Autumn and Animal Sex: Five Seasonal Changes in Reproductive Behaviour Across Species

Overview

The Conversation summarizes how autumn affects sexual behaviour across species, including humans, with a tour of diverse seasonal strategies in a range of organisms.

• Autumn triggers hormonal changes that shape mating and conceptions in people and animals.
• Red deer rut involves seasonal aggression to secure harems and spring births.
• Autumn bisexuality has been observed in several species and is not solely due to partner scarcity.
• Other organisms exhibit seasonal reproduction strategies including hydra, corals, and shrimp.

Introduction

As days shorten and temperatures drop in the northern hemisphere, many animals adjust their mating strategies. The article describes how autumn influences sexual behaviour in a wide range of species, from mammals to invertebrates, and even humans, through hormonal changes, shifts in social activity, and seasonal life-history timing. It emphasizes that seasonal mating is an adaptive response to the seasonal availability of resources, climate patterns, and life-history constraints.

Seasonal fighting: red deer rut

In red deer, autumn brings a rise in testosterone as days shorten, leading to aggressive competition for harems. Males lock antlers in fights to win access to females, and the holder of a harem fathers offspring. This timing aligns births with late spring when food is abundant, increasing offspring survival. The article notes that red deer are also responding to climate warming, bringing births forwards by almost two weeks.

Autumnal bisexuality across species

Although heterosexual mating dominates during reproductive seasons, autumn sees a reemergence of same-sex interactions in many species, including red foxes, red squirrels and bats. This seasonal bisexuality is observed as fluid switching between hetero- and homosexual couplings in several species. Interestingly, male walruses engage almost exclusively in homosexual behaviour until they reach maturity around four years old. After maturity, walruses shift to seasonal heterosexual couplings in the summer, then return to same-sex relationships in autumn.

Reproductive strategies and sex changes

Some freshwater species switch reproductive modes seasonally. The brown hydra reproduces asexually in spring and summer, then switches to sexual reproduction in autumn, producing dormant eggs that withstand winter and hatch when conditions are favorable. The northern shrimp begin life as small males and transition to females around autumn breeding season; the female stores sperm and carries eggs through the winter to hatch in spring, ensuring offspring survival. Coral gobies and other fish also exhibit sex changes in response to social and environmental cues, such as isolation of individuals or mating contexts, with potential reversals after breeding is complete.

Seasonal penises and courtship in birds

Duck species show dramatic seasonal changes in genital size; spring mating prompts large penises that may exceed body length, while autumn induces a reduction to around 10 percent of full size. Females have labyrinth-like vaginas that twist in the opposite direction to prevent fertilisation by unwanted males, relaxing the tract only when mating is desired. These adaptations illustrate how sexual selection shapes anatomy seasonally and across species.

Conclusion

The article closes by highlighting that autumnal shifts in sexual behaviour occur across the natural world, offering insights into the ecology and evolution of mating strategies and the diversity of life histories.