The frilled lizard (Agamidae: Chlamydosaurus kingii), part 1: an Australian genus, doubly odd beyond its cervical bluffing

@alexanderr

Please see:
https://www.researchgate.net/publication/230165917_Function_and_evolution_of_the_frill_of_the_frillneck_lizard_Chlamydosaurus_kingii_Sauria_Agamidae and https://academic.oup.com/biolinnean/article-abstract/40/1/11/2654260?redirectedFrom=fulltext&login=false and
https://academic.oup.com/biolinnean/article/129/2/425/5679583?login=false and https://www.semanticscholar.org/paper/Function-and-evolution-of-the-frill-of-the-lizard%2C-Shine/b105042138edb1962983700d01f1f5a68c4f138d
https://www.publish.csiro.au/wr/WR9890491
https://brill.com/view/journals/amre/26/1/article-p65_10.xml
https://pubs.aip.org/asa/jasa/article-abstract/152/1/437/2838531/The-acoustical-effect-of-the-neck-frill-of-the?redirectedFrom=fulltext
http://www.smuggled.com/Issue-14-24-26.pdf
https://www.researchgate.net/profile/Anthony-Griffiths-2/publication/321765547_Preliminary_investigations_on_the_reproduction_of_the_Frillneck_Lizard_Chlamydosaurus_kingii_in_the_Northern_Territory_A_Diverse_Discipline/links/5b4467860f7e9bb59b1b2edd/Preliminary-investigations-on-the-reproduction-of-the-Frillneck-Lizard-Chlamydosaurus-kingii-in-the-Northern-Territory-A-Diverse-Discipline.pdf

Chlamydosaurus kingii (https://www.inaturalist.org/taxa/31215-Chlamydosaurus-kingii and https://www.youtube.com/watch?v=rLY2gNiOFzk and https://www.australiangeographic.com.au/topics/wildlife/2019/08/the-science-behind-the-frill-of-the-frillneck-lizard/) is a distinctively Australian lizard.

This monospecific genus is well-known for an extreme anatomical structure: a foldable neck-frill, which is erectile by virtue of the opening of the mouth.

However, what is underappreciated is how odd this lizard is, in combining

specialisation for both arboreal (quadrupedal) locomotion and terrestrial (bipedal) locomotion, and
camouflage-colouration (when in trees, e.g. https://www.inaturalist.org/observations/141966420) and warning colouration (when displaying to potential predators).

POSTURES AND GAITS

Chlamydosaurus kingii is specialised for clinging inconspicuously to the boles of trees (https://www.inaturalist.org/observations/148718528
and https://www.inaturalist.org/observations/52991997) - which many photographs show it doing in the rainy season, as part of its sit-and-wait foraging strategy.

Furthermore, it spends the dry season on branches in the crowns of trees, where - in a metabolically quiescent state - it is further hidden by shading and plant-structural clutter.

What this means is that, for about half the year, the head is habitually oriented horizontally, while for the other half of the year, the head is habitually oriented vertically. During the former period, the animal tends to be semi-torpid and relatively inattentive, whereas in the latter period, the animal is alert, spotting its prey (mainly invertebrates) on the ground.

This implies that the eyes have a different relationship to the head during the two periods.

When C. kingii clings to boles, the rigidly vertical orientation of the head seems unremarkable.

However, this vertical orientation of the head tends to remain even when the animal stands quadrupedally on the ground (https://www.inaturalist.org/observations/138701495 and https://www.inaturalist.org/observations/137342571 and https://www.inaturalist.org/observations/124301082 and https://www.inaturalist.org/observations/120806132 and https://www.inaturalist.org/observations/66077775) - which it does mainly in the rainy season, when foraging on the ground.

The orientation tends towards the horizontal when C. kingii runs bipedally with the mouth open (https://www.youtube.com/watch?v=XAo09yYOpCU and https://www.youtube.com/watch?v=B1-U0rzMu1w and https://www.youtube.com/watch?v=OLuoExytqEA).

