Rock-dwelling agamids on two continents: Ctenophorus vs Agama, part 2: body mass, sociality, colouration, sympatry, and fecundity

...continued from https://www.inaturalist.org/journal/milewski/86303-rock-dwelling-agamids-on-two-continents-ctenophorus-vs-agama-part-1-illustrations#

The photos in part 1 give an impression of intercontinental similarity.

Females of rock-dwelling spp. of Ctenophorus (Australia) and Agama (southern Africa) seem so alike that, if the illustrations were unlabelled, most naturalists might assume them all to belong to a single fauna.

Given that the genera are, in fact, phylogenetically distinct (https://www.sciencedirect.com/science/article/abs/pii/S1055790314002267), the similarities would seem to indicate evolutionary convergence (https://en.wikipedia.org/wiki/Convergent_evolution).

However, the impression of convergence is misleading, for the following reasons:

• Closely related (confamilial) ancestors were probably recruited in the first place. The most recent common ancestor may already have been fairly similar, tens of millions of years ago (https://onlinelibrary.wiley.com/doi/abs/10.1002/jez.1133). Thus, shaping these lizards adaptively may not have required much evolutionary modification.
• More importantly, the lizards are actually not as similar as they seem. The superficial appearances are misleading.

It is true that rock-dwelling agamids on both landmasses have such similar diets that they could be considered to share a single trophic role. All the spp. studied here are semi-myrmecophages, in the sense that the most frequently eaten items are small colonial insects, particularly ants (Formicoidea).

Juveniles, which tend to range away from rocks into the surrounding terrain, tend to eat ants to the degree that they are functional myrmecophages (https://en.wikipedia.org/wiki/Myrmecophagy). Adults - particularly in the largest-bodied spp./mature males - eat enough coleopterans and orthopterans to relegate ants and termites to a minor proportion of the volume/mass of food eaten.

(The following show Agama atra eating ants (https://www.inaturalist.org/observations/34858600) and orthopterans (https://www.inaturalist.org/observations/11278625 and https://www.dreamstime.com/royalty-free-stock-photos-southern-rock-agama-image6046178).)

The dietary analogy applies not only intercontinentally but also within southern Africa. For example, Agama anchietae and A. planiceps are widely sympatric, yet have similar diets (https://www.researchgate.net/publication/335899948_Reproduction_predation_sexual_dimorphism_and_diet_in_Agama_anchietae_Reptilia_Agamidae_from_Namibia and
Heideman N J L Sept. 2002, Journal of Herpetology, A comparison if the breeding and nonbreeding season diets of Agama aculeata and Agama planiceps (Reptilia: Agamidae) in Windhoek, Namibia).

What is remarkable is that, despite this common denominator in diet, rock-dwelling agamids differ intercontinentally in

• body size (particularly mass),
• gregariousness/sociality,
• conspicuous colouration in females,
• sympatry (https://en.wikipedia.org/wiki/Sympatry#:~:text=In%20biology%2C%20two%20related%20species,common%20range%20exemplifies%20sympatric%20speciation.), and
• fecundity (particularly clutch-size).

BODY SIZE

Body mass is the single most biologically important aspect of any species of organism.

Therefore, any intercontinental difference in body masses among rock-dwelling agamids - particularly in adult females - would mean that the convergence/analogy is less than it seems from the photos presented in part 1.

Reviewing the literature, my findings are that rock-dwelling spp. of Ctenophorus, in Australia, are considerably smaller-bodied than rock-dwelling spp. of Agama, in southern Africa.

The first line of evidence is the snout-vent lengths presented in part 1, in which the maximum values in Australia and southern Africa are 10 cm and 12.5 cm respectively.

The intercontinental difference is clearest in mature males, which have snout-vent lengths several cm less in e.g. Ctenophorus decresii than in e.g. Agama planiceps.

However, it applies also, in some degree, to adult females. This is true overall, notwithstanding that A. anchietae and A. kirkii are smaller-bodied than A. atra and A. planiceps.

A second line of evidence is found in http://shaimeirilab.weebly.com/uploads/5/5/3/3/5533843/appendix_2_lizard_maximum_snout_vent_lengths.pdf.

