A comparison between rock hyraxes and rock-wallabies, part 1

Boulder outcrops look similar wherever in the world we find certain combinations of geology and erosion. Granitic rocks are particularly widespread, and their domed exposures are particularly uniform across all continents.

Crevices in rocks provide shelter from weather and predators for any vertebrate adapted to the risks of falling.

A single rock-dwelling life-form should in principle be suited to the similar boulder outcrops on all continents besides Antarctica. Although no rock-dwelling species spans the continents, we would expect the mammals intimately adapted to these similar environments to converge - regardless of their ancestries - into similar limb-shapes by the forces of nature,

However, rock hyraxes (Procaviidae: Procavia capensis https://www.inaturalist.org/taxa/43086-Procavia-capensis and https://www.youtube.com/watch?v=ERde-Z5dD4E and https://www.youtube.com/watch?v=P3FKlsbqQVU, and Heterohyrax brucei https://www.inaturalist.org/taxa/43088-Heterohyrax-brucei, totalling two species) of Africa and the Middle East differ from the rock-wallabies of Australia (Macropodidae: Petrogale https://www.inaturalist.org/observations?taxon_id=42916, totalling 16 species). The differences are so great that the concept of evolutionary convergence (unrelated lineages independently evolving similar traits as a result of adapting to similar environments) seems to have fallen off a cliff.

My leap of faith in this Post is that perhaps the puzzle of cause and effect would be solved by a more determined search for environmental differences between the continents.

Rock hyraxes resemble rodents only superficially, and are complicated to describe because they are not particularly similar to any other group of mammals. They share certain anatomical features - too nebulous even to summarise here - with their distant relatives, elephants. A more obvious resemblance is to rhinoceroses in the case of the cheek-teeth (premolars and molars) (https://www.pikist.com/free-photo-vlqbd and http://www.mammalogy.org/procavia-capensis-711).

Rock-wallabies are relatively easy to characterise, being essentially miniature versions of kangaroos that somehow maintain bipedal hopping on the most uneven of terrain. And it is the wallabies – in common with most other members of the kangaroo family – that have cheek-teeth somewhat convergent with those of elephants. In both rock-wallabies and modern elephants (Loxodonta and Elephas, https://www.inaturalist.org/observations?taxon_id=43692), the molars continue to erupt after adulthood is reached, forming a conveyor-belt in which the premolars are progressively lost by wear.

The body proportions and internal organs of rock hyraxes and rock-wallabies differ so much that these two groups of mammals seem as disparate as igneous and sedimentary rocks. There may be a bedrock of similarities between the hyraxes and the wallabies in terms of body mass (2.0-5.5 kg), relatively slow metabolism and growth, and limited brain size. However, these hardly add up to any overall evolutionary convergence. This is a problem for biologists, because it casts doubt on the notion of adaptation as the main shaping force of life.

Equally perplexing are the differences between hyraxes and various other climbing mammals. Unlike most climbers, rock hyraxes lack not only any claw or nail capable of gripping the substrate, but also any thumb or big toe. Rock-wallabies (and tree kangaroos, Dendrolagus) are also odd among climbers, because they retain the hopping locomotion of their relatives on flat ground with only minor modifications for negotiating the rocks, such as the bluntness of the hind claw.

Rock-wallabies retain not only the leg proportions but also the reproductive pouch and conveyor-belt dentition of other wallabies and kangaroos (https://www.publish.csiro.au/WR/WR9950547). Thus it seems as if the wallabies of flat terrain have evolved into rock-wallabies with only superficial modifications.

These disparities between rock hyraxes and rock-wallabies seem all the more puzzling when we consider that both are eaten by similar eagles, the African black eagle (Aquila verreauxii) in the case of the hyraxes and the wedge-tailed eagle (Aquila audax, https://www.inaturalist.org/taxa/5080-Aquila-audax, https://www.inaturalist.org/taxa/5077-Aquila-verreauxii) in the case of the wallabies. Why has culling by avian predators through time not shaped their rock-dwelling, rabbit-size mammalian prey into forms more similar than we find in reality?

My new explanation is that boulder outcrops in Africa and the Middle East are generally far more beset by mammalian predation than are boulder outcrops in Australia.  And that, as a result, the adaptive differences between rock hyraxes and rock-wallabies ultimately make sense as appropriate evolution – as opposed to reflecting some sort of fault-line in the natural order.

