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Fetuses Were Lighter in Red Deer Populations With a Higher Proportion of Adult Males

Red deer calf in Glen Garry, Scotland
Research · Reproduction & development

Among 1,120 red deer (Cervus elaphus) mother–fetus pairs in southwestern Spain, fetuses tended to be lighter for their length in populations with a higher proportion of adult males. The researchers used the proportion of adult males as an indicator of stronger male–male competition during the rut. They measured fetal body condition as weight relative to body length — how heavy a fetus was for its size — and also found effects of maternal traits, conception timing, fetal sex and weather.

The material came from free-ranging Iberian red deer populations in Sierra de San Pedro, Monfragüe and Sierra Morena and was collected over 19 years through regular population-management hunts. Researchers weighed, measured and sexed the fetuses and combined these data with maternal age, estimated conception date, weather and male age structure. The adult-male proportion was derived from the ages of males harvested in each population and season; previous comparisons with direct field observations had shown that this harvest-based measure closely tracked male age structure in the populations.

Several factors shaped fetal weight

Fetal length was the strongest predictor of weight, but several other factors also contributed. Male fetuses were heavier than females, while fetal weight increased with maternal age and earlier conception, particularly in males. In populations with a higher proportion of adult males, fetuses tended to be lighter; this pattern was especially evident among adult and older mothers under unfavourable weather conditions and later in gestation.

Graphs showing red deer fetal weight in relation to adult-male proportion and maternal age
The interaction between population structure and maternal age: predicted fetal weight fell as the proportion of adult males increased, most clearly for adult and older mothers. Figure 5 from Mateos et al. (2026), Behavioral Ecology and Sociobiology, CC BY 4.0. Source.

Social environment may affect maternal investment

The authors interpret the association with population structure as evidence that the social environment may influence maternal investment before birth. They point to interactions between females and competing males during the rut as one likely route: work in other ungulates has linked mixed-age male competition to more harassment and stress for females. In the Spanish study system, simple resource limitation was less consistent with the pattern because females in the fenced estates with more adult males were actually heavier, and available density and tooth-wear comparisons did not indicate poorer resource conditions there. The authors therefore regard male–female interactions under intense male competition as a plausible explanation and describe the result as suggestive of sexual conflict, not proof of a direct causal mechanism.

Sources and image credit

  1. Mateos, C., Pérez-González, J., de la Peña, E., Seoane, J. M., Alarcos, S., Sánchez-Prieto, C. B., & Carranza, J. (2026). Maternal effects of gestating red deer hinds on fetal body condition: Influence of individual and social environmental features. Behavioral Ecology and Sociobiology, 80, 18. https://doi.org/10.1007/s00265-026-03695-8Primary peer-reviewed open-access study, published 23 January 2026.

Image: Charles J. Sharp / Wikimedia Commons — juvenile red deer (Cervus elaphus) in Glen Garry, Highland, Scotland, on 15 July 2016. Illustrative photograph, not from the study. · CC BY-SA 4.0 · Source

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