An organ that allows certain birds to detect the movement of hidden prey by plunging their beaks into the ground was also present in early birds 70 million years ago, and probably first appeared in their dinosaur ancestors.
Special “remote touch” sensory receptors known as Herbst corpuscles, which are found within densely-packed pits in the beak’s tip, help birds detect the movement of worms in soil or small fish in water – even several centimetres away from the bird’s beak. This effectively gives birds a “sixth sense”, according to Carla du Toit at the University of Cape Town in South Africa and her colleagues.
To work out when the sixth sense evolved, du Toit and her colleagues studied the beaks of hundreds of modern and ancient birds, including four species of lithornithids, an extinct group of birds which lived alongside dinosaurs during the Cretaceous period.
Lithornithids belong to one of the two major types of birds alive today – the palaeognaths, which include kiwi, ostriches, and emus. The other major group is the neognaths.
By examining specimens of modern birds, the researchers identified distinct pitting patterns in the beak associated with Herbst corpuscles, says du Toit. The team then found those same patterns in lithornithid fossil beaks, which suggests that lithornithids had the same sensory abilities and were probe-foraging birds.
The discovery makes sense, because Herbst corpuscles are found in both palaeognaths such as kiwis, and in neognaths such as ibises. The two groups separated from one another more than 70 million years ago, which would suggest Herbst corpuscles evolved in the common ancestor of both bird groups.
In fact, the sensory structures might have evolved in dinosaurs, says du Toit. A “sixth sense” feature might have helped carnivorous theropods such as Neovenator find prey by probing their snouts into mud or murky water, she says.
The researchers hope in future to check whether pitting was present in the beaks of another ancient groups of winged reptiles – pterosaurs. If they are present there, it might suggest the sensory organs evolved earlier still, before even dinosaurs evolved. However, the poor quality of most fossils will make such analyses difficult, says du Troit.