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Sound production mechanisms in animals

Last updated:
22 June 2017
Elodie Briefer; Institute of Agricultural Sciences, ETH Zürich, Universitätstrasse 2, 8092 Zürich, Switzerland
The modes of production of sounds and the structures used in sound production in animals are very diverse, and range from vibration of the wings in fruit flies (Drosophila) to stridulation in crickets, snapping of the swimbladder in fish, tongue clicks in some bats and vibration of the tympaniform membranes in birds or of the vocal cords in mammals. Several seminal papers on the modes of acoustic sound production and its link with the acoustic structure of vocalisations, as well as on the information content of vocalisations, have been published in Journal of Zoology. This Virtual Issue gathers a selection of reviews and research papers, from various years, on the topic of sound production mechanisms.
In the first paper, Parmentier et al. (2008) compared the structures involved in acoustic communication in three species of pearlfish (Carapidae; Carapus boraborensis, Encheliophis gracilis and Carapus homei). The authors analysed the sound production system as well as the acoustic features of the sounds produced. They were able to relate each part of the sounds to the action of swimbladder muscles of the producer. They also show anatomical as well as acoustic differences between the three studied species.
With the second and third papers selected for this Virtual Issue, we move on to bird sound production. Thorpe (1958) and Warner (1972) are among the earliest detailed accounts on the topic of sound production mechanism in birds. Thorpe (1958) is a review paper describing the vocal apparatus of birds and comparing it with human vocal apparatus. Warner (1972) is a research paper comparing the anatomy of the syrinx in several passerine birds (songbirds). Warner’s main finding is that the only demonstrable vibratile areas, hence the only sound sources, in the syrinx are the internal tympaniform membranes. These findings would be later confirmed by many other studies.
Most bats produce echolocation signals, ranging from 20 to 200 kilohertz in frequency, using their larynx. However, some variation exists. For instance, a few species click their tongue, whereas horseshoe and leaf-nosed bats emit their echolocation calls through their nostrils, which are surrounded by a fleshy, horseshoe/leaf-like structure. In the fourth paper of this Virtual Issue, Robinson (1996) investigated the function of the noseleaf of horseshoe and leaf-nosed bats in echolocation. His results show that noseleaf width is determined by wavelength rather than body size, thus highlighting the importance of the noseleaf in shaping the echolocation signal.
The next paper selected for this Virtual Issue, Frazer Sissom et al. (1991), investigates purring in cats. The mechanisms of cat purring have turned out to be challenging to understand. Indeed, the very low fundamental frequency of purring, notably in domestic cats (around 25 Hertz), suggests alternative mechanisms of sound production than flow-induced vocal cord vibration, as only very long cords could produce such low frequency. The authors recorded domestic cats in a shelter. Their results suggest that purring could arise from the gating of respiratory flow by the larynx. This had been suggested also by Remmers and Gautier (1972), who showed that purring is in fact produced by active contractions of laryngeal muscles modulating the respiratory air flow passing through the vocal cords, as opposed to flow-induced self-sustaining oscillations found, for example, in humans.
According to the source-filter theory of voice production (Fant, 1960; Titze, 1994), the air flow coming from the lungs induces the oscillation of the vocal cords, thus producing the “source” sound. This sound is then filtered in the vocal tract (“filter”). Some frequencies, which correspond to the resonances of the vocal tract, will be amplified and other frequencies will be dampened. The source determines the lowest frequency of the voice (fundamental frequency) and its harmonics, while the filter determines the spectral peaks, called “formants”. The source-filter theory framework has been recently adapted to other animals. The sixth paper of this Virtual Issue, Taylor and Reby (2010), describes how mammal communication research can benefit from this framework by highlighting information, in vocalisations, about the sender’s characteristics. The seventh paper of the Virtual Issue, Briefer (2012), describes how this framework can help to find vocal correlates of emotional arousal and valence in mammals.
The eighth paper selected for this Virtual Issue, Fitch (1999), shows how the source-filter theory framework can be applied to birds, in order to explain the evolution of trachea elongation. More than 60 bird species possess an elongated trachea. Fitch suggests that this characteristic evolved through sexual selection to exaggerate perceived body size, as animals can produce vocalisations with lower formants than expected based on their body size.
The ninth paper in this Virtual Issue investigated the source of vocal production in muskoxen (Ovibos moschatus). Although highly sexually dimorphic (males are 1/3 heavier than females), both sexes of this species produce very low roars. Frey et al. (2005) found that roars in both sexes are characterised by a pulsed structure, with a pulse rate of 20 Hz on average. The larynx size of adult male and female are remarkably similar (i.e. almost identical larynx size and vocal cord length). Muskoxen thus differ from other species with similar mating systems (e.g. fallow deer, red deer, elephant seals), in which strong sexual dimorphism is accompanied by distinct acoustic differences.
The two last papers selected for this Virtual Issue on sound production mechanisms investigated the link between vocal tract length and formant frequencies in canids (Plotsky et al. 2013) and fallow deer (McElligott et al. 2006), respectively. Plotsky et al. (2013) present one of the first clear evidence that formants provide honest vocal cues about signaller size, by testing the link between vocal tract length and measures of body size in Portuguese water dogs (Canis lupus familiaris) and Russian silver foxes (Vulpes vulpes). McElligott et al. (2006) investigated the link between vocal tract elongation and formant frequencies in fallow deer (Dama dama). This species possess a mobile larynx that can be retracted during vocalizations. The authors show that fallow buck individuals can increase their vocal tract length on average by 52% during vocalization. This phenomenon could be used by males to exaggerate their perceived body size.
We hope that you will enjoy reading this collection of papers on various modes of sound production and adaptions.