Boulder outcrops in Africa and the Middle East are home to leopard (Panthera pardus, https://www.inaturalist.org/taxa/41963-Panthera-pardus), caracal (Caracal caracal, https://www.inaturalist.org/taxa/42042-Caracal-caracal, https://www.inaturalist.org/taxa/41834-Mellivora-capensis), honey badger (Mellivora capensis ), and other Carnivora. By contrast, Australia lacks precise counterparts for these predators.

Even the recently extinct thylacine (Thylacinus cynocephalus, https://animaldiversity.org/accounts/Thylacinus_cynocephalus/) and the more remotely extinct ‘marsupial lion’ were not associated with boulder outcrops, and there is certainly no suggestion of the 'marsupial lion' having been particularly predatory on wallabies. Thylacoleo carnifex (https://en.wikipedia.org/wiki/Thylacoleo_carnifex) had extremely specialised teeth that remaining a puzzle to this day.

Rock hyraxes are exposed to such a wide range of predators that, where particularly dense populations of carnivores occur, they can be locally exterminated despite hyraxes their apparent inaccessibility among the boulders. Such is the case in the southern parts of the Kruger National Park, where extensive granite boulder outcrops in the areas most heavily populated by the leopard, and its various abundant prey species of antelopes, are strangely devoid of the hyraxes so taken for granted in similar terrain over much of South Africa.

Although a felid is the likely culprit in this particular example, our chief suspects for the predator that has had the greatest influence on the evolution of rock hyraxes are baboons (Papio spp. such as P. ursinus, https://www.inaturalist.org/taxa/57556-Papio-ursinus). These are large omnivorous monkeys that congregate on boulder outcrops to sleep at night after foraging on surrounding plains by day.

Baboons, although not carnivorous, match or exceed the rock-scaling abilities of the hyraxes and are consequently in a position to kill them. Given that baboons are known occasionally to kill the infants of antelopes as well as adult cane rats (Rodentia: Thryonomys, https://www.inaturalist.org/observations?taxon_id=46277), which are similar in size to hyraxes, there is the possibility of a subtle, previously overlooked selective pressure on the hyraxes.

By contrast, the original mammal fauna of Australia – perhaps fortunately for rock-wallabies – lacks any mammals resembling primates, least of all baboons. Indeed, baboons have no climbing peers of equivalent body size in Australia, past or present, whereas in Africa there must have been very few boulder outcrops where rock hyraxes would have been free of baboons as co-tenants and part-time predators.

Although few observations have been made, I visualise that what appears to be a cosy sharing of refuges is in reality a taxing relationship on the smaller-brained mammals, particularly because of the intelligence and teamwork of baboons. I refer especially to mischievous and destructive juvenile males, which may behave like council-block or tenement gangsters towards the hyraxes.

Even as only occasional, opportunistic killers, and even if they harm the hyraxes for fun rather than from hunger, baboons are in a position to inflict hypothetical losses because of their sheer numbers and the fact that each evening and each morning, bored by the temporary lethargy of the group or bullied in its hierarchy, they may find themselves among – what is to them – the closest thing to food, toys, or soft targets (https://www.youtube.com/watch?v=Kbz4XOPYe4U and https://tenor.com/view/hyrax-hyrax-baboon-hyrax-vs-baboon-gif-15050092).

So, here is a more detailed comparison to see if predation explains why what we might call the rockrubbers of Africa and the Middle East are not mirrored by our rockhoppers of Australia. I consider hiding, alarming, escaping, defending, and reproducing.

Hiding is a way in which prey species can be adapted to evade pressure from predators. Rock hyraxes hide more effectively than rock-wallabies do, spending all night within crevices into which they can jamb their long, low bodies, and foraging as rapidly as possible in the immediate vicinity of their refuges.

Furthermore, rock hyraxes lack tails whereas rock-wallabies have extremely long tails. Being tailless is likely to serve the hyraxes, huddled deep in crevices, by denying baboons and other enemies any easy grasp on their bodies. And, although the hyraxes forage in the mornings and evenings, this does not pose a risk from baboons, which forage away from the boulder outcrops in daylight, and are therefore likely to be kilometres away at these times. So the hyraxes may be hiding not only in space but also in time from baboons.

