Birds keep their balance underfoot

Abstract

Every day is a balancing act for two-legged, land-dwelling animals traversing tree roots, rocks or any other obstacles they may encounter. The balance system of one nimble two-legged animal, the guinea fowl, is remarkably robust, allowing them to remain stable even when encountering the most extreme trips and slips: their leg muscles rapidly counteract a bad step in response to signals sent from nerve sensors embedded in the muscles. A team of researchers led by Monica Daley, now at the University of California, Irvine, USA, had previously conducted a series of experiments to learn how the guinea fowls’ muscles pull off this feat, but the role of the nerve sensors had eluded them. So, they decided to study how guinea fowl take obstacles in their stride when a critical sensory nerve is knocked out.They first trained the animals to run on a laboratory treadmill set to speeds ranging from 1.3 to 2.0 m s−1 – the equivalent to a brisk walk in humans. Then, to challenge the birds’ stability, they introduced 5 cm high hurdles on the treadmill at every 10th step taken by the animal. Once the birds were used to hurdling, the team severed the sensory nerve in the ankle muscle that, under normal circumstances, corrects the muscle length back to normal and maintains balance during running. In addition, the team implanted mechanical and electrical sensors in the ankle muscle to measure its force production and to gauge when nerve and brain signals activated the muscle. Finally, to find out how the birds fared when their nerve signals were cut, the team compared the birds’ responses with those of another group of birds with intact nerves.When the researchers looked at how the birds cleared the hurdles, they found that those with a severed nerve were still able to retain their stability, which underscores their remarkable agility. However, the birds that lacked sensory signals from the ankle muscle relied on generating a large burst of muscle force when stepping over the hurdle. This suggests that the birds compensated for the loss of the sensory feedback from the ankle nerve by using an alternative strategy to activate their muscles and maintain stability.To learn more about this substitute strategy, the team turned their attention to the muscle activations and found that the birds that lacked sensory nerve signals from the ankle activated their ankle muscles about 23 ms sooner and over a longer duration than the birds with an intact nerve. The shift in muscle activation timing suggests that the brain compensated for the loss of nerve signals by ramping up its signal to the ankle muscle. The compensation shows that the signal from the ankle muscle sensory nerve plays a role in maintaining balance – without the nerve signal, the brain must step in to compensate for the loss.Daley and her colleagues have shown that nerve sensors that monitor ankle muscle length contribute to maintaining balance in guinea fowl. The current results add to the understanding of how balance is maintained across two- and four-legged vertebrates and may even have implications for understanding how human balance problems arise from nerve injury or neurodegenerative disorders – because we, like guinea fowls, are only as stable as our next foothold.