Batman in the wind

Not only Drosophila manage to fly at constant speed – bats maintain their groundspeed even when wind conditions change.

Livingstone's fruit bat  By Ben Charles via Wikimedia Commons

Livingstone’s fruit bat
By Ben Charles via Wikimedia Commons

When the winds change, bats adapt their speed of flight so that they manage to fly at constant speed. Researchers tracked fruit bats in Ghana on their way from roost to feeding sites, and correlated their flight speed with wind conditions. Contrary to fruitflies, fruit bats compensate for wind changes the way we would expect: with tailwind, they reduce their flight speed, and with headwind, they increase their airspeed. Flying at constant speed likely allows bats to reduce the cost of travel. They may easily measure the total distance travelled, navigate more easily according to landmarks, and reach their feeding sites without having to search widely at the destination. Bats, migratory birds and insects all similarly manage to fly at constant speed, even in changing wind conditions.

Original research paper in the Proceedings of the Royal Society B.


Need for speed – Drosophila antennae regulate flight speed

Just relying on their eyes is not enough for the fruitfly Drosophila when zooming to that pitiful last banana on your counter. Wind-sensing antennae allow fruitflies to fly at constant speed, even when winds change.

Characteristically, fruitflies straight flight bouts are punctuated by quick changes in direction. Flies tend fly straight at a constant speed, even when wind speeds change. This suggests that flies actively regulate their groundspeed. The visual information of just seeing your kitchen pass by isn’t fast enough for the fly. 50 to 100 milliseconds have passed by the time the image reaches the fly’s brain and is processed so that it can respond, which roughly equates to 10 to 20 wing beats between seeing and reacting.

Sensors on the antenna of flies measure the speed of passing air with a delay of only about 10 milliseconds. In a paper published this week in PNAS, researchers let flies with clipped antennae fly in a wind tunnel, and compared how they fared in short and rapid gusts of wind with flies with intact wings.  Flies react to wind in an unexpected way, in the same direction as air drag – in headwind gusts, flies slow down, while in tailwind gusts, they fly faster. But afterwards, flies with intact antennae quickly return back to a set groundspeed.  Flies without antennae return to their groundspeed more slowly, and in steady wind, the groundspeed of flies without antennae changes greatly.

Drosophila melanogaster - sensing airspeed with their antennae. By André Karwath aka Aka (Own work) [CC-BY-SA-2.5], via Wikimedia Commons

Drosophila melanogaster – sensing airspeed with their antennae.
By André Karwath aka Aka (Own work) [CC-BY-SA-2.5], via Wikimedia Commons

The researchers show that airspeed measured by the antennae is combined with what the fly sees to achieve a steady groundspeed. Visual information is a baseline to regulate groundspeed to a specific velocity, but it has a long delay. So if windspeed changes quickly, the visual information is not fast enough to adapt flight speed rapidly and reliable: By the time the fly is able to react, the wind might have already changed again, and the adaptation overshoots. Airspeed measurement by antennae has a short delay, and gives a robust input that makes speed changes less variable. Combining airspeed with visual information allows flies to fly at a stable groundspeed, and rapidly react to wind speed changes.

Feeling the air and seeing the kitchen pass by, flies adapt their speed to changing winds, helping them to home in on your lonely banana.

If you would like to give your banana a better fate than being fly fodder, I suggest smitten kitchen’s recipe for chocolate banana bread.

Elephants – now in your language class

“Look, look over there, a group of elephants is coming!” Next time you are on a safari, you might not want to say these words if you’d like to snap a photo. Or at least not in Maasai: new research shows that elephants in the Amboseli National Park in Kenya are able to distinguish between Maasai and Kamba voices, and run away from Maasai men – but not from Kamba men, or Maasai women.

Young elephant in Madikwe Game Reserve, South Africa.

Young elephant in Madikwe Game Reserve, South Africa.

Probably everyone who has been face-to-face with these giant animals, looking so unlike other mammals, has been fascinated by elephants. But such encounters are not always without risk, not only for humans, but also for elephants themselves. Maasai whio herd their cattle in in the Amboseli National Park often come into conflict with elephants over access to water. Occasionally, elephants will kill cattle and even people, and Maasai warriors retaliate through – often lethal – spear attacks on elephants. Elephants therefore have a reason to be fearful of Maasai men, but not women. The Kamba, also living in the Amobseli National Park, are mostly farmers. As Kamba rarely engage in conflict with elephants, they do not usually pose a threat to elephants.

Elephants in Madikwe Game Reserve.

Elephants in Madikwe Game Reserve.

Researchers from the University of Sussex used this juxtaposition to test whether elephants in Amboseli are able to tell apart Maasai and Kamba, men and women, young and old, only by their speech. The researchers played recordings of “Look, look over there, a group of elephants is coming” recorded by Maasai and Kamba in their own languages to groups of female elephants, and observed the elephants’ reactions. Previous research has shown that elephants react to threat by bunching together in defense, smelling the air to investigate and retreating. Somewhat surprisingly, elephants distinguish between the Maasai and Kamba languages: When they hear a recording by Maasai men, elephants group together and smell investigatively, anticipating a threat. But when played a recording by Kamba men, these defense reactions are much rarer. Elephants also distinguish between men, women, and young boys, as they rarely react defensively when hearing a recording by Maasai women or Maasai boys, who herd cattle and frequently encounter elephants.

Elephants are considered one of the most cognitive advanced animals. The paper published this week in PNAS shows that elephants can also make very subtle distinctions between voices, even distinguishing between different languages, based on the level of threat speakers pose to them. The researchers also note that elephants react differently to human voices than to lion roars – while elephants retreat in response to Maasai men, they approach to recordings of lion roars. Retreating from humans probably allow elephants to avoid the threat, while aggressive mobbing can drive off lions. It is a remarkable achievement, which allows elephants to adapt to changes in the environment and the society they are faced with it. This off-sets at least some of the disadvantage they suffer from their long generation time and slow population growth in a changing environment. Let’s hope that by being clever, elephants will be around for a long time.

Research paper in PNAS: