Determining the purpose of the Cassowary’s casque.

Cassowaries, Casuarius spp., are large, flightless, Palaeognath Birds related to Emu’s and Ostriches, which inhabit the dense forests of northern Australia and Papua New Guinea. These Birds have a distinctive helmet-like casque, made up of bone with a keratinous covering, the purpose of which has eluded biologists for over 200 years, with suggestions having been made that the structure might be used in combat, as a sound-enhancing device to help the Cassowaries’ booming call carry further, to protect the head from damage when running at speed through vegetation, or possibly as a secondary sexual characteristic.

A Southern Cassowary, Casuarius casuarius. Lone Pine Koala Sanctuary.

In a paper published in the journal Scientific Reports on 13 February 2019, Danielle Eastick of the Department of Ecology, Environment and Evolution at La Trobe University, Glenn Tattersall of the Department of Biological Sciences at Brock University, and Simon Watson, John Lesku, and Kylie Robert, also of the Department of Ecology, Environment and Evolution at La Trobe University, publish the results of an investigation into the possibility that the casque of the Cassowary is a thermal window used in the regulation of body temperature.

The idea that Cassowaries might use their casques to aid temperature control was first made in 1994, in a paper by Polly Philipps of the University of Illinois, Urbana-Champaign, and Allen Sanborn of Barry University, as part of a wider study into temperature regulation in Palaeognath Birds. However, Philipps and Sanborn were only able to observe a single Cassowary, at an ambient temperature of lower than 30°C (i.e. lower than the average temperature in the areas where Cassowaries live), so while the idea that a Bird as large as a Cassowary (females can reach 160 cm in height and weigh as much as 60 kg), that lives in tropical forests and has a thick coat of black feathers, might use a well vascularised bony casque to help regulate its temperature seems good, there has been, to date, no proof of this.

Eastick et al. were able to examine 20 Cassowaries at a range of ambient temperatures, using a Testo 875i hand-held thermal imager. This revealed that at high temperatures Cassowaries shed the most heat through their casques, the tips of their bills and their legs (beaks and legs are known to be used as heat exchanges in a wide range of Birds), while at low temperatures (i.e. when the Birds were losing heat involuntarily) the Cassowaries lost the most heat through their necks, their eyeballs and the hindpart of their bills. This strongly suggests that when they are hot Cassowaries pump blood to the casque, neck and beak in order to facilitate heat loss, while when they are cold the most heat is lost from areas they cannot afford to cut the blood supply to. They also noted that when they were hot the Cassowaries frequently dipped their casques into water, something which would improve heat loss.

 

Thermal image of a Cassowary at an ambient temperature of 5°C. Eastick et al. (2019).

 

Thermal image of a Cassowary at an ambient temperature of 25°C. Eastick et al. (2019).

Thermal image of a Cassowary at an ambient temperature of 35°C. Eastick et al. (2019).

Eastick et al. also observe that as the temperature rises the rear of the casque begins to shed heat before the front, suggesting that the Cassowaries are able to fine-tune their temperature control by increasing the blood supply to different parts of the casque sequentially, something previously observed in the beak of the Toco Toucan, Rhamphastos toco, which is also used in temperature control.

A Toco Toucan, Rhamphastos toco. Bernard Dupont/Wikipedia.

They also note that casques and casque-like structures are seen in a wide variety of Birds living in tropical climates, including Hornbills, Bucerotidae, Helmeted Guineafowl, Numida meleagris, and the Maleo, Macrocephalon maleo. They also note that such structures are known to have been present in a wide range of Pterosaurs and Dinosaurs, making it possible that these animals also regulated heat in a similar way.

 Parasaurolophus cyrtocristatus, a Dinosaur with a prominant crest that may have been used in tempertature control. Wikimedia Commons.

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Butchery marks on the bones of Elephant Birds suggest Humans reached Madagascar thousands of years earlier than previously supposed.

Madagascar previously had a diverse Vertebrate Megafuana, that included Giant Lemurs, Hippopotamus, Giant Tortoises, and Elephant Birds (Aepyornithidae), an extinct group of Palaeognath Birds that included some of the largest Birds ever to have lived. Most of this fauna disappeared between about 2400 and 500 years ago, which has widely been taken as indicative of the arrival of Humans on the island. This ties in well with the archaeological record, with evidence of Human settlement appearing in rock shelters on the west coast of Madagascar about 3000 years ago, butchery marks on Lemur bones in the southwest of the island known from 2400 years ago, the first villages appearing about 1500 years ago, and extensive coastal settlements by 900 years ago. Some recent evidence has suggested that Humans may have occupied Madagascar much earlier, including Hippopotamus bones with butchery marks dating between 4288 and 4035 years ago and tool stone assemblages from the north of the island that may be over 4000 years old, but these pieces of evidence are not universally accepted, with the dates of the stone tools manufacture being disputed and the diagnosis of the marks on the Hippo bones as being of Human origin both being questioned.

In a paper published in the journal Science Advances on 12 September 2018, James Hansford of the Institute of Zoology at the Zoological Society of London, and the National Oceanography Centre at the University of Southampton, Patricia Wright of the Department of Anthropology and the Centre ValBio at Stony Brook University, Armand Rasoamiaramanana of Mention Bassins Sédimentaires Evolution Conservation at the University of Antananarivo, Ventura Pérez and Laurie Godfrey of the Department of Anthropology at the University of Massachusetts, David Errickson and Tim Thompson of the School of Science, Engineering and Design at Teesside University, and Samuel Turvey, also of the Institute of Zoology at the Zoological Society of London, describe the results of a review of cut marks on the bones of Elephant Birds held in museum collections in Europe, United States, and Madagascar, which suggests that Humans may have arrived on the island several thousand years earlier than previously thought.

