Thesis Format
Integrated Article
Degree
Master of Science
Program
Biology
Supervisor
Coltman, David
Abstract
When harvest criteria are based on phenotype, trophy hunting may select against the size of secondary sexual characteristics, including horns. Here I investigated the effects of trophy hunting on horn growth and morphology in thinhorn sheep (Ovis dalli). Trophy rams, harvested between 1996-2021 in the Yukon (n = 247), were genotyped from dried flesh or horn core specimens using a custom-designed 6050 SNP genotype-by-sequencing panel. With this panel, I calculated a genomic relationship matrix to estimate trait heritability, temporal changes in breeding values and the predicted response to selection based on age at harvest. Among measured horn characteristics, only growth was under negative directional selection because of harvesting; however, observed trends did not vary over this period, due to changes in the environment or bias in the sampling design. My study illustrates the potential and challenges of genotyping historical specimens to reveal the impacts of harvesting selection on trait variation.
Summary for Lay Audience
Eligibility for trophy-hunting herbivorous hooved mammals, defined as ungulates, often depends on the size of male sex-specific traits called secondary sexual characteristics (SSC), such as horns, antlers, and tusks. These size-specific harvesting criteria can result in serious consequences for maintaining genetically healthy wild populations. In ungulates, headgear is used for asserting dominance and mate competition, resulting in SSC being strongly linked to the individual’s fitness. Selectively harvesting individuals with certain traits, such that either survival or reproduction is impacted, results in artificial selection. Therefore, trophy hunting based on size can allow the harvest of adolescent males with fast-growing headgear before they can reproduce, potentially resulting in a decline in the size of SSC within a harvested population. In this study, I will investigate the genetic effects of trophy hunting on horn characteristics (e.g. horn growth and morphology) in thinhorn sheep and compare these genetic relationships to the observed trends in the trait.
The evolutionary response to harvesting on the horn characteristics was calculated for rams harvested between 1996-2021, from the 1986-2015 cohorts. Three methods were used to predict the response to harvesting selection: the univariate breeder’s equation, the secondary theorem of natural selection (STS) and the best linear unbiased predictor of estimated breeding values (BLUP EBV). The univariate breeder’s equation calculated the response by qualifying the heritability of the horn characteristics and the strength of harvesting selection. The BLUP EBV assessed the response by estimating temporal changes in breeding values, defined as a comparison between an individual’s genetic merit for a given trait to that of the population. The final STS approach required estimating the genetic covariance between the horn characteristics and fitness. Using these approaches, I found only horn growth was predicted to decline in size due to harvesting, however, observed trends in the phenotype did not change over the study period. While these discrepancies may be due to the environment compensating for a genetic decline in horn growth, there are also biases in the sampling design. Overall, my study illustrates the opportunities and challenges with using historical specimens to assess the impact of harvesting selection on trait variation.
Recommended Citation
Balsara, Mehra, "The evolutionary response to hunting-induced selection on horn characteristics in Yukon thinhorn sheep (Ovis dalli)" (2024). Electronic Thesis and Dissertation Repository. 10443.
https://ir.lib.uwo.ca/etd/10443
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This work is licensed under a Creative Commons Attribution 4.0 License.