This reference lists the maximum snout-vent lengths of the relevant spp. as follows (the measurements presumably refer to males, in all cases):

Ctenophorus:

decresii 9.0 cm
fionni 9.6 cm
ornatus 9.3 cm
rufescens 9.7 cm
vadnappa 9.0 cm

Agama:

anchietae 14.0 cm
atra 14.0 cm
kirkii 11.5 cm
planiceps 14.8 cm

It is evident that rock-dwelling spp. of Ctenophorus have snout-vent lengths less than 9.7 cm, whereas rock-dwelling spp. of Agama have maximum snout-vent lengths more than 11.5 cm.

As a third line of evidence, Meiri et al. (2020, Appendix 1 in https://eprints.whiterose.ac.uk/168862/1/GEB-2019-0253.R4_final.pdf) estimated body masses on the basis of the allometric relationship between snout-vent length and body mass in lizards generally.

The results are as follows (the first value is mean body mass of adult females, the second is maximum body mass of mature males):

Ctenophorus:

decresii 11.75 g, 24 g
fionni 8.9 g, 29.5 g
mirrityana 11.75 g, 24.5 g
ornatus 16.6 g, 28.1 g
vadnappa 9.3 g, 24 g

Agama:

anchietae 25.7 g, 95.5 g
atra 22.4 g, 95.5 g
kirkii 16.2 g, 51.3 g
knobeli - , 95.5 g
planiceps 23.4 g, 112.2 g (if these calculations apply, mature males weigh fourfold more than adult females in A. planiceps)

These results indicate that, among rock-dwelling agamids in Australia and southern Africa, adult females weigh

• 9-17 g in Ctenophorus, which is less than
• 16-26 g in Agama.

The corresponding comparison for mature males (maxima, not means) is

• 24-30 g in Ctenophorus, which is much less than
• 51 g (kirkii)-112 g (planiceps) in Agama.

In summary:
Although few direct measurements are available, several lines of evidence suggest that there is an intercontinental disparity in body masses. The difference seems to be

• in the case of the more central sex, viz. females, about two-fold (10 g in Australia vs 20 g in southern Africa), and
• in the case of mature males, about four-fold (with the exception of a relatively small-bodied sp., Agama kirkii, https://www.inaturalist.org/observations/103148911).

GREGARIOUSNESS/SOCIALITY

Rock-dwelling agamids are less gregarious/social in Australia than in southern Africa.

The Australian spp. tend to occur in pairs, with both a female individual and a male individual defending a shared territory.

This differs from the tendency documented in rock-dwelling Agama (with the possible exception of A. anchietae).

The southern African spp. have been described as not only polygynous but also colonial.

This is consistent with the above-mentioned differences in sexual dimorphism: mature males are smaller-bodied, relative to females, in rock-dwelling Ctenophorus than in rock-dwelling Agama.

CONSPICUOUS COLOURATION IN FEMALES

(By conspicuous, I mean colouration that advertises the individual sexually/socially, by virtue of either dark/pale contrast or bright hues such as blue, yellow, or orange.)

Females of rock-dwelling agamids lack conspicuous hues in Australia, but feature such hues in southern Africa.

No species of rock-dwelling Ctenophorus seems to have any conspicuous aspect - beyond ultraviolet patterns at the throat - to its colouration in females.

By contrast, females of rock-dwelling Agama in southern Africa feature noticeable hues. Although their colours are not as bold as in mature males, what is noteworthy is that the bright-hued patterns are different

• between the sexes, and
• between A. planiceps and the other spp.

The noticeable hues of females occur

• in A. anchietae, A. atra, A. kirkii, and presumably A. knobeli, only in adult females in breeding condition, but
• in A. planiceps, in females and juveniles, regardless of breeding condition (I do not know if juvenile males share the colouration of juvenile females).

For illustrations of conspicuous hues in females of A. anchietae, please see two detailed comments below by Johannes van Rooyen (@johannesvanrooyen ), who has been observant beyond anything that I have found in the literature.

Agama anchietae, A. atra, and A. kirkii share a certain pattern of feminine colouration, in breeding condition. In the case of Agama anchietae, this makes females the more boldly-coloured sex, because males tend to lack bright hues.

All of these spp. feature yellow in females. However, this hue occurs on the torso in most spp., vs the head in A. planiceps of Namibia and Angola.

