This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Following a surge of interest in deer hunting beginning in the '80s and '90s, strategies to manage deer populations evolved to increase the proportion of mature male deer. While the increase in male abundance has perhaps made for better hunting conditions, the overall mule deer population in western North America has continued to decline.
New BYU research appears to have figured out at least one potential reason why. A study published in the journal Ecological Solutions and Evidence, authored by BYU postdoctoral researcher Ranjana Pal along with professors Randy Larsen and Brock McMillan, finds that increasing the ratio of male mule deer is associated with decreasing female mule deer nutritional condition and lower population growth rates. "More males really don't help grow the population," said McMillan, a professor of plant and wildlife sciences.
"I think this is a game changer. Agencies and the public have continually pushed to decrease male deer harvest—which increases male-to-female ratios—but this paper suggests that is exactly opposite of what we should do for the mule deer population." Using 15 years of data on mule deer productivity and seven years of data on deer body reserve dynamics, the BYU trio teamed up with Utah's Division of Wildlife Resources and a team of co-authors to quantify the influence of decreasing male harvest on the population dynamics of free-ranging mule deer. Over the course of several years, the team annually captured female mule deer at the beginning of each winter using helicopter net-gunning, then recorded body mass, size, rump fat and other measurables before releasing the deer.
Their measurements revealed decreasing body fat accumulation in females—critical for their survival and the survival of their fawns—and lower female population growth the following year in areas managed for high male-to-female ratios. Summer and fall are a critical period for females to meet the energetic demands of lactation, mating and fat storage required for winter survival and reproduction. The researchers suggest increasing the proportion of males may create increased competition for forage during that time, even if population density remains constant.
"Males are 50% larger than females, so males are eating 50% more than females," McMillan said. "The fat reserves in a female influence the likelihood she makes it through any given winter ... skinny female mule deer will die, especially in strongly temperate regions." According to data from the team, some areas in Utah saw 50% mortality rates for adult females following the heavy 2023 winter. The researchers said the data also suggest decreasing female health conditions could be connected to more aggressive mating behavior (more males competing among one another) and evolving social interactions among the deer populations.
"Females have long been considered the primary drivers of population growth while the role of males has generally been overlooked," Larsen and colleagues wrote. "The biological consequences of the strategy to increase male proportions may involve demographic trade-offs that do not translate into long-term improvements." The findings also suggest other trickle-down effects from an increasing male-to-female ratio that would also negatively impact hunters: Males born to females in poor nutritional condition may experience lifelong consequences, including reduced growth and impaired development of antlers. "It's counterintuitive to think that harvesting more animals leads to better populations, but that's what our data suggest," McMillan said.
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