In celebration of Alfred Russel Wallace (1823-1913)

Last updated:
22 June 2017
Elina Rantanen
Journal of Zoology, Zoological Society of London
The selection of papers presented here, featuring birds, mammals, insects, fish and molluscs, reflect the breadth of Wallace’s contributions to zoology and biogeography. In addition to describing new species, Wallace included observations on their anatomy, appearance, behaviour and geographical distribution in relation to similar species and made suggestions as to how they should be classified. For example, it is fascinating to read Wallace, the ‘father of biogeography’, describe the distribution of different bird species on the Sula Islands in Indonesia and conclude that “it seems to me clear that the Sula Islands are really an outlying portion of Celebes, and must at some former period have had a much closer connexion with that great island than at present”. This clearly relates to his concept of the ‘Wallace Line’, a faunal boundary line that separates Asia and Australia, which is the result of historic variable land connectivity between islands in the region. In the same paper, and of particular interest to us at the Zoological Society of London, Wallace mentions that he named one of the 13 new species of birds he describes after Dr Sclater, “the indefatigable Secretary to the Zoological Society of London, to whose kind assistance and extensive knowledge of ornithology I am much indebted”.

Without a doubt, the jewel of this collection is the paper entitled ‘Narrative of Search after Birds of Paradise’, in which Wallace describes the toils and misfortunes (including disease, hunger and pirates!) he encountered during his expedition to New Guinea to collect specimens of birds of paradise, “this rare and beautiful bird”. This paper is truly fascinating: in it we can enjoy the poetic elegance of Wallace’s writing as he describes his admiration of these beautiful birds. Equally compelling is his account of the catastrophes he encountered, which allows us to experience the extent of Wallace’s dedication to his task.

We hope that you will enjoy reading this Virtual Issue and join us in celebrating Alfred Russel Wallace as one of the greatest zoologists of all time!

Journal of Zoology 50th Anniversary

Last updated:
22 June 2017
The Zoological Society of London has published scientific papers in zoology since 1830 in Proceedings and Transactions of the Zoological Society of London. These publications were merged in 1966 to form the Journal of Zoology. We are delighted to celebrate our anniversary year with this Virtual Issue, which includes 12 influential and highly cited articles. These fantastic contributions represent the breadth of research that we continue to publish in the journal to this day.