When rock hyraxes do venture out, rapid foraging is aided by features of their teeth and guts. Their cheek-teeth differ from those of rock-wallabies in having such sharp edges that it is the premolars, instead of the incisors, that are used to detach food. At the same time, the premolars and molars seem to occlude more precisely than those of rock-wallabies, and therefore chew efficiently with a few quick strokes of the jaws.

This emphasis on hasty foraging and hiding may help to explain why the stomachs and intestines of rock hyraxes are quite different from those of rock-wallabies, and differ from those of elephants or rhinoceroses. In both cases there are gut compartments that make the most from the limited amount of food found within reach of boulder outcrops. However, rock hyraxes have an arrangement unique among mammals: two stomachs with the second one located partway down the intestine (https://commons.wikimedia.org/wiki/File:GI_tract_Hyrax_Flower_et_al_1891.png and https://www.sciencedirect.com/science/article/abs/pii/0300962994902054 and https://pubmed.ncbi.nlm.nih.gov/8529006/ and https://blog.whyanimalsdothething.com/post/128333113261/suction-cups-fangs-and-a-grab-bag-of-digestive and https://journals.physiology.org/doi/pdf/10.1152/physrev.1998.78.2.393 and https://southafrica.co.za/hyrax-digestion.html).

Rock-wallabies, by contrast, have their stomach in the location and form expected for members of the kangaroo family: a large colon-like sac in which food ferments before entering the intestines (https://kangarooherman.weebly.com/digestive-system.html). My interpretation of this difference is that rock hyraxes use the first stomach partly for storage of hastily harvested material, reserving the second one for digestion. Rock-wallabies usually forage by night in the relative safety of a land with few predators, chew their food in relative leisure, and need no particular chamber for the initial storage of food.

Signalling alarm is a second way in which prey species can be adapted to pressure from predators. Rock hyraxes are not only vocal when spotting predators, but also have a wide range of alarm calls: a whistle for eagles, a grunt for snakes, and a loud or soft squeal for mammals (https://www.youtube.com/watch?v=mF3rPvzTPF4 and https://www.wired.com/2012/04/hyrax-song-complexity/ and https://www.wildsolutions.nl/vocal-profiles/hyrax-vocalizations/). Indeed, their vocal versatility of as many as 20 different sounds – considering the modest level of brain development of the hyraxes – is impressive among mammals. It remains unknown whether rock hyraxes use any special sound in fear of baboons approaching at close quarters. Oddly, this complexity seems to exceed that of baboons, which have relatively simple utterances despite their intelligence.

Furthermore, rock hyraxes go one step further than mere communication amongst their group; using soft calls to alert the whole family group with the acoustic advantages of rock walls, and issuing alarm calls loud enough to inform the predator itself that it has lost the element of surprise.

By contrast, rock-wallabies are quiet. I have not heard of any sounds of alarm made by them other than perhaps a thumping of the hind feet before fleeing.

When it comes to escaping, rock hyraxes use all four of their legs to flee, whereas rock-wallabies seem clumsy by comparison, fleeing on their hindlimbs. Rock-wallabies seem to prioritise conserving energy through limb-economy, whereas rock hyraxes seem to prioritise sure-footedness as they dash across and among rocks. This is not surprising given that the fleeing gaits used by mammals tend to show a compromise between efficiency and stability.

Rock-wallabies seem to be designed mainly around energetic efficiency. Whether ascending or descending, they minimise contact with the substrate when moving at maximum speed, by hopping bipedally and whipping the long tail for balance as they negotiate the boulders. This possibly gives them an advantage in fuel-economy because their bounce recycles energy in the ankle tendons.

Rock hyraxes, on the other hand, seem to be designed mainly around the risk of death. For them, bouncing would likely be a false economy in the tight dodging necessary when out-manoeuvering either a hungry leopard or a wanton thuggery of baboons. A visualisation of our rockrubbers and rockhoppers in such a scenario suggests that it is the wallabies that are more likely to misstep in full flight.

to be continued in https://www.inaturalist.org/journal/milewski/67341-a-comparison-between-rock-hyraxes-and-rock-wallabies-part-2...

Posted on June 8, 2022 07:10 PM by milewski