A few million years ago, Jamaica was home to one of the strangest boxers in the animal kingdom – a flightless bird called Xenicibis xympithecus that could batter its enemies with club-shaped wings.

Xenicibis is a large, extinct, flightless ibis. It was discovered by Storrs Olson from the Smithsonian Institution, who found some partial remains in a Jamaican cave in 1977. When Olson eventually saw the bird’s wing bones, he was baffled. They were so “utterly strange” that he thought the animal must have been suffering from some inexplicable disease.

Since then, Olson has found more remains including an almost complete skeleton. Now, he and his partner Nicholas Longrich from Yale University, have a very different view of the wing. They think it was a club. Weapons like clubs and bats have large weighted ends to deliver heavy impacts, and long handles to increase the speed of the swing. That’s exactly what you see in Xenicibis’s wing.

Its hand bone (the metacarpal) was massive, curved and inflated – perfect for inflicting strong blows. It sat at the end of a long ‘handle’, made up of the wrist and the forearm – perfect for creating a fast swing. The metacarpal is also hollow, just like many baseball bats are, allowing it to produce a stronger blow without adding too much weight. And its joints allowed it to swing its wing out very quickly, and extend it as far as possible, giving it speed and reach.

The bones are telling, but did Xenicibis really punch with its wing? It’s hard to be sure, especially because there are few modern birds with similar bones to compare against. However, Longrich and Olson have found some compelling evidence that the bird struck heavy blows with its wings At least two specimens of Xenicibis had arm bones that had broken and healed. The first had broken its upper arm (humerus) in two and the bones hadn’t knitted together properly. The second had fractured its hand, and a massive callus had grown over the front edge. These birds struck something with enough force to injure themselves.

Xenicibis might have used its wings to clobber enemies in defence. Unlike its living cousins, this ibis couldn’t fly. Many island birds lose the ability to fly because they aren’t threatened by any land predators. As a result, their wings become small and stunted, as in the kiwi or the flightless cormorant of the Galapagos. But prehistoric Jamaica had no shortage of predators, including a boa, an extinct monkey, and several birds of prey. Defence would have been important.

Alternatively, the bird could have boxed with its rivals. Longrich and Olson note that a couple of flightless birds have similar (but far less extreme) forearms, including the steamer duck and the extinct Rodriguez Island Solitaire. And both of these species occasionally use (or used) their wings in to beat other individuals in fights.

In fact, many birds use their wings as weapons (including some ibises). Some even have special adaptations for combat. Waterfowl in particular, such as geese, ducks and swans, have a wide variety of spurs, spikes and bony knobs on their wrists. They use these weapons in battle and conflicts can be very violent (although there’s some debate about whether swans can actually break a person’s arm). Xenicibis just expanded on a theme that’s common in the bird world and took it to an evolutionary extreme.

Reference: Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2010.2117 If the citation link isn’t working, read why here

More on bird wings:

• First birds were poor fliers – flaps would have buckled Archaeopteryx feathers
• Chasing daylight – tiny trackers reveal the incredible flight plans of the Arctic tern
• Sound the alarm – crested pigeons give off warning whistles simply by taking off
• Anna’s hummingbird outflies falcons and fighter pilots
• An insider’s look at the feather, a marvel of bioengineering

January 4th, 2011 by Ed Yong in Animal behaviour, Animal defences, Animals, Birds, Palaeontology | 6 comments | RSS feed | Trackback >