Scientists Discover Male Octopuses Use Specialized Arm to Detect Female Hormone Progesterone

Scientists have discovered that male octopuses employ a specialized arm, known as the hectocotylus, as a sensory organ capable of detecting the female hormone progesterone. This mechanism allows male octopuses to locate and inseminate mates, even without visual contact, providing insight into their reproductive strategies, particularly given their solitary nature.

Discovery of a Unique Sensory System

The precise mechanism by which the hectocotylus arm located a mate or delivered sperm was previously unclear. Research, published in the journal Science, indicates that this arm functions similarly to a tongue, equipped to detect progesterone.

Professor Nicholas Bellono of Harvard University, a senior author of the study, noted that this dual function as both a sensor and a mating organ is logical for solitary octopuses, enabling the arm to rapidly localize a female and initiate mating during brief encounters.

The research team included first author Pablo Villar.

Experimental Observations

The study involved California two-spot octopuses. Researchers separated male and female octopuses using an opaque barrier containing holes. Male octopuses were observed extending their specialized arm through these holes, locating the female's mantle, finding the egg-transporting tubes, and initiating mating. This behavior was consistently observed across multiple male-female pairs and occurred even in darkness, confirming that visual cues were not necessary for copulation. Mating attempts were not observed between male pairs.

Progesterone's Role in Mating

Further investigation revealed the presence of progesterone in the ovaries and skin of female octopuses. Experiments demonstrated that amputated specialized arms of male octopuses exhibited movement upon contact with progesterone, but not with other similar hormones. In subsequent barrier experiments, males readily explored and attempted to mate with tubes filled with progesterone, indicating that the hormone alone could trigger key aspects of mating behavior. Specific receptors on the tip of the specialized arm were identified as responsible for progesterone sensing.

Evolutionary Implications

These progesterone-sensing receptors show evidence of recent and rapid evolution across cephalopods. This suggests that different species may be attuned to distinct chemical signals, potentially encoding both sex and species identity. While the specialized arm in other cephalopod species also showed progesterone sensitivity, its sensitivity to other hormones varied.

The findings offer insights into how sensory systems evolve to maintain reproductive barriers or facilitate crossbreeding and the emergence of new species.

The discovery also highlights the importance of observational science, as the sensory function of the arm was revealed through direct observation of the animals.