Biology
Parental Investment and Offspring Sex Ratios in Red Deer
Quick fact
In red deer, hinds in good condition are more likely to give birth to sons, while those in poorer condition more often have daughters. This pattern is a classic example of the Trivers-Willard hypothesis, where maternal investment is biased toward the sex that offers the greatest reproductive payoff.
Why this is interesting
Why would a mother red deer sometimes produce more sons and sometimes more daughters? What does this tell us about her hidden strategy?
Read the full explanation
Understanding Parental Investment and Offspring Sex Ratios in Red Deer
In red deer, reproducing is a costly business, and mothers must decide how to allocate their limited energy. The key is that male and female calves have different potential for future reproduction. In red deer society, males compete fiercely for harems; only the largest, strongest stags get to mate, and they can sire many offspring. Females, on the other hand, almost always reproduce once mature. So, for a mother in good condition, it makes sense to invest more in a son—he has a good chance of becoming a dominant stag and producing many calves. A mother in poor condition, however, would be wiser to invest in a daughter, who is more likely to reproduce than a weak son. This is why we see more sons born to healthy, high-ranking hinds, and more daughters born to lower-condition hinds.
A deeper explanation
The Trivers-Willard hypothesis explains this as a conditional strategy: when maternal condition affects the future reproductive success of sons more than daughters, mothers in good condition should favor sons, and mothers in poor condition should favor daughters. In red deer, male reproductive success is highly variable because of intense competition for mates, while female success is more stable. A son born to a well-nourished, high-ranking hind can grow large and become a dominant stag, siring many calves. But a son born to a poor-condition mother is unlikely to compete successfully, wasting her investment. In contrast, a daughter is almost certain to reproduce, so she is a safer investment for a mother with limited resources. Over time, natural selection has favored mothers that adjust their offspring sex ratio according to their own condition. This is not a conscious decision but a physiological adjustment, possibly influenced by hormone levels or body fat. The result is that hinds in better condition produce more sons, and hinds in poorer condition produce more daughters.