Thursday, February 3, 2011

O Sole Mio-My Sun

Singing is ubiquitous in the animal kingdom. The reason for belting out a tune can vary widely among species. Frequently, the singing produced by animals is analogous to all the romantic images we have of males serenading and courting females. Perhaps most familiar to us are birdsongs. We have all been walking through the woods at one time or another and heard the sing-song sounds produced by birds. Or in my case, the lovely mockingbird sitting outside my window that begins his song at 4 o’clock in the morning. Songbirds, or oscines, belong to the suborder of passerine (perching) birds. Believed to have evolved over 50 million years ago, there are roughly 4000 species of songbird. Special to this group of birds is a well-developed vocal organ called a syrinx. In most cases it is the male that sings to announce his presence, to assert his rights over a piece of property (territory), to compete with other males, and to impress the female with his vocal prowess. Each species has its own kind of song. The Cornell Lab of Ornithology has a bioacoustics research program that not only maintains a database of bird songs, but also analyzes the acoustic world of many species. Within a species individuals often have their own unique song. Some individuals may have 20 or more songs in their vocal repertoire! Males often develop songs similar to their father or nearby males and females tend to prefer songs that are most similar to what they grew up hearing. Often many species of bird live together in the same area, so songs can help females identify which male belongs to her species. In North America some of the more common songs you may have heard come from the Northern cardinal and the Northern mockingbird (click on the link to hear the song). Other unusual ones like the white-fronted manakin and the resplendant quetzal are from more tropical regions. Some of the differences you may hear also have to do with the environment that different species live in. As sound moves through a tropical forest it ‘bumps’ into a lot more objects. Imagine trying to talk to a friend on the other side of a stream. The way many birds (and other animals) deal with this is by changing the frequency of their song.

What is more amazing among many songbirds is that the portions of the brain that are involved in producing song grow and shrink, depending on whether or not males and females are pairing up to raise a family. Usually a flood of sex steroid hormones (testosterone) triggers these changes in the brain of males, which is in turn controlled by the photoperiod, or the amount of sunlight. Don’t be fooled though by all the pretty little sounds you hear. It is not all lovey-dovey out there in the bird world, particularly when it comes to singing. Sure to us it can sound lovely, with crows being a notable exception, but sounds can be deceiving. Male singing is a display not just for females, but also for males. Songs are very expensive to produce and if a male does not have enough energy reserves, he simply cannot afford to sing as frequently. A female not only pays attention to the quality of a male’s song, but also how well his song competes with those of other males. Unlike the smashing together of heads we see in rams or the locking of antlers observed in elk, in birds there are special aggressive ‘displays’ centered around singing. For example, males will match their song to that of another male, they may match the frequency that another male is singing at, start singing while another male is singing, or they will even switch their song type all in an effort to ‘outsing’ the competition.

What about other species? We have all had the experience where we are just about to drift off to sleep when “chirp, chirp”...that’s right a cricket. Now if you were in China this would be excellent fortune! It turns out that not all species of cricket chirp, and of those that do, only males sing. As you might imagine if a male is spending a lot of energy singing he is doing it for two primary reasons, 1) to let other males know he is around and 2) to impress and woo the ladies. Again each species has its own unique chirp and the females find males of their species based on the way the male sings. A lot of information is contained in his song, which he produces by rubbing his wings together. Along the wing is a thick vein that acts like the bow of a violin. Depending on whether he is trying to call the female from far away, as far as a mile, or keep her attention when she is nearby, he produces a different song. How does a female cricket decide if she likes a particular male? Females generally want the best possible male and in crickets she can determine this by the quality of his song. One factor that seems to be very important to female field crickets (Gryllus bimaculatus) is age. It would appear that human females aren’t the only ones preferring the younger man! There are many potential benefits for females to mate with a younger male, but what is really amazing is that they can judge how old a male is by the quality of his song. It turns out that the muscles necessary to produce a high quality, energetic song break down with age so the younger, stronger males simply sing better than the old dudes.

Things change completely when we look at gibbons, a largely monogamous primate belonging to the lesser ape group. They inhabit the forests of Southeast Asia and are famous for their loud conspicuous songs that travel long distances through the forest. Every morning a bonded pair, or two gibbons who are in a ‘relationship”, wake up and sing a duet. As you can hear from the video it is incredibly loud, with a crescendo at the end as they turn towards each other…sigh. It is thought that singing together in the morning reinforces the bonds between the male and the female. Maybe we could learn something from the gibbon! Give it a try. Wake up next to your loved one tomorrow and start hooting and singing. For gibbons, though, singing is not only restricted to performing a duet with their partner. Like many animals, gibbons face danger from predators. The sophisticated songs of the gibbon act to put the predator on notice that it has been seen and to relay the information about what type of predator is nearby to other gibbons. They also have different songs depending on whether the predator is a snake, a large cat, or an aerial predator like a hawk.

The champion singer by far has to be the humble house mouse. Yes, you read that right, the mouse. Throughout history there have been a few examples of singing mice. In 1932, Dr. Lee Dice published a paper on the singing of mice. In it he reports that mice have been known worldwide to sing and yet not all mice apparently have this remarkable ability. When audible, the singing mouse sounds like a chirping cricket or canary. The particular mouse he studied was a male house mouse kept in a cage by someone. Dice mated this mouse yet none of the offspring sang quite like their father! What we now know is that all male mice will sing in response to the smell of a female and they are singing to attract the female. Some can be observed moving their mouth but because most of them are singing in the ultrasound range (think bats) we can’t hear them. Every now and then it seems that a male mouse sings in a lower range (e.g., the super sexy Barry White), which is why throughout history there is a legend about singing mice. In 2005 Holy and Guo published an article detailing the characteristics of male songs. You can follow the link to their paper to hear the mouse songs located at the end of the paper in the Supporting Information section (Audio file 1 is great!). What you will hear is that the songs of male mice are complex and sophisticated, complete with syllables and phrases. So maybe, just maybe, like Mighty Mouse he is singing that he will come and save the day!

Barnett, C.A. and Briskie, J.V. 2011. Strategic regulation of body mass and singing behavior in New Zealand robins. Ethology 117:28-36

Clarke E, Reichard UH, Zuberbühler K (2006) The Syntax and Meaning of Wild Gibbon Songs. PLoS One 1(1): e73. doi:10.1371/journal.pone.0000073

Dice, L.R. 1932. The singing of mice. Journal of Mammalogy 13:187-196.

Holy, T.E. and Guo, Z. 2005. Ultrasonic songs of male mice. PLoS Biol 3(12): e386. doi:10.1371/journal.pbio.0030386

Meitzen, J. and Thompson, C.K. 2008. Seasonal-like growth and regression of the avian song control system: Neural and behavioral plasticity in adult male Gambel’s white-crowned sparrows. General and Comparative Endocrinology 157:259-265.

Searcy, W.A. and Beecher, M.D. 2009. Song as an aggressive signal in songbirds. Animal Behaviour 78:1281-1292.

Tramontin, A.D., Hartman, V.N., Brenowitz, E.A., 2000. Breeding conditions induce
rapid and sequential growth in adult avian song circuits: a model of seasonal
plasticity in the brain. J. Neurosci. 20, 854–861.

Verburgt, L., Ferreira, M., and Ferguson, J.W.H. 2011. Male field cricket song reflects age, allowing females to prefer young males. Animal Behaviour 81:19-29.

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