
Age
Harvest year 9.262 1.583 <0.001
Harvest year
2
0.002 0.0004 <0.001
PDO 0.550 0.069 <0.001
Hunter origin
a
0.163 0.059 0.040
Horn base
Age 0.054 0.016 <0.001
Harvest year 3.657 1.714 0.033
Harvest year
2
0.001 0.0004 0.033
PDO 0.166 0.074 0.025
Hunter origin
a
0.308 0.063 <0.001
Horn length
Age 4.018 0.290 <0.001
Age
2
0.120 0.018 <0.001
Harvest year 20.541 3.518 <0.001
Harvest year
2
0.005 0.001 <0.001
Hunter origin
a
1.305 0.190 <0.001
a
The category of reference is nonresidents.
138 The Journal of Wildlife Management 78(1)

harvested sample. For example, the average horn length of
harvested 4-year-olds in Alberta was 16% greater than that of
all 5-year-olds live-captured at Ram Mountain. With
increasing age, more opportunities occur for rams with
different horn lengths to reach legal status, and the
coefficient of variation increases. The effect of age on
horn length in harvested rams should be weaker than in the
overall population; young rams would be a positively biased
sample, and old rams a negatively biased sample as longevity
declines with horn size because of the trophy hunt
(Bonenfant et al. 2009).
In British Columbia, bighorn rams with rapid horn growth
early in life were harvested at younger ages than those with
slower horn growth (Hengeveld and Festa-Bianchet 2011).
Hunters selectively removed the rams with the largest horns
before they could reach an age at which large horns increase
mating success (Coltman et al. 2002). Similar results from
Dall’s sheep in the Yukon support this contention and
suggest that hunter selection directly opposes sexual selection
(Loehr et al. 2006). Selective harvest may have led to the
artificial evolution documented in the isolated population
at Ram Mountain (Coltman et al. 2003). Because annuli
measurements were not available for rams harvested in
Alberta, we could not compare early horn growth with age at
harvest. The weak but negative relationship between harvest
age and horn base circumference, however, suggests that a
similar age-related selection may occur in Alberta; rams with
rapidly growing horns are harvested at a young age, whereas
those with smaller horns survive longer. Horn base
circumference normally increases through life (Jorgenson
et al. 1998).
Unexpectedly, nonresident hunters harvested rams with
horns slightly smaller than those harvested by residents.
Nonresident hunters employ guides and typically have access
to more remote areas. We suggest that they did not take
larger-horned rams than those shot by residents because few
large rams are available. Data on the proportion of legal rams
that survive the hunting season would be very informative
but are unavailable. The positive relationship between
licenses sold and total harvest by residents suggests that
some legal rams survive the hunting season, otherwise the
relationship between number of licenses and harvest would
reach a plateau. On the other hand, the negative effect of
number of licenses on success rate implies that hunters
compete for a limited pool of legal rams. More importantly,
once the number of licenses was accounted for, resident
success rate declined in recent years, suggesting a decrease in
the availability of legal rams.
A recent analysis of the Boone & Crockett records revealed
that the size of bighorn sheep horns submitted in recent
decades showed a slight increase (Monteith et al. 2013).
Submission to the Boone & Crockett book, however, is
voluntary and listing requires a minimum score. Rams
submitted for listing come from multiple jurisdictions with
different harvesting regimes and hunting pressure. Only
exceptionally large horns are listed, and ram age is not
included. Hunter-harvested rams overall provide a biased
estimation of temporal trends in horn size (Pelletier
et al. 2012), and the Boone & Crockett records are even
more biased. Their relevance to our results or to any actual
trends in horn or antler size of different species is unknown.
For example, our analysis of 7,100 harvested rams reveals a
temporal decline in horn size in bighorn rams in Alberta, yet
1 ram harvested in a previously unhunted area in central
Alberta in 2000 had the highest score ever recorded for a
Rocky Mountain bighorn sheep.
The relationships between horn size and legal status are
worthy of additional investigation because selective hunting
may affect the evolution of both horn size and shape (Festa-
Bianchet and Lee 2009). Although length is a major
determinant of legal status, differences in shape may also
affect it, as suggested for European mouflon (Garel
et al. 2007) and by the wide overlap of legal and not legal
rams with horns of 66–80 cm (Fig. 6A). Future research
Figure 6. Probability that a bighorn ram will be legal as a function of (A) horn length and (B) horn base circumference. Data are from 296 marked rams at Ram
Mountain, Alberta (1983–2006).
