Comparing courtship: men pump iron, and birds perfect songs

Breaking new ground 23. jan 2024 6 min Associate Professor Coen P.H. Elemans Written by Morten Busch

Recent research shows that songbirds engage in daily vocal training. This practice is vital for attracting mates, since female songbirds prefer males that exercise their voice. The study reveals that songbirds‘ vocal muscles rapidly deteriorate without regular use: use it or lose it. Uniquely, these muscles become weaker but more rapid with training. These findings are not only fascinating for understanding avian behaviour but also hold significant potential for advancing human speech therapy and vocal training.

Did you ever wonder why birds sing so much? Humans exercise to get strong and look good. Birds practice their tunes every day to keep their vocal muscles in great shape. Think of it as hitting the gym but for their voice! A recent study of avian communication reveals the importance of these daily singing rituals.

“Our study challenges long-held notions about birds’ vocalisation. We have been observing the muscles animals use to vocalise for a long time. They are very specialised. Our observation underscores the complexity of the birds’ songs and their daily training routine, similar to that of athletes,” explains Coen Elemans, Professor, Department of Biology, University of Southern Denmark, Odense.

The research highlights the critical role of vocal exercise in maintaining the complex muscle system required for melodious calls. It also reveals that female songbirds prefer males that consistently practise.

“The females will pick the males that train, and this clearly motivates the males to sing – because the females like it,” notes Coen Elemans.

Temporarily kept in the dark

To explore bird singing, researchers from Denmark collaborated with teams from Leiden University in the Netherlands, the University of Umeå in Sweden and the University of Vermont in the United States, focusing on the zebra finch (Taeniopygia castanotis). These songbirds have 5–9 pairs of muscles around their syrinx, enabling them to produce various sounds. The brain of a songbird holds four times as many neurons per cm3 as those of cats or dogs.

“I have been studying the very specialised muscles animals use to vocalise for a long time. Songbirds achieve the most rapid muscle speeds possible, and they get faster during their growth. Our main question was: what is driving this? We did several experiments, which I think nicely show that training drives this,” says Coen Elemans.

The researchers’ first definitive experiment was to sever the brain’s link to these muscles.

“We saw within days that the muscles declined in strength and speed. Everything shifts. But this is a very unnatural intervention,” explains Coen Elemans.

The next step was to prevent the birds from singing to determine whether singing drives vocal muscle performance. The birds were temporarily housed in dark conditions, removing their motivation to sing completely.

“A few times a day, we gave them feeding sessions. As soon as they started singing behaviour, we would just move or wave, and they would not start singing. This worked for the first two days, when eating was their main priority, but after three days, their motivation to sing was extremely high,” says Coen Elemans.

Males sing; females do not

Even though it has long been known that songbird singing is controlled by rapid vocal muscles, researchers still know very little about how these muscles might respond to exercise, like we know for human leg muscles.

“The experiments show that if songbirds do not use their vocal muscles, these muscles weaken and slow down within days. Notably, skipping singing for just a week reduces muscle strength by 50%. If they do not use them, they lose them quickly,” says Coen Elemans.

In zebra finches, the syrinx is the same in males and females as juveniles. But as male finches start learning to sing, around 20 days after hatching, the muscles in their syrinx grow in size and speed, whereas the female’s syrinx remains largely unchanged.

“The singing behaviour is completely sexually dimorphic in this species. Males sing; females do not. This is also reflected in the brain; males have specific pathways for producing song, their primary function, whereas females lack these,” explains Coen Elemans.

This is more than just hormones. Previous studies manipulating hormones did not fully account for the differences in syrinxes between males and females.

“Our current hypothesis is that the brain plays a key role in these differences. We believe the brain’s sexual differences drive this behaviour. So, if we stimulate a female’s syrinx, we might get it to develop like a male’s. Conversely, removing stimulation from a male’s syrinx resulted in a female-like state,” says Coen Elemans.

Taught them how to push buttons

The researchers went full circle. They prevented the birds from singing and investigated the physiology, the proteomics and other things. However, they also noticed that the lack of training of the male syrinxes led to changes in the songs themselves.

