Why do some animal species have different male and female secondary sexual characteristics?

The presence of different secondary sexual characteristics in males and females, a condition known as sexual dimorphism, is primarily driven by sexual selection. This evolutionary process occurs when individuals of one sex compete for access to mates or are chosen by the opposite sex based on specific traits.12

These traits often arise because they provide a reproductive advantage that outweighs the survival risks associated with them, such as increased visibility to predators or the physical burden of carrying large ornaments. There are two main drivers for these differences:12

• Intrasexual Competition: Males often compete with other males for access to females. This has led to the evolution of increased size, strength, and weaponry, such as antlers or horns, which are used to establish dominance or win physical contests.123
• Intersexual Choice: Females, who often make a higher initial parental investment in offspring, are frequently more selective. They may prefer males who exhibit traits that signal fitness, such as bright coloration or elaborate displays.13

In species where both sexes share equally in raising offspring, the sexes tend to be less differentiated. Conversely, when one sex invests more in offspring, the other sex typically evolves to be more competitive and adorned to secure mating opportunities.3