Changes in horn size of Stone's sheep over four decades correlate with trophy hunting pressure

Abstract

16Selective harvest may lead to rapid evolutionary change. For large herbivores, trophy hunting 17 removes males with large horns. That artificial selection, operating in opposition to sexual 18 selection, can lead to undesirable consequences for management and conservation. There have 19 been no comparisons of long-term changes in trophy size under contrasting harvest pressures. 20 We analyzed horn measurements of Stone's rams (Ovis dalli stonei) harvested over 37 years in 21 two large regions of British Columbia, Canada, with marked differences in hunting pressure to 22 identify when selective hunting may cause a long-term decrease in horn growth. Under strong 23 selective harvest, horn growth early in life and the number of males harvested declined 24 respectively by 12% and 45% over the study period. Horn shape also changed over time: horn 25 length became shorter for a given base circumference, likely because horn base is not a direct 26 April 23, 2015 HARVEST PRESSURE AND CHANGE IN HORN GROWTH 2 target of hunter selection. In contrast, under relatively lower hunting pressure, there were no 27 detectable temporal trends in early horn growth, number of males harvested or horn length 28 relative to base circumference. Trophy hunting is an important recreational activity and can 29 generate substantial revenues for conservation. By providing a reproductive advantage to males 30 with smaller horns and reducing the availability of desirable trophies, however, excessive 31 harvest may have the undesirable long-term consequences of reducing both the harvest and the 32 horn size of rams. These consequences can be avoided by limiting offtake. 33 34

Mountain sheep management using data versus opinions: A comment on Boyce and Krausman (2018)

“…The large overall phenotypic decline in horn size at Ram Mountain was partly due to a doubling of population size (Jorgenson et al ). Several hunted populations of mountain sheep have shown similar declines in male horn size, consistent with an evolutionary effect, without major increases in density (Hengeveld and Festa‐Bianchet , Festa‐Bianchet et al , Douhard et al ).…”

When does selective hunting select, how can we tell, and what should we do about it?

“…For example, these measurements revealed that early horn growth was strongly correlated with age at harvest in two species of mountain sheep (Hengeveld & Festa‐Bianchet , Douhard et al. ). Stone's sheep Ovis dalli stonei rams harvested at 6–7 years of age had grown twice as much horn during their second and third year of life as those shot at 9–10 years (Douhard et al.…”

“…Stone's sheep Ovis dalli stonei rams harvested at 6–7 years of age had grown twice as much horn during their second and third year of life as those shot at 9–10 years (Douhard et al. ), demonstrating a very intense selective pressure against rams with fast‐growing horns. Hunter preferences are another source of bias, and are difficult to quantify (Mysterud et al.…”

Testing the importance of harvest refuges for phenotypic rescue of trophy‐hunted populations

“…That analysis examined how length varied as a function of circumference. In Stone's sheep Ovis dalli stonei , intense selective harvest led to a decrease in the horn length to circumference ratio, likely because thicker horns are effective in male–male combat but longer horns increase the chance that a male would be 'legal' (Douhard et al, ). Indeed, base circumference is not directly targeted by hunters (Pelletier et al, ).…”

“…Rates of phenotypic change due to anthropogenic selective pressures are higher than those induced by natural selection (Darimont et al, ) because harvest‐induced selective pressures are often stronger than natural selection (Conover, Munch, & Arnott, ). Selective harvest may thus have undesirable evolutionary consequences, such as shorter horns in wild sheep (Douhard, Festa‐Bianchet, Pelletier, Gaillard, & Bonenfant, ; Garel et al, ; Hengeveld & Festa‐Bianchet, ; Pigeon et al, ). Decreasing horn growth over time was also documented by Crosmary et al () for trophy‐hunted male impala Aepyceros melampus and sable antelope Hippotragus niger in Zimbabwe.…”