“But then, I think the fun part is that we also recorded the songs and looked at all the changes that occur. You and I could barely hear a difference between the songs, but we saw clear effects when we analysed our song recordings,” explains Coen Elemans.

The amplitude of the sound declines after not training, but the effects on frequency vary between birds. Humans need equipment to tell the difference between the songs of a trained and an untrained male, but the researchers wanted to test whether the female zebra finches could detect the differences.

“We taught them how to push buttons. One button played one song and the other button played a different song. Then they just sit there. They get such strong feedback from listening to the song. They love it and press the buttons hundreds of times per day. The experiment showed that the females preferred the trained version of the same male and the same song. The females will select the males that train,” says Coen Elemans.

Similar to listening to sports

Apparently, for the birds, voice is extremely important to attract females. They compete with certain vocal athletics to attract females.

“It is similar to human sports. So, the males were clearly motivated to train, because the females like the singing better,” says Coen Elemans.

Thus, vocal quality acts as a reliable indicator of a male’s fitness. The researchers still do not fully understand how the females can tell the difference. “We cannot distinguish what frequency changes are caused by different muscle changes. But the females can detect this. This preference indicates that vocal output carries information about recent exercise history. Singing is crucial for songbirds. They sing to impress future partners, to defend their territory and to maintain social bonds,” explains Coen Elemans.

The necessity of daily vocal exercise for maintaining peak performance provides new understanding of why birds sing regularly, even under adverse conditions. The findings paint a complex picture of the interaction between physical exercise, vocal performance and social interactions in zebra finches, but the findings are also important to understand the development of the birds’ muscles.

“The muscles controlling their syrinx change, both in mass and speed, during their postnatal development. These muscles eventually become some of the most rapidly contracting muscles in any vertebrate. After a bird is born, its muscles change and grow. We have now showed that this happens because the bird’s brain and body work together when it exercises,” says Coen Elemans.

Slow to fast

Human limb muscles typically get slower with exercise, but songbird vocal muscles behave differently.

“Something odd happens when these songbirds exercise their singing muscles. Unlike limb muscles, which usually get stronger and slower with exercise, their vocal muscles get weaker but faster,” notes Coen Elemans.

This is the opposite of what happens with regular muscles in the human body.

“We think this unique way of muscle training might not just be for birds but for all vertebrate animals, including humans, because the origin and way vocal muscles grow, and change is similar for all vertebrates,” says Coen Elemans.

The new study shows that the mechanisms driving the development of the bird’s singing muscles are different and part of a different group, like cranial facial muscles in humans, which are differently organised, but very few experiments have been performed on this range of muscles – among humans.

“Speech is very important for us; there is a lot of information in our speech. Surprisingly, we know very little about the effects of exercise on these kinds of muscles and whether they even react to training in humans. Strength training and endurance training turn fast fibres into slow fibres that are much stronger or more resilient to fatigue. Here, we saw the opposite. If you train them a lot, they get faster,” explains Coen Elemans.

Implications for speech therapy

Thus, the new study not only advances knowledge of bird communication but also has potential implications for understanding human speech development and disorders.

“Because studying the physiology of human larynx muscles is so challenging, therapeutic intervention today is based on what is known about the exercise physiology of leg muscles. And no professional singer dares to let researchers come even near their precious voice box. Therefore, these findings in birds can have major consequences for speech therapy and vocal training in humans,” notes Coen Elemans.

The parallels between the vocal training in birds and speech development in humans open new avenues for research, especially in voice therapy and rehabilitation.

“We know that people do not speak for a few days after surgery; they have problems speaking, and part of that is their vocal folds. The muscles may also need some retraining. You cannot perform these measurements among humans very easily. We can devise specific experiments in songbirds and then measure humans more precisely,” says Coen Elemans.

People with Parkinson’s already get treated with speech therapy, but there is no physiological basis for understanding why this helps.

“We know that it works because the voice improves. We do not know the physiological basis for this. Much evidence, for example, indicates that early-onset Parkinson’s can be detected in speech way before it can be detected in handshakes or motion. There are many signatures of aspects of speech – also among humans. So, surprisingly, songbirds may be our best allies to further improve human voice training and rehabilitation,” concludes Coen Elemans.

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