Platypus Melanosomes: A Bird-Like Mystery Challenges Evolutionary Beliefs

Platypuses possess a unique cellular characteristic: pigment-producing structures called melanosomes that are more similar to those found in birds than in other mammals. This finding, published in Biology Letters, raises new questions about the evolution of color.

For over 200 years, the platypus has been known for its unusual combination of features. Its peculiarities include females producing milk without nipples, males having venomous spurs, sensing electricity, glowing under UV light, and possessing a high number of sex chromosomes.

Unraveling a Long-Held Belief

For more than 50 years, it was believed that hollow melanosomes occurred exclusively in birds, while mammal melanosomes were consistently solid. Researchers investigated whether platypus melanosomes, the tiny pigment-making factories within animal cells, might also differ from this established understanding.

Unexpected Discoveries

Jessica Leigh Dobson from Ghent University and her team utilized high-resolution microscopy to examine melanosomes within platypus hairs.

Their findings contradicted previous understanding, revealing that platypus melanosomes are hollow and spherical. Leigh Dobson described this as "totally unexpected."

Leigh Dobson described this as "totally unexpected," noting that this combination had not been observed in any other known species.

Color, Shape, and Further Questions

In birds, hollow melanosomes are organized into nanostructures that produce iridescent colors and increase brightness. However, platypuses are brown, leading to further research questions regarding the correlation between melanosome shape and color in this species.

Hollow melanosomes were not identified in closely related species like long- or short-beaked echidnas, nor in any of the approximately 120 other mammal species examined.

Evolutionary Hypotheses: An Aquatic Adaptation?

One hypothesis suggests that hollow melanosomes may have served as an adaptation for an aquatic lifestyle in the platypus's ancestors, providing insulation. Echidnas, evolving into land-dwelling species, might have subsequently lost these features, while platypuses, remaining aquatic, retained them.