The larynx 'evolves faster in primates' than other mammals

Humans and other primates have evolved ‘significantly larger’ voice boxes than other mammals to help with social interactions, study shows

  • The voice box evolves faster in primates than in other mammals including tigers
  • Primate larynges also undergoing faster rates of evolution than other mammals
  • New study claims to be the first large-scale study into the evolution of the larynx 

Humans and other primates have evolved ‘significantly larger’ voice boxes than other mammals to help with social interactions, a new study shows. 

Compared with other mammals such as cats, the voice box, or larynx, of primates such as gorillas and chimpanzees is more than a third larger in relation to their body size. 

They also found that primates’ voice boxes undergo faster rates of evolution, and are diverse in function and more variable in size.

Researchers made CT-scans of specimens from 55 different species, including primates and other mammals, and produced 3D computer models of their larynges. 

The research claims to be the first large-scale study into the evolution of the larynx, where tissue vibrations produce sounds for vocal communication.

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Pictured, a chimpanzee. The researchers made CT-scans of specimens from 55 different species, and produced 3D computer models of their larynges

‘Our study also shows that differences in larynx size predict changes in voice pitch, highlighting the larynx’s crucial role in vocal communication,’ said study author Dr Daniel Bowling at Stanford University in California. 

‘This is demonstrated by the rich and varied calls produced by many primate species.’ 

The larynx has three main functions – protecting the airway during feeding, regulating the supply of air to the lungs and vocal communication. 

Because of its important role in facilitating social behaviour, through vocalisation, it has long been believed that the larynx is a key area of evolution.

This is thought to be particularly true for species with highly developed vocal communication systems – best exemplified by humans, but also true of other primates, from gorillas to gibbons. 

3D image of a gorilla larynx. Tissue vibrations in the larynx produce most sounds that comprise vocal communication in mammals

For the study, 3D computer models of the larynges of 55 different species were analysed alongside detailed measurements, including body length and body mass.

The primates ranged in size from a pygmy marmoset (Cebuella pygmaea) weighing just 3.8 ounces (110g), to a Western gorilla (Gorilla gorilla) weighing approximately 264 pounds (120kg). 

These were compared to the order of mammals known as carnivorans, which include cats, dogs, wolves, bears and hyenas. 

The carnivoran specimens in this study spanned from a 9.8 ounce (280g) common dwarf mongoose (Helogale parvula) to a 396 pound (180kg) tiger (Panthera tigris).

A gorilla skull scanned for the study, within which is the scan of the larynx. The larynx has three main functions: protecting the airway during feeding, regulating the supply of air to the lungs, and vocal communication

For a given body length, primate larynges are on average 38 per cent larger than those of carnivorans, and that the rate of larynx evolution is faster in these species, the study found. 

There is also more variation in larynx size relative to body size among primates, indicating that primates have greater flexibility to evolve, the specimens demonstrated. 

Carnivorans follow more of a fixed larynx-size to body-size ratio, meaning the size of larynx is more consistently proportioned through this order compared with primates. 

Larynx size was also found to be a good predictor of the call frequency of a species, as in the ‘pitch’ of the call.

For example, howler monkeys produce extremely low frequency roars, while tarsiers, small leaping primates, produce ultrasound calls, which are so high pitched that humans cannot hear them.

‘As larynges increase in size across species, we get an even bigger effect on call frequency in primates than in carnivorans,’ study author Dr Jacob Dunn, also at Anglia Ruskin University, told MailOnline. 

This phylogenetic tree of 55 species in the study. Carnivorans (red) exhibited smaller larynges than expected based on body size, whereas primates (blue) exhibited larger larynges

In order to achieve the variation observed in residual larynx size, the larynges of multiple primate lineages underwent rapid diversification throughout history.

This has left howler monkeys with big larynges and smaller body size, compared to the western gorilla, which ranks high for both larynx and body size. 

The results imply ‘fundamental differences’ between primates and carnivorans in the forces constraining larynx size, while highlighting an evolutionary flexibility in primates.

This can help explain why they have developed complex and diverse uses of the vocal organ for communication. 

‘This study demonstrates clear differences in the evolution of the larynx between groups of mammals,’ said Dr Dunn.

‘Specifically, we have shown for the first time that the primate larynx is larger, less closely linked to body size, and under faster rates of evolution than the carnivoran larynx, which is a well-matched comparison group, indicating fundamental differences in the evolution of the vocal organ across species.’

