The 3 Key Differences Between Male and Female Dolphins

Quick Take

• Telling a male dolphin from a female by sight is harder than you'd think, and for most species size won't help you at all. See why looks deceive →
• Male dolphins form social alliances that rival the complexity of human political structures, and this behavior is entirely driven by one goal. Explore male alliance behavior →
• Researchers use a surprisingly simple trick to definitively sex a dolphin, and it involves two everyday symbols you already know. See the sexing method →
• Male and female dolphins reach reproductive maturity at different ages, and the gap between them is wider than most people expect. Check the maturity age gap →

Dolphins are highly intelligent and social marine mammals that belong to the toothed whale group. There are over 40 distinct species of dolphins, ranging from the small Maui dolphin (Cephalorhynchus hectori maui) to the massive orca (Orcinus orca). Dolphins inhabit a wide variety of aquatic environments, including oceans and freshwater rivers. Depending on the species, it can be challenging to differentiate between males and females because they look very similar. However, there are some features that can reveal the sex of a dolphin. Continue reading to learn about the three key differences between male and female dolphins: appearance, evolutionary behavior, and physiology.

Appearance

Most dolphin species are sexually monomorphic, meaning males and females look nearly identical from the surface and cannot be reliably distinguished by size or shape alone. Because there are no visible external sex characteristics, the only way to definitively determine a dolphin’s sex is by getting a clear look at its underbelly.

While in most dolphin species males and females are of a similar size, there are exceptions. The orca is the largest species in the dolphin family, Odontoceti. Adult male (bull) orcas can measure up to 26 feet long and weigh as much as 12,000 pounds, whereas female orcas, called cows, are noticeably smaller at 22 feet long and weighing no more than 8,000 pounds.

Therefore, although appearance may help determine the sex of a dolphin in certain species, it is in no way a key determinant in others.

Evolutionary Behavior

Evolutionary pressures regarding reproduction and offspring survival have caused female and male dolphins to develop highly contrasting social and pod behaviors.

Females and their offspring typically form highly stable, lifelong matrilineal pods. These groups are core for socializing, coordinating cooperative hunting, and raising calves. The older females often act as matriarchs, passing down critical cultural knowledge, like foraging techniques and specific communication dialects to the younger generation. While female offspring may stay with their mothers for life, male offspring typically leave the pod upon reaching sexual maturity.

Male dolphins form social alliances to secure mates and defend against rivals. These alliances can range from pairs or tight-knit groups of 3–4 individuals to super-alliances of up to 14 members, making them one of the few human-like social structures found outside of primates.

A male dolphin’s investment in his offspring is virtually nonexistent across all species. In no dolphin species do males actively raise their young. Parenting is left exclusively to the mothers and their extended social pods.

Physiology

The only foolproof way of determining the sex of a dolphin is by examining its genitalia. However, dolphins do not have visible external genitalia; their reproductive and excretory organs are concealed within the lower underbelly, near the tail. The layout of these hidden slits allows researchers to easily determine the dolphin’s sex.

Females have a single continuous genital slit that is flanked by two smaller mammary slits, which resemble a division sign (÷). Males have two separate slits (a genital slit and an anal slit) that look like an exclamation mark (!).

Dolphins exhibit sexual bimaturism, meaning the two sexes achieve reproductive capabilities at different ages. While exact timelines vary significantly depending on the species and geographic location, females typically become sexually mature at an earlier age than male dolphins.

In most dolphin species, females reach sexual maturity between 5 and 13 years of age, with larger species requiring more time. Male dolphins are a bit slower, becoming sexually mature between 8 and 14 years of age.