Higher male share linked to poorer female mule deer health

At the start of each winter, the team flew a helicopter over female deer, captured them with nets, recorded body mass, size, rump fat and other measurements, then released them.

Females accumulated less body fat in areas managed for high male-to-female ratios. In the following year, those areas also showed lower female population growth.

Body fat is not a decorative measure. It provides energy for winter survival and reproduction, and poor female condition can endanger both a female and her fawns.

Why more males could mean less forage

Female mule deer face heavy summer and fall demands: producing milk for fawns, mating and storing fat for winter survival and reproduction. The authors suggest that more males can intensify competition for food, or forage, during those months, even when total deer density remains constant.

In McMillan’s explanation, males are 50% larger than females and therefore consume 50% more forage. He said a female’s fat reserves affect her chance of surviving any winter; a skinny female can die, especially in strongly temperate regions.

That makes forage competition a plausible mechanism, not a proven step in the chain. The proposed link still needs direct testing across landscapes with different food supplies and winter conditions.

Some Utah areas also saw 50% mortality among adult females after the heavy 2023 winter. The data additionally suggest that poorer female health may connect with more aggressive competition among males during mating and with evolving social interactions among deer. Neither the winter deaths nor those behavioral links prove that a high male-to-female ratio caused them.

What was overlooked?

“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.”

Here, a demographic trade-off means a gain in one measure can bring a loss elsewhere. Increasing the number of males may produce attractive hunting ratios while leaving females, fawns and later population growth under greater strain.

The researchers’ warning also reaches the next generation. Males born to females in poor nutritional condition may experience lifelong reduced growth and impaired antler development, potentially hurting hunters as well as the population.

McMillan called the male-harvest implication counterintuitive: harvesting more animals can sometimes lead to better populations, at least according to the data. He said the research may explain part of the mule deer decline seen over the past 30 years.

What does this change for mule deer management?

The immediate change is a serious challenge to a decades-old assumption: that protecting more males and harvesting fewer of them will reliably increase mule deer numbers. The study instead directs attention toward female nutrition and the demographic costs of a high male-to-female ratio.

It does not establish an optimal harvest ratio, and the findings do not amount to a universal policy switch. Managers still need local evidence about female condition, food supply, mortality and herd structure before changing rules.

For hunters, the practical warning is simple: a stronger buck-to-doe ratio may not mean a healthier herd. Poorly nourished females may produce sons with slower growth and impaired antler development.

The study appeared as Ranjana Pal et al., “Dams in distress,” in Ecological Solutions and Evidence (2026). Its DOI is 10.1002/2688-8319.70353.