What does not seem to have been pointed out before is that the eyelids - as well as presumably the eyeballs - swivel with changes in the orientation of the head, from vertical to horizontal.

Various animals, including caprin bovids (https://unsplash.com/photos/eHN5Q5NYEeE and https://www.shutterstock.com/image-photo/beautiful-soay-sheep-ovis-aries-head-1189301416 and https://www.shutterstock.com/image-photo/group-two-soay-sheep-their-offspring-2292568177), are capable of swivelling the eyeballs within the eye-sockets, compensating for shifts in orientation from horizontal to vertical.

Such swivelling of the eyeball itself is not apparent in photographs of C. kingii, because the pupil is circular.

This has not been recorded in any mammal or bird, and it suggests that the whole complex, of eyeball plus lids, swivels in the orbits

Turning now to gaits:

Despite its arboreal specialisation, C. kingii tends to adopt upright bipedality when running (https://www.gettyimages.com.au/detail/video/track-right-as-frilled-lizard-runs-on-hind-legs-in-stock-video-footage/1B02409_0002 and https://gdeichmann.photoshelter.com/image/I0000P8K3H95toi8 and https://gdeichmann.photoshelter.com/image/I0000smri2ParobQ) and walking on the ground.

Several Australian agamids tend to run bipedally. However,

none of these - apart from C. kingii - is arboreal, and
C. kingii is unique among agamids in being able to walk bipedally.

The upright bipedality of C. kingii seems to be facilitated by the proportionately long neck of the species, which allows the animal to lean backwards in maintaining balance.

What this means is that, in their own ways, both the eyes and the cervical vertebrae are adaptively modified in ways consistent with the dual locomotory specialisation of the species.

COLOURATION

The conspicuous colouration of C. kingii is displayed while bluffing would-be predators, as well as when interacting socially/sexually (https://www.youtube.com/watch?v=bkz9PCcRNYE).

The full anti-predator display - which is largely bluff, because the C. kingii has neither formidable teeth nor venom - consists of

erection of the frill,
opening of the mouth,
lashing of the tail, and
approaching and even contacting the human figure.

The colouration of C. kingii varies

ontogenetically (with infants having consistent camouflage-colouration, whether perched or among leaf-litter on the ground),
according to substrate (somewhat analogous with chameleons),
according to mood/season, and
geographically, with the frill tending to be red in the west, yellow in the east (with orange in the intermediate area), and whitish in the southeast (see photos at the end of this Post).

There seems to be scant sexual dimorphism in colouration in C. kingii, despite the fact that mature males can weigh twice as much as adult females, and have the head and frill disproportionately large.

If this is true, it is at odds with the family Agamidae, in which many or most spp. have males considerably brighter-hued than females.

I have yet to understand the adaptive value of this feature, which

plays a negligible part in the display of the erect frill, and
is ambivalent w.r.t. functioning conspicuously (e.g. for communication intraspecifically among individuals with the frill folded) or inconspicuously (by disrupting the figure, in resemblance of a dapple of sunshine).

GEOGRAPHICALLY-CORRELATED VARIATION IN HUE OF FRILL

red hue (Kimberley of Western Australia):
https://www.inaturalist.org/observations/901378
https://www.inaturalist.org/observations/901384
https://www.inaturalist.org/observations/145822596
https://upload.wikimedia.org/wikipedia/commons/7/7f/Frilled-lizard500.jpg
https://www.inaturalist.org/observations/67514916

yellow hue (northernmost Queensland):
https://www.inaturalist.org/observations/145745999
https://www.inaturalist.org/observations/110824795

dark spot on frill:
https://www.inaturalist.org/observations/7534956

maximum display:
https://upload.wikimedia.org/wikipedia/commons/d/dc/Chlamydosaurus_kingii.jpg

to be continued in https://www.inaturalist.org/journal/milewski/82751-the-frilled-lizard-agamidae-chlamydosaurus-kingii-part-5-why-no-african-counterpart#...

Publicado el 02 de agosto de 2023 a las 10:26 PM por