In the case of A. planiceps, the conspicuous colouration of females and juveniles is not as gaudy as that of mature males. However, what makes it remarkable is that its pattern is

• distinctively feminine/juvenile on the head, and
• not restricted to breeding condition/season.

(For the gaudy conspicuousness of males of A. planiceps, please see part 3.)

Agama anchietae and A. planiceps, although broadly sympatric, differ in sociality and sexual dimorphism.

What makes A. planiceps odd is that the bright hues in females and juveniles consist of yellow spots on the head (https://www.inaturalist.org/observations/182622561) and orange patches on the thorax (https://www.inaturalist.org/observations/9032284).

The hues in females and juveniles of A. planiceps are puzzling, because they

• seem to compromise the otherwise camouflaged effect, and
• remain vivid irrespective of the fluctuation of darkening/paling of the torso according to day/night and basking activity (https://www.inaturalist.org/observations/135030700).

(The following shows the hues of female/juvenile A. planiceps at their dullest: https://www.inaturalist.org/observations/32480455.)

It seems that A. planiceps is so social that this compromises the anti-predator camouflage of females, regardless of breeding condition. The hues possibly aid social advertisement, beyond the sexual selection that has produced gaudiness in males.

SYMPATRY

There is a biogeographical difference between Ctenophorus and Agama, as follows.

In the Australian genus, the various rock-dwelling spp. tend to be allopatric, replacing each other in various parts of the continent.

The main exception is that C. modestus and C. vadnappa are partly sympatric in the Flinders Ranges (https://en.wikipedia.org/wiki/Flinders_Ranges) of South Australia (http://danielhoops.com/blog/2014/6/4/rock-dragon-group-2). Here, they segregate according to the darkness of the rocks.

By contrast, in the southern African genus, there is major overlap, in distribution and habitat, between

• A. anchietae and A. planiceps (in northern Namibia), and
• A. anchietae and A. atra (in southern Namibia and Northern Cape province of South Africa).

FECUNDITY

Based on https://eprints.whiterose.ac.uk/168862/1/GEB-2019-0253.R4_final.pdf, rock-dwelling agamids in Australia lay fewer eggs at a time than do those in southern Africa.

In the relevant spp. of Ctenophorus, clutch-size is about 5 (3-5.5). By contrast, in the relevant spp. of Agama, clutch-size is about 11 (6.5-17.5).

This means that clutch-size in the Australian rock-dwelling agamids is generally less than half that in the southern African rock-dwelling agamids.

OVERALL DISCUSSION

The textbook-concept of evolutionary convergence is one in which different phylogenetic lineages, subject to natural selection under similar environmental regimes on different landmasses, come to resemble each other in more than a superficial way.

What is surprising about rock-dwelling agamids is that they seem to turn this concept on its head.

The same family, namely Agamidae, has long been shared between Australia and southern Africa. The phylogenetic constraints would seem to be minimal, and the environmental similarities maximal. Therefore, it seemed reasonable to expect particularly close similarity in this comparison.

However, in reality, rock-dwelling agamids differ so much intercontinentally that it seems that, if anything, evolutionary divergence has occurred. This applies to body size (including sexual dimorphism) and reproductive mode, which are basic to the niche of any species.

Females and juveniles of A. planiceps have conspicuous colouration in a way beyond any emulation of masculine advertisement; the following shows the sexual difference in bright-hued patterns (https://www.inaturalist.org/observations/96081518). This degree of sexual dimorphism in colouration is not seen in any rock-dwelling species of Ctenophorus.

If the comparison is focussed on females (thus factoring out some of the vagaries of sexual selection that tend to shape males in a distracting/confusing way), what emerges is that the typical Australian rock-dwelling agamid differs from the typical southern African rock-dwelling agamid in being

• smaller-bodied (albeit with a proportionately longer tail in some spp.),
• less gregarious/social, and less sexually dimorphic in body size
• less conspicuous (in hues visible to the human eye) in females, even in breeding condition,
• less likely to share its range with a rock-dwelling congener, and
• less fecund, at least in terms of the number of offspring produced per reproductive bout.

to be continued in https://www.inaturalist.org/journal/milewski/86484-rock-dwelling-agamids-on-two-continents-ctenophorus-vs-agama-part-3-variation-in-masculine-colouration#...

Posted on October 30, 2023 01:17 AM by milewski