Koala Bellows Deciphered

The pitch of male koalas' mating calls is about 20 times lower than it should be, given the Australian marsupial's relatively small size. Now, researchers reporting in the journal Current Biology have discovered their secret: koalas have a specialized sound-producing organ that has never before been seen in any other land-dwelling mammal. The key feature of this newly described organ is its location outside the voice box, what scientists call the larynx (Figure 1).

Koala_anatomy

Figure 1. The velar vocal folds (dark red) of male koalas are located at the intersection between the oral and nasal portions of the pharynx just opposite to the laryngeal entrance. Oral tract in light blue, nasal tract in yellow, soft palate in light red, laryngeal vocal folds in green, and arytenoid cartilage in blue. (From Charlton et al., 2013).

"We have discovered that koalas possess an extra pair of vocal folds that are located outside the larynx, where the oral and nasal cavities connect," says Benjamin Charlton of the University of Sussex. "We also demonstrated that koalas use these additional vocal folds to produce their extremely low-pitched mating calls."

The koala's bellow calls are a continuous series of sounds produced on inhalation and exhalation, similar to a donkey's braying, Charlton explains. On inhalation, koala bellows sound a bit like snoring. As the animals exhale, the sound is more reminiscent of belching. And, as Charlton says, "they are actually quite loud."

They are also incredibly low-pitched, more typical of an animal the size of an elephant. Size is related to pitch in that the dimensions of the laryngeal vocal folds normally constrain the lowest frequency that an animal can generate. As a result, smaller species will typically give calls with higher frequencies than larger ones.

Koalas have bypassed that constraint by putting those vocal folds in a new location. Charlton describes the folds as two long, fleshy lips in the soft palette, just above the larynx at the junction between the oral and nasal cavities. They may not look all that different from the laryngeal vocal folds of other mammals, but their location is highly unusual.

"To our knowledge, the only other example of a specialized sound-producing organ in mammals that is independent of the larynx are the phonic lips that toothed whales use to generate echolocation clicks," Charlton says.

The combination of morphological, video, and acoustic data in the new study represents the first evidence in a terrestrial mammal of an organ other than the larynx that is dedicated to sound production. Charlton says that he and his colleagues will now look more closely at other mammals to find out whether this vocal adaptation is truly unique to koalas.

Source: Modified from materials provided by Cell Press.

Reference

Charleton, B.D., Frey, R., McKinnon, A.J., Fritsch, G., and D. Reby. 2013. Koalas use a novel vocal organ to produce unusually low-pitched mating calls. Current Biology, 23:R1035-R1036.
http://dx.doi.org/10.1016/j.cub.2013.10.069

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Hyena Scent Posts Use Symbiotic Microbe Messengers

Twitter limits human communication to a mere 140 characters. Animals’ scent posts may be equally as short, relatively speaking, yet they convey an encyclopedia of information about the animals that left them.

In the current issue of the
Proceedings of the National Academy of Sciences, a Michigan State University researcher shows that the detailed scent posts of hyenas (Figure 1) are, in part, products of symbiotic bacteria, microbes that have a mutually beneficial relationship with their hosts.

“When hyenas leave paste deposits on grass, the sour-smelling signals relay reams of information for other animals to read,” said Kevin Theis, the paper’s lead author and MSU postdoctoral researcher. “Hyenas can leave a quick, detailed message and go. It’s like a bulletin board of who’s around and how they’re doing.”

hyena-scent-communication_lg
Figure 1. A spotted hyena scent marking. (courtesy of Michigan State University)

Interestingly, it is the bacteria in pastes – more diverse than scientists had imagined – that appear to be doing the yeoman’s job of sending these messages.

“Scent posts are bulletin boards, pastes are business cards, and bacteria are the ink, shaped into letters and words that provide information about the paster to the boards’ visitors,” Theis said. “Without the ink, there is potentially just a board of blank uninformative cards.”

Theis, who co-authored the study with
Kay Holekamp, MSU zoologist, studied multiple groups of male and female spotted hyenas and striped hyenas in Kenya.

By using molecular surveys, they were afforded unprecedented views of the diversity of microbes inhabiting mammals’ scent glands. The researchers were able to show that the diversity of odor-producing bacteria in spotted hyena scent glands is much greater than historical studies of mammals had suggested (Figure 2).


scentpost
Figure 2. Variation in the bacterial communities and volatile fatty acid (VFA) profiles of the pastes of immigrant male, lactating female, and pregnant female spotted hyenas in the Talek clan. (A and C) A plot showing variation in the structure of paste bacterial communities among Talek clan members. (B) A heat map of the mean abundances of the prominent bacteria in the pastes of Talek hyenas. (D) A heat map of the mean percent abundances of VFAs in the pastes of Talek hyenas. From Theis et al. 2013)

The diversity, however, still consistently varies between hyena species, and with sex and reproductive state among spotted hyenas, Theis added. Importantly, the variation in scent gland bacterial communities was strongly correlated with variation in the glands’ odor profiles, suggesting that bacteria were responsible for the variation in scent.

For the current paper, Theis’ team was the first to combine microbial surveys and complementary odor data from wild animals. The studies’ findings leave Theis anxious to return to the field.

“Now I just need to get back into the field to test new predictions generated by this study,” Theis said. “The next phase of this research will be to manipulate the bacterial communities in hyenas’ scent glands to test if their odors change in predictable ways.”


Source: Modified from materials provided by Michigan State University.

Reference

Theis KR, Venkataraman A, Dycus JA, Koonter KD, Schmitt-Matzen EN, Wagner AP, Holekamp KE, & Schmidt TM (2013). Symbiotic bacteria appear to mediate hyena social odors. Proceedings of the National Academy of Sciences of the United States of America PMID: 24218592


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