Women in Zoology

Last updated:
17 May 2017
Jon Bielby
Institute of Zoology, Zoological Society of London, Regent’s Park, London, UK
In my short scientific career I have been fortunate enough to work with some great scientists. I’ve worked with a Fellow of the Royal Society who revolutionised the way we measure extinction risk, a disease-ecologist whose research is cited in parliamentary debates on one of the most controversial environmental and agricultural issues in UK politics, and an epidemiologist whose research informs policy on the efficacy of interventions on diseases such as dengue fever, foot-and-mouth disease, pandemic influenza, and severe acute respiratory syndrome (SARS).

I’m not listing these examples to introduce a virtual issue about my C.V. and the excellent people I have worked with: I am listing these examples because this virtual issue of the Journal of Zoology is about women in science, and all of the scientists I have mentioned above are senior women scientists in the UK.

In considering the background to this virtual issue I have come to realise that my experience of working with so many women in advanced career positions is unusual and atypical. In doing so, I have side-stepped a problem of great importance to contemporary science with the EU and UK (my apologies to JZO’s global readership, but I limit my statistics and statements on gender inequality to the UK and EU, and I’m therefore committing another type of inequality myself by omitting data from everywhere outside of Europe…I apologise for this). The fact is that science in the EU is imbalanced, with gender inequality being present in STEM subjects at each level beyond that of the PhD, and being particularly obvious at more senior levels - the very levels at which decisions are made and future directions are set.

At junior levels within STEM subjects (science, technology, engineering and maths), the playing field appears to be even. Throughout the EU the proportion of female students and graduates exceeds that of their male counterparts, while at the level of the PhD and PhD graduates the gender of candidates is roughly 50:50. However, above and beyond the Doctoral training things start to become skewed, and by the time we reach the very highest career-levels, women represent only 18% of professors within the EU 1  and 17% within the UK 2.

The gender-bias at the highest levels is, at least in part, due to the loss of skilled and experienced young women from STEM careers, which may occur for a number of reasons. Career progression within STEM subjects is often associated with long working hours and time away from home for data collection and long-running experiments. Such expectations and a lack of institution-based support for scientists (of either gender) mean that many young scientists find it very difficult to make critical life-choices and create a better work-home balance. As a result many people, particularly women, are lost to the STEM system, deciding to take their skills and experience with them to more supportive, flexible sectors.

What of it? Does this inequality affect how science runs and progresses? Well, yes, it does; it affects it to a large degree. Aside from the moral and philosophical arguments for increasing equality and support in STEM subjects, there are also very strong economic arguments for reducing the loss of STEM trained scientists and increasing gender equality in science. In the UK we have a deficit of skilled STEM workers. If we are to meet this shortfall, we desperately need to boost the number of people staying in STEM subjects, particularly women, at every level of experience and seniority. Additionally, there is a massive cost to losing STEM-workers at each level of the career ladder. According to a 2005 report issued by the UK Higher Education Funding Council for England, the costs of supervising and, more importantly, training a PhD student can be as high as £87,000 3. With inflation that figure could now be into six-figures. Given the high proportion of graduates leaving STEM subjects, a great deal of money, skill and experience is lost from the system, which is an inefficient way to run a sector in which funding is tight to begin with. Additionally, there is the unknown cost to scientific progress of systematically losing scientific knowledge and talent in general, and specifically to gender bias.

Discussing the ways and means of motivating people to stay in STEM subjects and rendering gender inequality a thing of the past is beyond the scope of this introduction, and there are many other better-informed people than me who have written widely on the subject 4,5,6,7,8,9. Instead, having raised the problem, I shall leave the elephant (the metaphorical one – it’s always good to qualify that phrase in this line of work) firmly in the middle of the room and introduce the papers forming this virtual issue of the Journal of Zoology.

In compiling this virtual issue I have trawled the archives and have attempted to choose papers from some of the most notable female scientists and personalities from the history of the ZSL, the Journal of Zoology, and zoological research in general. And here they are:

Emily Sharpe, 1891, Descriptions of New Butterflies collected by Mr. F. J. Jackson, F.Z.S., in British East Africa, during his recent Expedition. – Part 1

This paper represents the earliest paper first-authored by a woman in the archives of the Journal of Zoology. Although the specimens were collected by a man, in the 19th Century it became increasingly common for women to describe the specimens, thereby doing the actual science.