Festa-Bianchet et al. Trends in Horn Size and Age of Harvested Bighorns 139

should assess whether horn length varied according to the
age at which a ram first became legal, because as horns grow,
they may change in shape as well as in size.
Cohorts born in the late 1970s generally led to greater ram
harvests than more recent cohorts, and many of the rams
from earlier cohorts were harvested at young ages. Harvest
biases affect cohort analyses as well; rapid horn growth will
lead to more harvested rams because more rams will become
legal at a young age, and be shot before they are exposed to
much natural mortality. Differences in horn growth among
cohorts will also bias the apparent relationship between age
and horn size because more large rams will be shot early in
life from cohorts with rapid growth. Data on horn size for
older rams may therefore originate mostly from slow-
growing cohorts.
The idea that killing large-horned rams may favor small-
horned rams remains controversial (Loehr et al. 2006). In
some ungulates, relationships between horn or antler size,
male age, and reproductive success are not as strong as in wild
sheep, so that the evolutionary effects of selective removals of
large-horned males may be weak (Rughetti and Festa-
Bianchet 2010, Rivrud et al. 2013). Some analyses in
Coltman et al. (2003) may have overestimated the genetic
component of the temporal decline in horn size
(Postma 2006). We attempted to control for environmental
conditions during early development by using summer PDO.
Our results suggest that favorable environmental conditions
during the first 4 years of life lead to a decrease in the age at
which rams are harvested, possibly increasing the selective
effect of the trophy hunt. To properly manage bighorn sheep,
we must distinguish between selection and evolution.
Although our analysis does not establish that evolution of
smaller horns is responsible for the observed decline in both
horn size and harvest of trophy rams in Alberta, it implicates
artificial selection as 1 cause of those declines.
MANAGEMENT IMPLICATIONS
Unlimited availability of resident licenses, combined with a
legal minimum horn curl definition to limit harvest may or
may not have evolutionary consequences, but inevitably leads
to selection; rams surviving the hunt on average have smaller
horns than the pre-hunt population. Rams typically can
become legal at any age after 4 years, and horn size increases
with age. Therefore, a heavily hunted population with low
escapement will have fewer and smaller rams than one that is
lightly hunted. The rut starts in late November, so only rams
that survive the hunting season can breed. The evolutionary
effect of selective hunting would be correlated with harvest
pressure on large-horned rams, and may be partly countered
by post-hunt immigration from refuge areas (Tenhumberg
et al. 2004). The minimum-curl regulation with unrestricted
entry allows any Alberta residents to hunt bighorn sheep, and
assumes that escapement of adult males will avoid any impact
on population dynamics. Our analysis suggests that this
management strategy reduces the availability of trophy rams
and may have an undesirable genetic impact. Adaptive
management would involve a reduction in ram harvests, in
line with bighorn management in other jurisdictions (Festa-
Bianchet and Lee 2009). Our work underlines the usefulness
of long-term records of age and horn size of harvested bovids,
which would be improved by measuring horn increments in
addition to length and circumference. Even when harvested
animals are a biased sample, once that bias is taken into
account, long-term monitoring allows the detection of trends
in age and horn size that are of interest to managers.
ACKNOWLEDGMENTS
We gratefully acknowledge the support of the Natural
Sciences and Engineering Research Council of Canada for
our long-term research in evolutionary ecology. Funding was
also provided by the Universite
´de Sherbrooke, the Alberta
Conservation Association, and Alberta Fish and Wildlife.
We are particularly grateful to the Alberta Fish and Wildlife
personnel who measured horns and entered data. Critical
comments from E. Bruns, D. Coltman, A. Mysterud, and K.
Smith improved an earlier draft of the manuscript.
LITERATURE CITED
Allendorf, F. W., and J. J. Hard. 2009. Human-induced evolution caused by
unnatural selection through harvest of wild animals. Proceedings of the
National Academy of Sciences 106:9987–9994.
Bonenfant, C., F. Pelletier, M. Garel, and P. Bergeron. 2009. Age-
dependent relationship between horn growth and survival in wild sheep.