The earliest Elephant Bird bones showing signs of butchering came from the Christmas River Site near a tributary of the Ihazofotsy River and Ilakabe village in southern Madagascar. This site preserves a bone bed with numerous Vertebrate species, including Crocodiles; Tortoises; Carnivorans, Giant Lemurs, Dwarf Hippopotamus, and the Elephant Bird Aepyornis. The bones were deposited in a wetland between about 9000 and 11 000 years ago, and no previous evidence of Human activity has been found there. Hansford et al. found evidence of butchery in the form of linear grooves on the distal aspect of the lateral condyle and medial condyle of the of the central trochlea of a a single Aepyornis maximus specimen dated to between 10 721 and 10 511 years ago. The penetrating marks are intact and well defined, consistent with kerfs made by single-bladed, sharp lithic tools and multiple cutting actions intended to disarticulate the central phalanges.

Aepyornis maximus tarsometatarsus. (A) Distal aspect of Aepyornis maximus tarsometatarsus from Christmas River, showing five cut marks: three (TM-1 to TM-3) on the central trochlea (digit III), one (TM-4) on the medial trochlea (digit II), and one (TM-5) on the lateral trochlea (digit IV). (B) Cross section of TM-1 at ×30 magnification, illustrating depth using a topographic height colour scale. Ventura Pérez in Hansford et al. (2018).

These findings suggest that Humans may have reached Madagascar 6000 years earlier than even the most generous previous estimate, which provides a puzzle for palaeontologists and archaeologists studying the island, as it suggests that (assuming that the Humans who made the marks did not themselves become extinct shortly after) Humans coexisted with the Vertebrate Megafuana of Madagascar for most of the Holocene, before switching to a lifestyle that rapidly wiped out all of the islands large animals, either through overhunting or habitat destruction.

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Blindness in wild Kiwi.

Birds are considered to have the best vision in any living Vertebrate group, with almost all Birds having large eyes relative to their size and densely packed retinal neurons, along with other specialisations found in different species and groups. This is closely tied to the ecology of Birds, which typically fly, are visual foragers and rely on markings and display to find mates. However, one group of Birds, the flightless Kiwi, Apteryx spp., of New Zealand, appear to break with this pattern, having small eyes relative to their size, under-developed visual regions of the brain and the smallest visual field of any Birds. Kiwi also have a very different feeding ecology to other Birds, being nocturnal foragers on the floor of canopy forests, with highly developed senses of smell hearing and touch, particularly at the tip of the long bill, which has a unique set of mechanoreceptors (touch neurons), raising questions as to how much value vision is to a Kiwi at all.

In a paper published in the journal BMC Biology on 12 September 2017, Bret Moore of the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California-Davis, Joanne Paul-Murphy of the Department of Medicine and Epidemiology, also at the University of California-Davis, Alan Tennyson of the Museum of New Zealand Te Papa Tongarewa, Christopher Murphy of the Department of Surgical and Radiological Sciences at the University of California-Davis, and the Department of Ophthalmology & Vision Science at the University of California-Davis, describe the incidence of widespread visual impairment, including blindness, in a population of wild Okarito Brown Kiwi, Apteryx rowi.

An Okarito Brown Kiwi, Apteryx rowi, on Mana Island, New Zealand, in June 2017. Leon Berard/New Zealand Birds Online.

Out of a population of 160 Kiwi examined, 53 were found to have some form of visual impairment, and four were completely blind. The Birds were found to suffer from a variety of conditions, including corneal opacification and shrunken fibrotic globes. Many of these infections appeared chronic in nature, suggesting that they were long-term conditions that had not had any undue impact on the Bird’s overall health. This included three of the completely blind specimens, which appeared healthy at the time of inspection, and which were shown by radio-tagging to survive for at least four years after the initial inspection, with one of these Kiwi going on to form a pair-bond with a visually healthy individual. 

Normal and pathologic findings for the anterior segment of the Okarito Brown Kiwi. Complete ophthalmic examinations consisted of slit lamp biomicroscopy, direct ophthalmoscopy, and streak retinoscopy. Lack of vision was interpreted by no response to light or motion, combined with the severity of ocular lesions (e.g. inability to visualize intraocular structures beyond the abnormal ocular tissue, such as marked corneal or lens opacification). (a) Normal anterior segment. Note the small palpebral aperture (mean diameter 8.53 ± 0.50 mm SD, n = 9 birds). (b) Nuclear sclerosis: a normal aging change in the lens associated with changes in lens protein composition. Nuclear sclerosis generally has minimal visual consequences in animals. (c) Buphthalmia with marked corneal edema. This animal was blind bilaterally but was in good physical condition. (d) Phthisis bulbi (a globe shrunken with fibrosis). Potential causes include any chronic inflammatory or glaucomatous process or severe trauma. (e), (f) Resorbing, end-stage cataracts. (g) Anteriorly luxated cataract. h Inferiorly luxated cataract. Moore et al. (2017).

From this Moore et al. conclude that the loss of vision has no real impact on Kiwi. They suggest that while the Birds may use vision to determine the difference between night and day, useful for determining when to forage, but do not appear to need working vision to obtain food (indeed they cannot see the tips of their beaks, their principle foraging tool), nor to obtain a mate. They suggest that Kiwi may be in the process of losing their vision by regressive evolution, i.e. vision is no longer being selected for, so the visual abilities of the Birds are slowly degenerating as deleterious mutations build up in the DNA regions which govern vision.

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