Edith Durham, 1896, Notes on the Mode of Feeding of the Egg-eating Snake

This paper represents another very early publication by a female scientist, and is a highly personal choice. One of my very earliest memories is of looking through an animal encyclopaedia and seeing and being fascinated by pictures of an egg-eating snake. For someone who was already amazed by reptiles and amphibians, this was an incredibly strong imagine and it is amazing to think that it was first described in the Journal of Zoology well over a century ago.

Dorothea Bate, 1907, On Elephant Remains from Crete, with Description of Elephas creticus, sp. n

Dorothea Bate was a palaeontologist who worked at the NHM from the age of 19. Dorothea was an unconventional character, who once commented that her education was only briefly interrupted by having to go to school. Dorothea was nominated as a Fellow of the Royal Geographic Society in 1948 and published prolifically, both generally and also specifically in the Journal of Zoology, throughout her career.

Nellie Eales, 1925, External Characters, Skin, and Temporal Gland of a Foetal African Elephant

Dr Nellie Eales was the first woman to be awarded a PhD, in her case from the University of Reading. In a historical context, this paper was published before women had the same voting rights as men in the UK (parity being granted in 1928), which is amazing: “Here’s your PhD, but you can’t vote on how the country is run” (actually, in 1925 in the UK if Nellie was over thirty years of age and lived in a house worth over £5 she would have been able to vote – young and poor women couldn’t between the years of 1918 and 1928).

Joan Beauchamp Procter, 1928, On a living Komodo Dragon Varanus komodoensis Ouwens, exhibited at the Scientific Meeting, October 23, 1928

Joan Procter was the Curator of Reptiles at ZSL, who regularly travelled into the field to study and collect specimens to bring back to the ZSL. This is an early example of the synthesis of research science and the Living Collection that ZSL strives to support to this day.

Sidnie Manton, 1973, Arthropod phylogeny – a modern synthesis

Sidnie Manton was the first female Fellow of the Royal Society to have published in the Journal of Zoology. She was awarded her Fellowship in 1948 making her the seventh woman to be elected into that prestigious role. Incidentally, Sidnie’s sister, Irene, was also awarded one, in 1961 for her services to botany: how about that for a high-achieving family? Sidnie was awarded the ZSL Frink medal in 1977.

Georgina Mace, 1988, The genetic and demographic status of the Western lowland gorilla (Gorilla g. gorilla) in captivity

Georgina Mace was the first woman to be the Director of Science at the Zoological Society of London. In addition to her work on the evolution of life histories in small-mammals, and on the genetics of captive populations of threatened species (such as this JZO paper), Georgina was responsible for revising the IUCN Red List in order to make extinction risk assessments based on clear, standardised, quantitative criteria. Georgina was elected a Fellow of the Royal Society in 2002, and is also a Commander of the British Empire.

Adrienne Zihlman, Mary Morbeck, and Jane Goodall, 1990, Skeletal biology and individual life history of Gombe chimpanzees

This paper includes two very high-profiled women in science. Adrienne Zihlman is a professor of anthropology who has spent her career researching issues related to human origins and evolution, and has a particular interest and has published widely on the role of women in human evolution and adaptation.

Dame Jane Goodall is best known for her ground-breaking work on the chimps of Gombe Stream National Park. In addition to communicating her findings through scientific publications, she has produced a number of popular science books and is perhaps best known as an activist and advocate for conservation and animal welfare.

Miriam Rothschild,1992, Neosomy in fleas, and the sessile life-cycle

Miriam Rothschild was the first woman to be made an honorary Fellow of the Zoological Society of London. She was a prolific scientist who published over 300 papers and a great number of books during her career. Amongst other achievements, she was the person who identified how the jumping mechanism of the flea worked (thereby providing the mechanism underpinning the pub quiz question “which animal can jump the highest relative to its size?”). Miriam was elected a Fellow of the Royal Society in 1985 and was made a Dame in 2000. One of her quotations, which is a fantastic mantra for any scientist, was “I must say, I find everything interesting”.