|A ruby-throated hummingbird Archilochus colubris feeds on mimosa flowers. |
Photograph by George Grall newswatch.nationalgeographic.com
Sensory systems define an animal's capacity for perception and can evolve to promote survival in new environmental niches. We have uncovered a noncanonical mechanism for sweet taste perception that evolved in hummingbirds since their divergence from insectivorous swifts, their closest relatives. We observed the widespread absence in birds of an essential subunit (T1R2) of the only known vertebrate sweet receptor, raising questions about how specialized nectar feeders such as hummingbirds sense sugars. Receptor expression studies revealed that the ancestral umami receptor (the T1R1-T1R3 heterodimer) was repurposed in hummingbirds to function as a carbohydrate receptor. Furthermore, the molecular recognition properties of T1R1-T1R3 guided taste behavior in captive and wild hummingbirds. We propose that changing taste receptor function enabled hummingbirds to perceive and use nectar, facilitating the massive radiation of hummingbird species.
The makings of a powerful sweet tooth
The main attraction of nectar, a hummingbird favorite, is the sweet taste of sugar. Oddly, though, birds lack the main vertebrate receptor for sweet taste, TIR2. Baldwin et al. show that a related receptor, TIR1-T1R3, which generally controls savory taste in vertebrates, adapts in hummingbirds to detect sweet (see the Perspective by Jiang and Beauchamp). This repurposing probably allowed hummingbirds to specialize in nectar feeding and may have assisted the evolution of the many and varied hummingbird species seen today.
Maude W. Baldwin, Yasuka Toda, Tomoya Nakagita, Mary J. O'Connell, Kirk C. Klasing, Takumi Misaka, Scott V. Edwards and Stephen D. Liberles. 2014. Evolution of Sweet Taste Perception in Hummingbirds by Transformation of the Ancestral Umami Receptor. Science. 345(6199); 929-933. DOI: dx.doi.org/10.1126/science.1255097
Scientists Solve Mystery of How Hummingbirds Taste Sweetness
How hummingbirds evolved to detect sweetness