by ICB blogger Etti Cooper, Bates College.
Hummingbirds are familiar fauna throughout the Americas; as charismatic and elegant as they are tiny, they often stop for a meal at a hummingbird feeder and flit away just as quickly as they appear. But Dr. Anusha Shankar, who presented her recent work at the 2022 SICB meeting, wants you to know that hummingbirds are much more complex than they appear. Shankar notes that “you can’t really judge them by the cover”: not only are hummingbirds surprisingly aggressive with one another, they are also shockingly resilient despite their apparent fragility. When collecting them in the field, hummingbirds “just sit there in your hand,” she explains. “I haven’t had hummingbirds freaking out as much as I’ve had other birds freaking out from being caught in the net.”
Dr. Anusha Shankar (Photo by Jen Shook, National Geographic)
Early in her career, Shankar never saw herself studying birds. Initially interested in snakes and insects, she began her work with hummingbirds upon starting her PhD in 2012. After shifting her focus to studying Seasonal Affective Disorder in Nile grass rats as a postdoctoral fellow, Shankar ultimately returned to working with hummingbirds at the Cornell Lab of Ornithology at Cornell University in 2020, enthralled by the unique energetic and metabolic properties of these tiny birds.
Because hummingbirds are so small—the bee hummingbird Mellisuga helenae can perch on a pencil eraser!—they have incredibly high metabolic rates, which means that they must consume an extraordinary number of calories per day. As Shankar explains, if a human were to survive on potato chips alone, we would need around 15 small bags per day to reach our caloric requirement. Now imagine a human-sized hummingbird; it would need a whopping 600 small bags per day. This high caloric requirement means that hummingbirds must feed many times per day, which begs an important question: how do they survive the night without feeding?
Hummingbirds are able to enter a state called torpor, which is very similar to hibernation. But rather than hibernating to survive a long winter like many mammals and reptiles, hummingbirds enter torpor just to survive the night. When in torpor, the heartbeat slows significantly, and the bird draws just one breath every ten to fifteen seconds. However, hummingbirds that enter torpor dispense with a function that is considered to be crucial for survival in birds and mammals: thermoregulation.
Birds and mammals are endotherms, which means that they are able to maintain a constant internal body temperature through metabolic processes alone. Unlike in reptiles, amphibians, fishes, insects, and other taxa, the temperature of the surrounding environment has little bearing on body temperature in endotherms. But hummingbirds in torpor, as opposed to hummingbirds in a traditional sleep state, do not actively regulate their own body temperature. In fact, they have been described as “micro-endotherms,” blurring the line between the endotherms—birds and mammals—and the ectotherms, or everyone else.
A display of aggression in Santa Lucia, Ecuador (Photo by Anusha Shankar)
Shankar and colleagues employ fascinating laboratory techniques in order to identify when the transition between states occurs. The difference in body temperature between sleeping hummingbirds and hummingbirds in torpor is an important tool used to determine which state a particular bird is in. Shankar and colleagues use images generated by thermal cameras to pinpoint exactly when an individual enters torpor. A warm, sleeping hummingbird glows red and orange against a deep blue background; when using torpor, the bird begins to blend into the blue background as its internal temperature decreases and eventually equilibrates with the ambient temperature.
Recently, Shankar has discovered that many hummingbirds are able to employ shallow torpor, in which some thermoregulation is still present, in addition to deep torpor. However, there is still a lot to learn. “We know so much more [in mammals],” Shankar explained, when asked how studying birds differs from her work with rodents. “[The fields of mammal and bird physiology] are advancing at very different rates because we think of rodents as a good proxy for humans, and so that gets more funding and more interest in so many ways.”
But Shankar’s past experiences with mammalian systems are serving her well. Currently, Shankar is investigating the underlying gene expression involved in the use of torpor. One strategy is to “look at what genes are important in mammals, and look for similarities among the same genes in birds,” she explains. In fact, the idea for this project came to her when working in a lab that studied Nile grass rats as well as hibernating Arctic ground squirrels. “I think there’s a lot we can learn by talking to each other,” she notes. At the broadest scale, Shankar wants to know which physiological processes are “non-negotiable” when hummingbirds utilize torpor.
The takeaway? Hummingbirds are just as beautiful as they are puzzling from a physiological standpoint. While researchers, including Shankar, have worked hard to demystify their fascinating lifestyles, many unanswered questions remain. Perhaps most importantly, little is known about the effects of human interaction on hummingbirds. Shankar notes that some birds seem to be overwintering in snowier regions of the US in response to the availability of hummingbird feeders. While it is generally understood that red dyes in artificial nectar are harmful, whether feeders are generally more beneficial or detrimental to wild populations remains to be seen.
It is evident that investigating hummingbirds’ status as “micro-endotherms” and their unique use of torpor will allow us a deeper understanding of the physiological and metabolic processes used by endotherms and ectotherms alike. Equally evident is the fact that we still have a lot to learn.
Read Shankar’s recent publication
here, and find her on her website or on Twitter at @nushiamme.
Shankar A, Cisneros INH, Thompson S, Graham CH and Powers DR. 2022. A heterothermic spectrum in hummingbirds. Journal of Experimental Biology. In press.
Connect with sci comm blogger Etti Cooper who is interested in plasticity in thermal tolerance and metabolism in insects, reptiles, and fishes. Connect with her on Twitter at @EttiCooper.
Read other ICB hummingbird papers:
Bene“fit” Assessment in Pollination Coevolution: Mechanistic Perspectives on Hummingbird Bill–Flower Matching
Locomotion and Energetics of Divergent Foraging Strategies in Hummingbirds: A Review
Get SICB student fund hummingbird merch
with art by our Assistant Editor Devaleena Pradhan via https://sicb-artinbiology.pixels.com/