Wild Sheep Horn Growth: The Evolution of Science on the Evolutionary Impacts of Harvest

A January 2016 publication in Evolutionary Applications provides further evidence that intense selective hunting leads to artificial evolution in horn size¹. This is the latest in a series of publications since 2003 that address the potential for evolutionary impacts of trophy hunting in wild sheep, based on long-term data sets initiated in the 1970s. Each new insight has countered criticisms, reinforcing the conclusion that, like it or not, intense trophy hunting causes evolution of small horns in mountain sheep².


Harvest does not necessarily produce evolutionary change in all exploited species. The key is that there must be a link between traits that are naturally favoured in evolutionary processes and the artificial forces imposed on traits targeted in selective harvest. In wild sheep, the question of whether or not hunting can drive evolutionary change is particularly compelling because natural and artificial selection are opposing forces on the same physical trait: large horns.

A crucial point in understanding evolution in horn size is knowledge of how horn growth and the social dynamics of wild sheep are linked to natural selection in reproductive success. The strength of these linkages is what makes artificial selection relevant to wild sheep management – more so than for other species which don’t have the same linkages between social dynamics and natural selection.

Evidence for the trade-offs between natural and artificial selection on wild sheep horn growth has been unfolding over 30 years of detailed observation and analytical research. Earlier work unraveled the intricacies of bighorn population ecology, physiology, social behaviour, reproductive success, and heritability of horn size, leading more recently to questions about whether hunting regulations sufficiently protect dominant breeders. The foundations for this line of inquiry are long-term data sets that describe the lineage and life histories of generations of wild sheep, and harvest statistics collected over decades.

For an in-depth look at the research, see the chronology of published articles at the end of this post. Here is the abridged version of the story.




Wild sheep populations are socially-structured (there are patriarchs, matriarchs, and followers) and polygynous (one male will breed many females). Horn size is a hereditary trait, and large horns in older males are directly linked to social rank and reproductive success. Dominant males monopolize mating opportunities; younger and smaller males participate in the rut but are less successful in terms of number of offspring annually.

While younger males may collectively father about half the lambs born each year, dominant breeders are a few large-horned, older (age 7+ yrs) rams with very high reproductive success. In a population of 100–120 sheep at Ram Mountain, Alberta, the most dominant male fathered one third (35.5%) of lambs annually, producing a quarter (26.1%) of all lambs over a 6 yr study of breeding success. In wild sheep, high quality males have larger horns, higher mating success, higher survival rates, higher offspring birth weights, and more male offspring – loaded with “good genes”.




Most hunting regulations are based on horn size and require a minimum amount of horn growth before the sheep can be harvested. Typically, this means that hunting is restricted to males with minimum 3/4, 4/5th or Full Curl horns, reinforcing why legal harvest of wild sheep is often referred to as “trophy hunting”. Originally, the rationale was that minimum curl regulations would protect sheep until they were mature, hunters would have opportunities for coveted larger animals, and hunters would be taking older males that had lower natural survival rates.

The problem is that these hunting regulations contradict sheep biology; the evolutionary forces that promote reproductive success for large, older rams are opposed by hunting pressure on large, older rams. Because more than 75% of horn growth occurs in the first few years of life, a fast-growing ram can be legal to harvest at 3 or 4 yrs old under 3/4, 4/5th and Full Curl minimum regulations, but wouldn’t have maximum breeding success until at least 7 yrs of age. More importantly – and this is the key point here – high harvest pressure under minimum curl regulations will remove fast-growing rams before they get maximum reproductive opportunity. The result? Documented declines in average horn size over time in areas with high harvest pressure.


Critics who dismiss this work as irrelevant or interpret the findings as advocacy for anti-hunting agendas are missing the point. The potential for evolutionary changes in hunted species has biological, social and economic implications. It is worthwhile understanding the trade-offs imposed by harvest regulations, so that management can focus on long-term conservation goals, and harvest decisions in the field can build on a deeper appreciation of wild sheep population dynamics.


What can hunters do to reduce the potential for harvest impacts?


Given the choice between 2 sheep that have the same horn length, choose the older sheep. Taking an older, slower-growing sheep during your hunt will have less impact on population genetics than taking a fast-growing younger sheep. Carl Sorensen provides the best advice I’ve seen on learning to read age and horn growth differences in the field; you can find his article here³. His article is specific to Stone’s sheep, a species of thinhorn. Thinhorn and bighorn differ slightly in horn shape, but the same horn growth principles apply to both.


Comments? Questions? Leave a reply below


Web links
  1. http://onlinelibrary.wiley.com/doi/10.1111/eva.12358/abstract Pigeon, G., Festa-Bianchet, M., Coltman, D. W. and Pelletier, F. (2016). Intense selective hunting leads to artificial evolution in horn size. Evolutionary Applications.
  2. Marco Festa-Bianchet’s summary www.ecoevoevoeco.blogspot.ca/2016/01/like-it-or-not-intense-trophy-hunting.html (Jan 25 2016)
  3. Carl Sorensen’s article http://bchuntingblog.com/blog/2012/06/16/judging-stones-sheep-in-the-field/


