Final answer:
In red deer, natural selection has favored traits that allow females with low fat reserves to selectively abort male offspring requiring extensive parental care. Male reproductive strategies are also influenced by physiology, as males prioritize mating with fecund females, particularly when sperm supplies are low.
Step-by-step explanation:
Researchers studying red deer (Cervus elaphus) have observed a correlation between the body condition of yearling females, particularly their fat reserves, and the sex ratios of their offspring. It appears that natural selection has favored traits that prevent pregnancies in female elk with low fat reserves, leading to the selective abortion of male offspring. This is because male calves require more parental care and resources compared to female calves, which could be a strain on females with lower body condition. Female elk with higher fat reserves are more likely to carry a pregnancy to term.
Additionally, studies have shown that male elk behavior in mate selection is influenced by their physiological state. Males with depleted sperm reserves, due to prior matings, tend to prefer larger, more fecund females. The selection of such partners enhances their chances of offspring since larger females have higher fecundity. Conversely, this selective pressure also likely impacts the females, as those that can avoid the additional energy expenditure of rearing males when in a resource-deprived state would have increased survival and reproductive success.