Editor, Earth News
Staying cool, or keeping warm?
It is one of the simplest, but most enigmatic mysteries of nature: just why do flamingoes like to stand on one leg?
The question is asked by zoo visitors and biologists alike, but while numerous theories abound, no-one has yet provided a definitive explanation.
Now after conducting an exhaustive study of captive Caribbean flamingoes, two scientists believe they finally have the answer.
Flamingoes stand on one leg to regulate their body temperature, they say.
Matthew Anderson and Sarah Williams are comparative psychologists based at Saint Joseph's University in Philadelphia, US who are interested in the studying the evolution of behaviour.
"Flamingoes captured my attention for a variety of reasons," says Anderson.
"Scientifically speaking, their highly gregarious nature makes them an ideal species for investigating social influences on behaviour.
"Aesthetically speaking, they are large, beautiful, and iconic.
"Perhaps most importantly, I was very surprised to discover how little systematic, hypothesis-driven empirical research had been conducted on flamingoes."
Anderson and Williams's research began by studying laterality in flamingoes: whether they show any preference over which side of their bodies they use for various tasks, just as a human may be right or left-handed.
They found that flamingoes prefer to rest with their heads on one side more than the other, and that which side a flamingo rests its head determines how aggressive it is toward others in the flock.
TO THE LEFT, OR RIGHT?
Flamingoes do not prefer to rest on one leg more than the other
However, most flamingoes do prefer to rest their head to the right
Those that prefer to rest their heads to the left are more likely to be involved in aggressive encounters with other birds
That lends support to the idea that being right-handed, or right-headed, in this way, helps promote social cohesion in flamingoes
That led the researchers to investigate whether flamingoes also prefer to stand on one leg more than the other, and from there, why they stand on one leg at all, empirically testing the question for the first time.
To investigate, Anderson and Williams spent several months observing the habits of captive Caribbean flamingoes (Phoenicopterus ruber) at Philadelphia Zoo, Pennsylvania, each of which carries a leg band that allows individuals to be identified.
At first, they examined whether standing on one leg helps reduce fatigue in the birds' legs, or helps flamingos escape from predators more quickly, by shortening the time to take flight.
Both are commonly proposed as reasons for unipedal resting in flamingoes.
The scientists ruled out each as a benefit of standing on one leg, as their research showed it took flamingoes longer, and therefore more energy, to move forward after resting on one leg than after resting on two.
The birds also showed no preference for which leg they stood on.
Nor did standing on one leg help the birds balance when conditions were windy, another proposed idea.
However, the researchers did find that flamingoes prefer to stand on one leg far more often when they are standing in water than when standing on land, they report in the journal Zoo Biology.
"As water invariably draws away more body heat, this result supports the thermoregulation hypothesis," says Anderson.
In short, the birds stand on one leg to conserve body heat. If they put two legs in the water, rather than one, they would lose more heat than is healthy, particularly as they spend so much time wading.
"The results provide definitive evidence that thermoregulation is a principal function of unipedal resting in flamingoes," Anderson confirms.
The birds also likely alternate which leg they stand on to avoid one leg becoming too cold.
"If they stood on one leg consistently, they would risk greater loss of body heat and potential tissue damage in the cold," says Anderson.
The researchers also discounted some other more outlandish theories, such as one that suggests standing on one leg helps flamingoes circulate blood better by limiting the Effects of gravity on their circulatory systems.
But they don't eliminate the idea that there may be added benefits as well as conserving body heat.
"Given the wading lifestyle of flamingoes, perhaps unipedal resting helps reduce fungal or parasite load as well," says Anderson.
Others birds, such as herons, storks, ducks and many others also often stand on a single leg in water, perhaps for the same reasons as flamingoes.
But as flamingoes tend to spend much longer filter feeding in water than these other birds, this remains speculation, Anderson says.