Evolution of the Research

Reproductive success of dominant males
Ram reproductive success was first quantified in two populations in Alberta and one in Montana (Hogg and Forbes 1997, Coltman et al. 2002). At the time, these were the only published data on male mating success of any mountain ungulate.
Mating in bighorn sheep: frequent male reproduction via a high-risk ‘unconventional’ tactic. Hogg, J. T. and Forbes, S. H. · January 1997· Behav Ecol Sociobiol. 41: 33-48 ABSTRACT
Age-dependent sexual selection in bighorn rams. D W Coltman · M Festa-Bianchet · J T Jorgenson · C Strobeck · Feb 2002 · Proceedings of the Royal Society B: Biological Sciences ABSTRACT
Sexual selection and social rank in bighorn rams. Fanie Pelletier · Marco Festa-Bianchet · Mar 2006 · Animal Behaviour ABSTRACT
Male mating effort in a polygynous ungulate. Fanie Pelletier · John T. Hogg · Marco Festa-Bianchet · Sep 2006 · Behavioral Ecology and Sociobiology ABSTRACT
Sexually antagonistic association between paternal phenotype and offspring viability reinforces total selection on a sexually selected trait. Alexandre M Martin · Marco Festa-Bianchet · David W Coltman · Fanie Pelletier · Feb 2014 · Biology letters ABSTRACT
Paternal reproductive success drives sex allocation in a wild mammal. Mathieu Douhard · Marco Festa-Bianchet · David W. Coltman · Fanie Pelletier Jan 2016 · Evolution ABSTRACT


Genetic markers for inheritance of horn size
Quantitative genetics of lifehistory traits in a longlived wild mammal Denis Réale · Marco Festa‐Bianchet · Nov 2000 · Heredity ABSTRACT
Selection and genetic (co)variance in bighorn sheep David W Coltman · Paul O’Donoghue · John T Hogg · Marco Festa-Bianchet · Jul 2005 · Evolution ABSTRACT
Genetic linkage map of a wild genome: genomic structure, recombination and sexual dimorphism in bighorn sheep Jocelyn Poissant · John T Hogg · Corey S Davis · Joshua M Miller · Jillian F Maddox · David W Coltman · Sep 2010 · BMC Genomics 11: 524 ABSTRACT
QTL mapping for sexually dimorphic fitness-related traits in wild bighorn sheep J Poissant · C S Davis · R M Malenfant · J T Hogg · D W Coltman · Aug 2011 · Heredity ABSTRACT
Whole genome resequencing uncovers molecular signatures of natural and sexual selection in wild bighorn sheep Marty Kardos · Gordon Luikart · Rowan Bunch · Sarah Dewey · William Edwards · Sean McWilliam · John Stephenson · Fred W Allendorf · John T Hogg · James Kijas · Oct 2015 · Molecular Ecology ABSTRACT


What does hunting have to do with evolution of horn size?
Undesirable Evolutionary Consequences of Trophy Hunting David W Coltman · Paul O’Donoghue · Jon T Jorgenson · John T Hogg · Curtis Strobeck · Marco Festa-Bianchet · Dec 2003 · Nature ABSTRACT
Age-related Horn Growth, Mating Tactics, and Vulnerability to Harvest: Why Horn Curl Limits may Select for Small Horns in Bighorn Sheep Marco Festa-Bianchet · David W. Coltman · John T. Hogg · Jon T. Jorgenson · Jan 2006 ABSTRACT
Guns, Sheep, and Genes: When and Why Trophy Hunting May Be a Selective Pressure Marco Festa-Bianchet · Chapter · Feb 2009 ABSTRACT
Not surprisingly, no inheritance of a trait results in no evolution Philip W Hedrick · David W Coltman · Marco Festa-Bianchet · Fanie Pelletier Full-text · Article · Nov 2014 · Proceedings of the National Academy of Sciences  ABSTRACT


How do different harvest regulations influence the possibility of evolutionary changes in horn size?
In BC hunting is managed under Full Curl, ¾ Curl and Any Ram regulations with an unlimited number of permits (general open season) or limited number of permits (Limited Entry Hunt). Most Alberta sheep are hunted under unlimited 4/5 Curl regulations.
Harvest Regulations and Artificial Selection on Horn Size in Male Bighorn Sheep in British Columbia Pamela E. Hengeveld · Marco Festa‐Bianchet · Jan 2011 · Journal of Wildlife Management PDF
Decrease in Horn Size and Increase in Age of Trophy Sheep in Alberta Over 37 Years Marco Festa-Bianchet · Fanie Pelletier · Jon T. Jorgenson · Chiarastella Feder · Anne Hubbs · Jan 2014 · Journal of Wildlife Management ABSTRACT


Do protected areas help provide “genetic rescue”?
Can phenotypic rescue from harvest refuges buffer wild sheep from selective hunting? Fanie Pelletier · Marco Festa‐Bianchet · Jon T. Jorgenson · Chiarastella Feder · Anne Hubbs · Sep 2014 · Ecology and Evolution ABSTRACT


Do we see the same evidence in Stone’s sheep harvest?
Changes in horn size of Stone’s sheep over four decades correlate with trophy hunting pressure Mathieu Douhard · Marco Festa-Bianchet · Fanie Pelletier · Jean-Michel Gaillard · Christophe Bonenfant · Jun 2015 · Ecological Applications  PDF


Do recent world record sheep disprove that hunting causes genetic harm?
Data from selective harvests underestimate temporal trends in quantitative traits Fanie Pelletier · Marco Festa-Bianchet · Jon T Jorgenson · May 2012 · Biology Letters ABSTRACT
Record books do not capture population trends in horn length of bighorn sheep Marco Festa-Bianchet · Susanne Schindler · Fanie Pelletier · Oct 2015 · Wildlife Society Bulletin ABSTRACT


What happens when harvest stops?
Intense selective hunting leads to artificial evolution in horn size Gabriel Pigeon · Marco Festa-Bianchet · David W. Coltman · Fanie Pelletier · Jan 2016 · Evolutionary Applications PDF


Pamela Hengeveld

Wild sheep specialist and founder of WILDLIFE.FISH. Also known as the quiet neighbour with the really big freezer.

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