As temperatures around the world shift dramatically, wildlife are often being forced to relocate to find suitable habitats – and scientists are working hard to try and understand how many species might struggle with trying to find a new home.
Animals going to higher ground face two issues: colder temperatures, and thinner, less oxygen-rich air (so it's harder to breathe). In a new study, a group of Anna's hummingbirds (Calypte anna) were taken on a trip some 1,200 meters (4,000 feet) above their normal habitat.
Curiously, the hummingbirds' metabolic rates actually decreased as they hovered. They also flew shorter durations with less efficiency, most likely for want of more oxygen.
While future temperatures could be warmer, for now the cooler elevations have a chilling effect on the hummingbird's sleep patterns. When the birds snoozed, they entered a kind of mini-hibernation more often, which also depressed their metabolism by around 37 percent on average.
The team behind the study says that in the case of hummingbirds at least, going to higher ground would pose a significant challenge.
"Our results suggest lower oxygen availability and low air pressure may be difficult challenges to overcome for hummingbirds shifting upslope as a consequence of rising temperatures, especially if there is little to no long-term acclimatization," write the researchers in their published paper.
These birds are already having to move their homes in response to rising temperatures, and can currently be found at elevations of between 10-2,800 meters (that's 33-9,186 feet). That covers quite a distance and range of temperatures, but the research team was interested to see if there was an upper limit.
For this study, 26 hummingbirds were relocated from all across that current elevation range, and they all struggled more or less equally to adapt. However, the study did find that those from higher elevations tended to have larger hearts for better circulating oxygen around the body.
The researchers used a variety of methods to measure sleep levels and metabolic rate in the hummingbirds, including syrup-filled funnels to get the birds to eat while monitoring their oxygen consumption at the same time.
Carbon dioxide production during sleep was also recorded, another indicator of metabolic rate. The hummingbirds spent at least 87.5 percent of the night in a state of torpor or energy-saving mini-hibernation, compared with 70 percent normally. Again, this was consistent no matter what elevation the hummingbirds had been taken from.
"It means that even if they're from a warm or cool spot, they use torpor when it's super-cold, which is cool," says ecologist Austin Spence from the University of Connecticut.
Hummingbirds make great study subjects in this case because of their high-energy lifestyle. They're able to cope with a variety of weather conditions, but it seems as though relocating to higher ground might be beyond them – unless they do it slowly enough for their bodies to be able to adapt.
However, species don't necessarily have to go to higher ground to find cooler temperatures, because they can also change their latitude – and the researchers think these hummingbirds may eventually have to venture further north.
The study authors also suggest that future studies and models shouldn't simply look at temperature as a trigger for species shifting location. Other factors, including water and oxygen availability, also need to be taken into consideration.
"To fully understand a species' capacity to shift in response to a warming climate, it is critical to both assess its physiological performance within its current range and compare this with performance beyond its current distribution," write the researchers.
The research has been published in the Journal of Experimental Biology.