Journal of Animal Ecology 78:161–171.
Coltman, D. W. 2008. Evolutionary rebound from selective harvesting.
Trends in Ecology and Evolution 23:117–118.
Coltman, D. W., M. Festa-Bianchet, J. T. Jorgenson, and C. Strobeck.
2002. Age-dependent sexual selection in bighorn rams. Proceedings of the
Royal Society of London B 269:165–172.
Coltman, D. W., P. O’Donoghue, J. T. Jorgenson, J. T. Hogg, and M.
Festa-Bianchet. 2005. Selection and genetic (co)variance in bighorn sheep.
Evolution 59:1372–1382.
Coltman, D. W., P. O’Donoghue, J. T. Jorgenson, J. T. Hogg, C. Strobeck,
and M. Festa-Bianchet. 2003. Undesirable evolutionary consequences of
trophy hunting. Nature 426:655–658.
Darimont, C. T., S. M. Carlson, M. T. Kinnison, P. C. Paquet, T. E.
Reimchen, and C. C. Wilmers. 2009. Human predators outpace other
agents of trait change in the wild. Proceedings of the National Academy of
Sciences 106:952–954.
Edeline, E., A. Le Rouzic, I. J. Winfield, J. M. Fletcher, J. B. James, N. C.
Stenseth, and L. A. Vøllestad. 2009. Harvest-induced disruptive selection
increases variance in fitness-related traits. Proceedings of the Royal Society
B 276:4163–4171.
Estes, J. A., J. Terborgh, J. S. Brashares, M. E. Power, J. Berger, W. J. Bond,
S. R. Carpenter, T. E. Essington, R. D. Holt, J. B. C. Jackson, R. J.
Marquis, L. Oksanen, T. Oksanen, R. T. Paine, E. K. Pikitch, W. J.
Ripple, S. A. Sandin, M. Scheffer, T. W. Schoener, J. B. Shurin, A. R. E.
Sinclair, M. E. Soule
´, R. Virtanen, and D. A. Wardle. 2011. Trophic
downgrading of Planet Earth. Science 333:301–306.
Festa-Bianchet, M., D. W. Coltman, L. Turelli, and J. T. Jorgenson. 2004.
Relative allocation to horn and body growth in bighorn rams varies with
resource availability. Behavioral Ecology 15:305–312.
Festa-Bianchet, M., and R. Lee. 2009. Guns, sheep and genes; when and
why trophy hunting may be a selective pressure. Pages 94–107 in B.
Dickson, H. J, and B. Adams, editors. Recreational hunting, conservation
and rural livelihoods: science and practice. Wiley-Blackwell, London,
United Kingdom.
Garel, M., J.-M. Cugnasse, D. Maillard, J.-M. Gaillard, A. J. M. Hewison,
and D. Dubray. 2007. Selective harvesting and habitat loss produce long-
term life history changes in a mouflon population. Ecological Applications
17:1607–1618.
Hengeveld, P. E., and M. Festa-Bianchet. 2011. Harvest regulations and
artificial selection on horn size in male bighorn sheep. Journal of Wildlife
Management 75:189–197.
140 The Journal of Wildlife Management 78(1)

Hutchings, J. A. 2009. Avoidance of fisheries-induced evolution: manage-
ment implications for catch selectivity and limit reference points.
Evolutionary Applications 2:324–334.
Jachmann, H., P. S. M. Berry, and H. Imae. 1995. Tusklessness in African
elephants—a future trend. African Journal of Ecology 33:230–235.
Jorgenson, J. T. 2008. Rocky mountain bighorn sheep status report—
Alberta. Biennial Symposium of the Northern Wild Sheep and Goat
Council 16:30–36.
Jorgenson, J. T., M. Festa-Bianchet, and W. D. Wishart. 1998. Effects of
population density on horn development in bighorn rams. Journal of
Wildlife Management 62:1011–1020.
Law, W., and J. Salick. 2005. Human-induced dwarfing of Himalayan snow
lotus, Sassurea laniceps (Asteraceae). Proceedings of the National Academy
of Sciences 102:10218–10220.
