Woodpeckers grunt like tennis players
Woodpeckers really know how to punch above their weight. Woodland birds can attack a tree at around 15 miles per hour with their powerful beaks. To achieve this, woodpeckers essentially transform themselves into hammers, strengthening the muscles in their head, neck, abdomen, and tail to keep their bodies completely rigid as they hammer the wood.
While each impact is caused by the hip flexor muscles and the front of the neck, biologists have learned that there is a greater respiratory force here. Just like tennis stars who grunt to synchronize and stabilize their bodies and hit a ball, woodpeckers also synchronize their breathing with their movements when they hit wood. The results are detailed in a study published today in the Journal of Experimental Biology.
“What’s exciting about woodpeckers is that they take the pecking that we see all birds doing and take it to the extreme,” says Nicholas Antonson, study co-author and NSF postdoctoral researcher at Brown University and an integrative organismal biologist. Popular science. “These extreme performances are interesting to study in biology because they allow us to better understand how ordinary movements are organized and coordinated when subjected to high-intensity tasks.”
The world of a woodpecker
Woodpeckers are well adapted to their life among trees, from beak to toe. Two of their toes point inward and two backward with sharp, pointed claws. With these feet, they can climb tree trunks to find food and shelter.
Most bird species peck to find food. However, woodpeckers also use their characteristic pecking behaviors to dig into nests and use these drums to let other animals know what species they belong to and that they are ready to defend their territory.
“These interactions with the drummers are similar to the way other birds sing, and woodpeckers can assess the quality of each based on their skills as drummers,” Antonson explains. “They also generate very high impact forces when engaging in these behaviors, making them particularly difficult to produce.”
Ace Grunters
To learn more about how woodpeckers use these muscles when drilling into a tree, Antonson and the team captured eight wild downy woodpeckers (Picoids pubescens). For three days, they filmed the birds with high-speed video, recording the moment they drilled and tapped on a piece of hard wood. While recording, the team measured electrical signals in the muscles that control the birds’ heads, necks, abdomens, tails and legs to see when they contracted when the birds hammered with their beaks.
Additionally, they recorded the atmospheric pressure in a section of the airways of six birds and the amount of air that two of them exhaled through their voice boxes. This helped them track the birds’ breathing before returning to the wild.
The team found that the front hip flexor and neck muscles are essential. They propel the birds forward while digging their beaks into the wood. The other muscles seem to play a more supportive role. Antonson says the birds tilt their heads back and rely on three muscles at the base of the skull and the back of the neck. They also seem to prepare their entire body to transform into a hammer. And breathing helps.
“The picks exhale with each hit,” says Antonson. “We show that they exhale the moment they make contact with the wood, similar to how a tennis player grunts when hitting the ball. This is likely because this athletic strategy stabilizes their core and can increase their hitting power in both contexts.”
The birds also perfectly synchronized their breathing with each impact, at rates of up to 13 strikes per second. They inhaled a mini-breath (about 40 milliseconds) between each rapid stroke. However, it’s a little hard to hear, as the growls are drowned out by the drums.
As batsmen, picks are not mere “one-hit wonders” either. They will fine-tune the power of their impacts, depending on whether they tap softer to send a message or drill hard. When the team compared the strength of muscle contractions when woodpeckers pecked, they found that the front hip flexor muscle contracted harder while the birds foraged. This contraction helped the birds exert a stronger impact. The muscles then relax with gentler tapping.
“It’s a full-body effort that requires close coordination of muscles from the tail to the hips, abs, neck and head, in precise timing with breathing that hammers them like nature’s hammer and makes these strikes so effective,” says Antonson.
The rhythm continues
In future studies, the team hopes to see how this drum varies among other woodpecker species. Some may strike even faster than the minor peaks studied in this study.
“One species is capable of drumming at a frequency of up to 40 beats per second,” says Antonson. “It would be exciting to see if the muscular coordination and exhalation with each strike that we observed in our study is maintained at an even more extreme level, or if this species potentially uses a different physiological strategy.”
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