A comparison of life-history strategies between two rock-dependent lizards coexisting in the Cape Point area of South Africa

Please see https://www.inaturalist.org/journal/milewski/86215-the-reptile-fauna-of-the-southwestern-tip-of-africa-part-1-checklist-of-families-and-species-in-the-cape-point-area#

Cape Point (https://www.shutterstock.com/image-photo/black-girdletailed-lizard-cordylus-niger-on-391665787) lies at the southwestern tip of Africa.

This Post focuses on two species of lizards, belonging to different families, which coexist here (https://www.shutterstock.com/image-photo/black-girdled-lizard-southern-rock-agama-470332649). Both are rupicolous (https://www.merriam-webster.com/dictionary/rupicolous#:~:text=%3A%20living%20among%2C%20inhabiting%2C%20or%20growing%20on%20rocks).

https://www.inaturalist.org/observations/114579698
https://www.inaturalist.org/observations/63963234
https://www.dreamstime.com/stock-photo-black-girdled-lizard-southern-rock-agama-rocky-outcrop-over-ocean-africa-image76030679

At present, there are 723 observations of Cordylus niger (https://www.inaturalist.org/taxa/33062-Cordylus-niger and https://nextgenherpetologist.co.za/2018/09/24/black-girdled-lizard-cordylus-niger/) and 526 observations of Agama atra (https://www.inaturalist.org/taxa/31150-Agama-atra), making these the most frequently-observed reptiles in the Cape Point area.

Both of these lizards

• are common in the fynbos ecosystem of this area,
• take refuge from predators in crevices among rocks of sandstone (https://stock.adobe.com/au/search?k=cordylus&asset_id=210300852 and https://www.shutterstock.com/image-photo/black-girdled-lizard-basking-on-rock-1588076845),
• are somewhat colonial (with up to 165 individuals per hectare) and territorial, and
• advertise themselves by 'bobbing' (https://www.youtube.com/watch?v=e-5M-I8v5QI and https://www.pond5.com/stock-footage/item/167792327-agama-doing-pushups-then-running-away and https://www.pond5.com/stock-footage/item/97639668-southern-rock-agama-lizard-nodding-top-rock).

This leads me to ask:
How do these two lizards coexist? How do they differ in their niches and life-history strategies, within a single habitat?

Both lizards eat mainly invertebrates (https://www.inaturalist.org/observations/104874559 and https://www.inaturalist.org/observations/33258400 and https://www.inaturalist.org/observations/69937836). A small proportion of their diets consists of plant matter.

However, C. niger tends to eat relatively large items, often grasshoppers (https://www.inaturalist.org/taxa/688780-Scintharista-saucia and https://www.inaturalist.org/taxa/693223-Thericlesiella-meridionalis and https://www.inaturalist.org/taxa/424335-Eyprepocnemis).

By contrast, A. atra tends to eat ants (https://hal.science/hal-02990303/document and https://journals.co.za/doi/pdf/10.10520/AJA00445096_2027 and https://scholar.sun.ac.za/items/c7c76dd9-00a7-48c7-b4c2-c2d9cfb9e55a).

(The most frequently-observed species of ants in the Cape Point area are https://www.inaturalist.org/taxa/426236-Crematogaster-peringueyi and https://www.inaturalist.org/taxa/428267-Lepisiota-capensis and https://www.inaturalist.org/taxa/426182-Camponotus-niveosetosus and https://www.inaturalist.org/taxa/428219-Anoplolepis-steingroeveri.)

This is consistent with

• the greater ability of A. atra to range from the crevices to open ground, where insects such as ants are likely to be found, and
• the possibly greater affinity of A. atra for brightly-lit, sunny conditions.

Cordylus niger relies on using its keeled scales (https://www.inaturalist.org/observations/186089617 and https://stock.adobe.com/au/search?k=cordylus&asset_id=81880784) to wedge itself inextricably into crevices. Thus, it does not range far from the rocks.

By comparison, Agama atra runs speedily enough to make a dash for the rocks. It may also be more adept than C. niger in leaping.

Cordylus niger, to a greater degree/extent than A. atra, can lose and regrow its tail after close escapes from predation (https://www.inaturalist.org/observations/12063858 and https://www.shutterstock.com/image-photo/black-lizard-cordylus-niger-south-africa-633626588 and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8395757/).

(It is somewhat paradoxical that, in Cordylus, the tail is subject to autotomy despite being invested in armour. The main function of the tail is to shield the animal further, when it has retreated as deeply as possible into a crevice. However, the risk shifts if the predator manages to grasp the tail. Once this happens, the armour on the tail becomes a liability, and the tail, or part of it, can be shed. As far as I know, cordylids are the only lizards, worldwide, that combine caudal autotomy with caudal armour.)

Cordylus niger is less sexually dimorphic than A. atra (https://journals.co.za/doi/abs/10.10520/AJA00445096_798).

Cordylus niger has few offspring and is ovoviviparous, with a measure of parental care in that the infants stay near the sheltering-places of the adults. By contrast, A. atra has many offspring (https://www.inaturalist.org/observations/71285965) and is oviparous.

Each adult female of C. niger produces only 1-3 offspring per year (https://www.sanbi.org/animal-of-the-week/black-girdled-lizard/). By contrast, that of A. atra can produce clutches of 7-12 eggs, twice per year, totalling 14-24 offspring per year.

(Please see https://journals.co.za/doi/abs/10.10520/EJC19129 and https://www.inaturalist.org/taxa/1367368-Agama-bibronii)

This means a 4-7-fold difference in fecundity, based on clutch/litter size and frequency of breeding.

This difference is partly reflected in the fact that the offspring of C. niger are proportionately larger, at birth, than the hatchlings (https://www.inaturalist.org/observations/107542883 and https://www.inaturalist.org/observations/107478662) of A. atra.

Adults of C. niger (3-19 cm) are smaller than those of A. atra (20-25 cm), not all of which can be accounted for by the difference in the lengths of the tails. Yet the newborns of the former species are larger (7-8 cm) than the hatchlings (5-6 cm) of the latter species.

I have perused the photos in iNaturalist, to find the most pregnant-looking individuals of each species.

Cordylus niger:
https://www.inaturalist.org/observations/158552209
https://www.inaturalist.org/observations/158615525
https://www.inaturalist.org/observations/152666455
https://www.inaturalist.org/observations/140000602

Agama atra:
https://www.inaturalist.org/observations/108670252
https://www.inaturalist.org/observations/147757992
https://www.inaturalist.org/observations/143788547
https://www.inaturalist.org/observations/142390614

To my eye, it seems that it is A. atra that shows the greater distension of the torso.

So, C. niger has a reproductive strategy of few but large, well-protected offspring, and differs from A. atra in being the more precocial species.

Both the adults and the infants/juveniles of C. niger are relatively well-defended, by both their armoured skins and their restriction to the immediate vicinity of crevices.

Is there any information in the literature on comparative rates of growth?

Cordylus niger may live up to 25 years, which is about twice as long as in Agama (https://www.lllreptile.com/articles/188-spiderman-agama/#:~:text=Size%20and%20Longevity&text=Agama%20species%20have%20been%20known,if%20housed%20and%20maintained%20properly.).

On the above basis, I hypothesise that pace of life is faster is A. atra than in C. niger, involving metabolic rate, activity, amounts of food consumed, dietary quality, rates of reproduction and growth, and longevity.

If so, there is some consistency with patterns hnown in animals beyond lizards. It is common for armoured animals to have relatively slow pace of life, and relatively great lifespans.

Posted on October 27, 2023 10:14 PM by milewski