Population density affects sex ratio variation in red deer

Area of Science:

• Ecology
• Reproductive Biology
• Mammalian Studies

Background:

• Mammalian populations often exhibit skewed sex ratios at birth, but these patterns are inconsistent.
• Multiple mechanisms influencing sex ratio likely exist, with their effects modulated by environmental conditions.

Purpose of the Study:

• To investigate how environmental conditions, specifically population density, affect the relationship between maternal dominance and offspring sex ratio in red deer (Cervus elaphus).
• To explore the mechanisms underlying sex ratio variation in response to environmental changes.

Main Methods:

• Analysis of long-term data on red deer populations, focusing on sex ratios, population density, and environmental variables like winter rainfall.
• Comparison of findings with existing literature on ungulate sex ratio dynamics.

Main Results:

• The previously observed adaptive relationship where dominant female red deer produced more males was absent at high population densities.
• Offspring sex ratio (proportion of males) decreased with increasing population density and winter rainfall, indicators of nutritional stress.
• These changes correlated with reduced fecundity, suggesting differential fetal loss as the primary mechanism.

Conclusions:

• Environmental factors, particularly nutritional stress linked to high density and rainfall, can override pre-implantation mechanisms influencing sex ratio, such as maternal dominance effects.
• The dominant mechanism influencing sex ratio can shift within approximately two generations.
• Positive associations between maternal quality and male offspring proportion appear limited to ungulate populations below their carrying capacity.