A fascination with dinosaur skeletons has led Jonathan Codd of Bonn University to new insights into birds' breathing physiology. Codd was puzzled by the role of velociraptors' uncinate processes, `small ossified structures projecting like little handles from their ribs.' Reflecting on the evolutionary link between dinosaurs and extant birds, he hoped that our feathered friends might provide him with some clues regarding the function of these strange structures. When Codd investigated the muscles associated with the uncinate processes of giant Canada geese, he found that these bony projections are integral to bird breathing mechanics(p. 849).
Codd explains that vertebrate respiratory and locomotor systems are mechanically linked, so muscles attached to the rib cage could either help an animal breathe or stabilise it during walking. Together with Steve Perry he headed across the Atlantic to team up with Dave Carrier, a vertebrate morphologist at the University of Utah who studies the relationship between respiratory and locomotor systems. To determine whether the three muscles associated with birds' uncinate processes primarily assist during breathing or walking, they decided to measure muscle activity and breathing in standing,sitting and running geese.
Muscle activity is normally recorded by sticking needle electrodes into muscles. `But you can't be sure that the needle is in the right place' Codd says. So he surgically implanted patch electrodes directly onto the three muscles, a technique that `allows you to be 100% certain that you have the right muscle.' To record the birds' breathing, Codd needed to measure pressure differences in their air sacs, `bags that act as bellows to pump air in and out of the birds' lungs.' Dona Boggs, a bird respiration expert, helped Codd insert differential pressure transducers into the birds' air sacs. Geese can have nasty tempers, but the team used this to their advantage: they measured the large inspirations and expirations associated with the birds' threatening hisses. If the three muscles were involved in breathing, the team reasoned they should see muscle activity during hissing.
Sure enough, just before the birds hissed, the team noticed a burst of electrical activity from the appendicocostal muscle and larger inspiration,indicating that this muscle has an inspiratory function. And they saw bursts of activity from the external oblique muscle during the large expirations associated with hissing, suggesting that this muscle plays a role in expiration. But they only saw short bursts of activity in the external intercostal muscle while the geese were running, not when the birds were hissing, so the team concluded that this muscle has a locomotor function. Clearly, only two of the three muscles associated with birds' uncinate processes help the animals breathe.
The team were particularly intrigued to see increased appendicocostal activity in sitting geese. Codd explains that when birds sit, their sternum movement is restricted, making breathing difficult. `The increased appendicocostal activity we noticed suggests that when the sternum's movements are restricted, like when birds are sitting on their nests, this muscle causes the rib cage to flare sideways and draw air into the birds' lungs' he says. So thanks to this muscle, birds can breathe easy during those long periods of egg incubation.