Loehr, J., J. Carey, M. Hoefs, J. Suhonen, and H. Ylo
¨nen. 2006. Horn
growth rate and longevity: implications for natural and artificial selection
in thinhorn sheep (Ovis dalli). Journal of Evolutionary Biology 20:818–
828.
Loehr, J., J. Carey, R. B. O’Hara, and D. S. Hik. 2010. The role of
phenotypic plasticity in responses of hunted thinhorn sheep ram horn
growth to changing climate conditions. Journal of Evolutionary Biology
23:783–790.
Loison, A., M. Festa-Bianchet, J.-M. Gaillard, J. T. Jorgenson, and J.-M.
Jullien. 1999. Age-specific survival in five populations of ungulates:
evidence of senescence. Ecology 80:2539–2554.
Mainguy, J., S. D. Co
ˆte
´, M. Festa-Bianchet, and D. W. Coltman. 2009.
Father-offspring phenotypic correlations suggest intralocus sexual conflict
for a fitness-linked trait in a wild sexually dimorphic mammal. Proceedings
of the Royal Society of London B 276:4067–4075.
Monteith, K. L., R. A. Long, V. C. Bleich, J. R. Heffelfinger, P. R.
Krausman, and R. T. Bowyer. 2013. Effects of harvest, culture and climate
on trends in size of horn-like structures in trophy ungulates. Wildlife
Monographs 183:1–26.
Mysterud, A. 2011. Selective harvesting of large mammals: how often
does it result in directional selection? Journal of Applied Ecology 48:
827–834.
Mysterud, A., C. Bonenfant, L. E. Loe, R. Langvatn, N. G. Yoccoz, and
N. C. Stenseth. 2008. The timing of male reproductive effort relative to
female ovulation in a capital breeder. Journal of Animal Ecology 77:
469–477.
Mysterud, A., P. Trjanowski, and M. Panek. 2006. Selectivity of harvesting
differs between local and foreign roe deer hunters: trophy stalkers have the
first shot at the right time. Biology Letters 2:632–635.
Pelletier, F., M. Festa-Bianchet, and J. T. Jorgenson. 2012. Data from
selective harvests underestimate temporal trends in quantitative traits.
Biology Letters 8:878–881.
Pe
´rez, J. M., E. Serrano, M. Gonzale
´z-Candela, L. Le
´on-Vizcaino, G. G.
Barbera, M. A. de Simon, P. Fandos, J. E. Granados, R. C. Soriguer, and
M. Festa-Bianchet. 2011. Reduced horn size in two wild trophy-hunted
species of Caprinae. Wildlife Biology 17:102–112.
Pinheiro, J. C., and D. M. Bates. 2000. Mixed-effects models in S and S-
PLUS. Springer-Verlag, New York, New York, USA.
Postma, E. 2006. Implications of the difference between true and predicted
breeding values for the study of natural selection and micro-evolution.
Journal of Evolutionary Biology 19:309–320.
R Development Core Team. 2012. R: a language and environment for
statistical computing. R Foundation for Statistical Computing, Vienna,
Austria.
Rivrud, I. M., C. Sonkoly, R. Lehoczki, S. Csanyi, G. O. Storvik, and A.
Mysterud. 2013. Hunter selection and long-term trend (1881–2008)
of red deer trophy sizes in Hungary. Journal of Applied Ecology 50:
168–180.
Rughetti, M., and M. Festa-Bianchet. 2010. Compensatory growth limits
opportunities for artificial selection in Alpine chamois. Journal of Wildlife
Management 74:1024–1029.
Swain, D. P., A. F. Sinclair, and J. M. Hanson. 2007. Evolutionary response
to size-selective mortality in an exploited fish population. Proceedings of
the Royal Society B-Biological Sciences 274:1015–1022.
Tenhumberg, B., A. J. Tyre, A. R. Pople, and H. P. Possingham. 2004. Do
harvest refuges buffer kangaroos against evolutionary responses to selective
harvesting? Ecology 85:2003–2017.
Wishart, W. 2012. Bighorns and little horns revisited. Biennial Symposium
of the Northern Wild Sheep And Goat Council 15:28–32.
Associate Editor: David Euler.
SUPPORTING INFORMATION
Additional supporting information may be found in the
online version of this article at the publisher’s web-site.
Festa-Bianchet et al. Trends in Horn Size and Age of Harvested Bighorns 141