Baboons of the primate order of mammals. Primates is the order to which people belong and is divided into two suborders – The Prosimii, who have longer snouts than their relatives, and the Anthropoidae

Future studies could examine larynx variation among the 20-odd mammalian groups, aside from primates and carnivorans.    

The study has been published in the journal PLOS Biology.

Last month, researchers in Switzerland revealed that humans develop fine motor skills later than other primates because we have bigger brains that take longer to develop. 

In comparison, squirrel-like tamarins achieve their full potential when it comes to mastering objects quicker, but don’t have the skills of more sophisticated primates, they revealed in Science Advances.

WHAT ARE THE DIFFERENT ORDERS OF MAMMAL? 

Rodentia (gnawing mammals)

Includes mountain beavers, chipmunks, squirrels, marmots, field mice, lemmings, muskrats, hamsters, gerbils, Old World mice, rats, geomyidae (gophers), and Dipodidae (jerboas). 

1,700 species, the largest mammalian order. 

Chiroptera (bats)

There are two suborders of bats, the only mammals that can fly. 

Suborder Megachiroptera contains one family, the Pteropodidae (flying foxes, Old Worm fruit bats). 

Suborder Microchiroptera contains 17 families, including mouse-tailed bats, sheath tailed bats, hog-nosed or butterfly bats, bulldog or fisherman bats, slit-faced bats, false vampire bats, and horseshoe bats. 

19 families, 178 genera, 926 species (the second largest mammalian order).  

Primates

The order to which people belong is divided into two suborders – The Prosimii, who have longer snouts than their relatives, and the Anthropoidae.

The first group includes the tree shrew, lemurs, aye-ayes, lorises, pottos, and tarsiers. 

The anthropoids include marmosets, New World monkeys, baboons, Old World monkeys, gibbons, gorillas, chimpanzees, orangutans and Hominidae (human beings). 

Carnivora (meat eaters)

There are two suborders of these toe-footed creatures. They include wolves, dogs, jackals, foxes, bears, giant pandas, coatis, raccoons, lesser pandas, and Mustelidae (martens, weasels, skunks, otters).

They are all part of one superfamily that is characterised by long snouts and unretractable claws.

Felidae (cats, lions, cheetahs, leopards) Hyaenidae (hyenas), and Viverridae (mongooses, civets), all have retractable claws. 

There are between 240-270 recognised species in this order, depending on the source. 

Artiodactyla (even-toed hoofed animals)

Hoofed animals with an even number of toes include those that ruminate, or digest their food in four-chamber stomachs and chew cuds, and those that do not ruminate. 

Those that ruminate include giraffes, deer, moose, reindeer, elk, pronghorn antelope, and cattle, bison, yaks, waterbucks, wildebeest, gazelles, springboks, sheep, musk oxen, goats. 

Nonruminators include pigs, peccaries, hippopotamuses, and camels, llamas.

Lagomorpha (pikas, hares, and rabbits)

Two families make up this order: Ochotonidae (pikas) and Leporidae (hares and rabbits of all sorts). 

Insectivora (insect-eaters) 

The three members are the families Talpidae (moles), Soricidae (shrews), and Erinaceidae (hedgehogs). 

Cetacea (whales and purpoises)     

Two suborders of Order Cetacea are the toothed whales, which have regular conical teeth, and the baleen, or whalebone, whales, which have irregular whalebone surfaces instead of teeth.

Toothed whales include sperm whales, narwhals, belugas, porpoises, and dolphins, killer whales. 

Baleens include gray whales, right whales, fin-backed whales, hump-backed whales). 

Marsupialia (pouched animals)  

Included among these are rat opossums, true opossums, native cats, native mice, marsupial moles, numbats, bandicoots, koalas, wombats and kangaroos, wallabies.

Perissodactyla (odd-toed hoofed animals)

The two suborders, Hippomorpha and Ceratomorpha, include creatures that have an odd number of toes. 

Families in this order are the Equidae (horses, donkeys, zebras), the Tapiridae (tapirs), and the Rhinocerotidae (rhinoceroses)   

Monotremata (egg-laying mammals)

These more primitive mammals make up the families Tachyglossidae (echidnas, also called spiny anteaters) and Ornithorhynchidae (platypuses). 

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