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Bighorns and Little Horns Revisited
Abstract
During the 1960s a series of horn measurements of bighorn rams (Ovis canadensis) from the eastern slopes of Alberta was recorded. The horn base circumferences of rams from the chinook belt south of the Bow River were significantly larger than ram horns to the north. A subsequent series of horn base measurements up to forty years later had the same results. However, there were some notable exceptions in central and northern Alberta. Ram horn bases increased significantly following a controlled ewe removal program in central Alberta on Ram Mountain and decreased to former levels after cessation of ewe removals. Ram horns at northern coal mine reclamation sites had larger horn bases than ram horn measurements prior to reclamation. BIENN. SYMP. NORTH. WILD SHEEP AND GOAT COUNC. 15: 28-32
... Nevertheless, under a management strategy of nearly unregulated and highly selective harvest, only 3 cm of the >15-cm reduction in age-corrected horn-length over time was attributed to genetic effects (Pigeon et al. 2016); analyses of which also have been challenged (Coulson et al. 2017). Instead, direct reductions in nutrient availability from a 2.4Â increase in population abundance primarily were responsible for the phenotypic reductions in horn size (Jorgenson et al. 1993bWishart 2006;Pigeon et al. 2016;Festa-Bianchet 2017). ...
... Nonetheless, the idea that mountain sheep, large herbivores also regulated by density-dependent factors, would not respond to female harvest in a similar manner, or that their populations are somehow not subject to the same effects of resource limitation, is paradoxical. Like other large herbivores, mountain sheep are constrained by density-dependent processes, and harvest of females therefore, can be an effective tool to increase relative availability of nutrients by decreasing population density (Jorgenson et al. 1993b, Wishart 2006. ...
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.
... Because of differences in climate, soil, and vegetation conditions, bighorn rams generally have lower body and horn growth north of the Bow River in Alberta (Wishart 1969;Jorgenson 2008). Northern populations are therefore less likely to provide exceptionally well-developed rams as reflected in the lower overall proportion of rams aged 4 or 5 years in the harvest. ...
Trophy hunting can affect weapon size of wild animals through both demographic and evolutionary changes. In bighorn sheep (Ovis canadensis Shaw, 1804), intense harvest of young males with fast-growing horns may have partly driven long-term decreases in horn size. These selective effects could be dampened if migrants from protected areas, not subject to artificial selection, survived and reproduced within hunted populations. Bighorn rams undertake long-distance breeding migrations in the weeks preceding the late-November rut. We analysed records of >7800 trophy bighorn rams shot from 1974 to 2019 in Alberta, Canada, to test the hypothesis that high harvest pressure during breeding migrations was correlated with a greater decrease in horn size. We compared areas with and without a pronounced harvest peak in late October, when male breeding migrations begin. Areas without a pronounced harvest peak in late October, which likely experienced a lower harvest rate, showed a similar temporal decline in horn size, but no increase in age at harvest suggesting a possibly weaker decline in horn growth. Our study suggests that unselected immigrants from protected areas could partly buffer the effects of intense trophy hunting only if harvest pressure was reduced when breeding migrations commence.
... Clearly, population density was the largest contributing factor to the decline in male horn length on Ram Mountain, and this was caused mostly by management that allowed for an overpopulation of mountain sheep with serious consequences for the nutrition available to the sheep occupying Ram Mountain (Jorgenson et al. 1998, Wishart 2006. Alpine plants have inherently slow growth rates Poorter 1992, Atkin et al. 1996) and herbivory alters alpine vegetation (Evju et al. 2009), with a consequence of slow recovery of nutrition and thus phenotypic characteristics (Monteith et al. 2018). ...
We respond to Festa‐Bianchet (2019) and caution against using interpretations from the unique Ram Mountain history to justify management of mountain sheep throughout their range. Because harvest management at Ram Mountain is atypical, it is not useful in informing the management of most mountain sheep herds.
... Finally, we invited Monteith et al. (2018) to examine whether the practice of restricting harvests to mature male mountain sheep is sound herd management. Previous studies have reported that harvests of females can keep population size low enough to sustain good range condition (Jorgenson et al. 1993a(Jorgenson et al. , b, 1998Wishart 2006), whereas harvesting only a small number of mature males cannot achieve herd size objectives. The resistance to harvesting females is reminiscent of the strong push back to harvesting white-tailed deer (Odocoileus virginianus) females in Wisconsin during the 1940s (Leopold 1992); convincing mountain sheep hunters to kill females likewise will require hunter education. ...
... Together, temporal trends in age and horn size of harvested animals may provide information on agespecific 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. ...
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.
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