A comparison between rock hyraxes and rock-wallabies, part 2
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...continued from https://www.inaturalist.org/journal/milewski/66984-a-comparison-between-rock-hyraxes-and-rock-wallabies-part-1
Not only do rock hyraxes (https://www.flickr.com/photos/berniedup/40155879563/in/photostream/) flee quadrupedally, they have feet specialised in ways unknown in any marsupial or carnivore, for grip on uneven surfaces. The bare, smooth, rubbery soles (https://www.mindenpictures.com/stock-photo-cape-rock-hyrax-procavia-capensis-adult-close-up-of-front-foot-naturephotography-image80155913.html and http://www.zootorah.com/assets/media/HyraxLayout.pdf and https://mammiferesafricains.org/2013/02/daman-des-rochers/ and http://cctv.cntv.cn/2014/09/16/VIDE1410822362803906.shtml) combine pliability with surprising toughness. This is evident in the fact that the regurgitated pellets of eagles and the faeces of leopards still contain the recognisable soles of rock hyraxes, as intact as the indigestible hooves of any small antelope in the diet of these predators.
The rubbery adhesiveness of these durable soles is partly owing to their clamminess; their clinging to the rock surface is a subtle form of suction-cupping. Long claws would impede such suctioning and accordingly the stumpy digits of the hyraxes have short, blunt nails (https://www.flickr.com/photos/berniedup/32178776167) except for one grooming claw on each hind foot. To promote adhesion, the soles perspire despite an inability to sweat elsewhere on the body, and despite the scarcity of drinking water typical in the habitats of rock hyraxes.
The following show the soles of a rock-wallaby, Petrogale penicillata: https://twitter.com/mtrothwell/status/519737786216112130?lang=bg and https://www.rockwallaby.org.au/brush-tailed-rock-wallaby-information/brush-tailed-rock-wallaby-images-photos/.
Escaping is also aided by specialisations in the skeletal structure of the limbs of our rockrubbers, the rock hyraxes.
Unlike either their mammalian predators or rock-wallabies, rock hyraxes completely lack collar bones (i.e. clavicles, one on each side, left and right), and their wrist and ankle bones swivel to retain contact with rock surfaces whatever the angles of the legs. Rock-wallabies do not use the wrists while fleeing and have tightly connected ankle bones that allow each achilles’ tendon to act as a spring.
While the wrist bones of rock hyraxes are particularly adept at swivelling, those of rock-wallabies are no more able to swivel than our own.
The result of the peculiar anatomy of their ankles and wrists (https://www.pnas.org/doi/pdf/10.1073/pnas.87.12.4688) is that the hyraxes can easily keep contact between their paws (notwithstanding the reduced number of digits) and the rockiest of terrain where even the most acrobatic baboon would be slightly delayed.
Defending is a fourth way in which prey species can be adapted to pressure from predators.
True to form, rock hyraxes are better equipped than rock-wallabies to bite back (https://www.thedodo.com/in-the-wild/rock-hyrax-facts). The second incisors of the hyraxes serve as tusks in both sexes (https://www.mindenpictures.com/stock-photo-cape-rock-hyrax-procavia-capensis-skulls-adult-male-with-ridge-on-naturephotography-image80094391.html). Except for the occasional stripping of bark when normal foods are unavailable, these are not used in foraging. Sharpness is maintained by honing the edges of the upper and lower incisors against each other. This parallels the mechanism in their likely enemies: male baboons, which hone their upper canines against the first lower premolars.
Both rock hyraxes (https://www.gettyimages.com.au/detail/photo/male-rock-hyrax-yawning-close-up-royalty-free-image/200023200-001 and https://www.agefotostock.com/age/en/details-photo/rock-hyrax-procavia-capensis-somerset-nj-mouth-open-showing-teeth/AAM-AAES66037 and https://bilderreich.de/1330-6972/image-rock-hyrax-teeth.html and https://www.alamy.com/rock-hyrax-procavia-capensis-bloemfontein-south-africa-image6902283.html) and male baboons (https://dissolve.com/stock-photo/Chacma-baboon-yawning-Kruger-National-Park-royalty-free-image/101-D25-28-182) frequently yawn to show off the intact, sharp edges of their front teeth.
And unlike either rock-wallabies or baboons, even the premolars of the hyraxes remain sharp enough potentially to complement the tusks in a desperate bite. This is suggested by the fact that, particularly when agitated by a predator, rock hyraxes noisily grind their empty jaws, which simultaneously sharpens the premolars and reminds attentive adversaries that front teeth and cheek-teeth alike are kept sharp for self-defence.
Rock hyraxes have particularly well-developed muscles to drive their defensive bites.
The temporalis muscle (i.e. the muscle connected the lower jawbone to the temple of the skull, https://en.wikipedia.org/wiki/Temporalis_muscle) is proportionately far larger in rock hyraxes than in most herbivorous mammals. Exceptions are pigs (Suidae), peccaries (Tayassuidae), and camels (Camelidae), all of which bite in self-defence. This muscle tends to drive powerful puncturing bites. Compare this with another jaw muscle, the masseter (https://en.wikipedia.org/wiki/Masseter_muscle), which tends to be responsible for the routine grinding of food, and is massive in even the most harmless of herbivores (https://pubmed.ncbi.nlm.nih.gov/6854654/ and https://www.jstor.org/stable/2400390).
The defences of rock-wallabies are, instead, unremarkable ((https://link.springer.com/chapter/10.1007/978-3-030-68398-6_11).
and https://www.southernbiological.com/anatomy-models/zoology/mammals/tq259-rock-wallaby-skull-teaching-quality/ and https://www.dinosaurcorporation.com/wallaby.html). The premolars, which are used like millstones rather than with shear-precision, are likely to be blunter than those of rock hyraxes, and therefore less versatile in self-defence. As I mentioned in part 1, the cheek-teeth (https://collections.museumsvictoria.com.au/specimens/123468) emerge throughout adult life, with the grinding surfaces wearing away at the front of the row and being replaced at the back of the row.
Even the front teeth of rock-wallabies lack a sharpening mechanism and are used mainly to detach food. And their temporalis muscle seems modest in size, suggesting limited bite-force.
Reproducing is the fifth and final way in which prey species can be adapted to pressure from predators. Here again, rock hyraxes differ from rock-wallabies.
If the eutherian (usually described as ‘placental’ although certain marsupials do in fact possess a placenta) mode of reproduction is generally more protective than the marsupial mode against predation, then the gestation of rock hyraxes is particularly indicative of the threat of enemies such as baboons as well as wild felids of various sizes. Rock hyraxes protect their offspring in the womb far longer than do rock-wallabies, their gestation period being seven months which is remarkably long for a mammal of their body size. By contrast, the gestation of the wallabies is remarkably short at one month.
In fact, rock hyraxes gestate as long as the hippopotamus does (https://en.wikipedia.org/wiki/Hippopotamus), whereas the wallabies gestate more briefly than do some rats. The result is that each newborn hyrax weighs about 200 grams, so well-developed that it looks like a miniature adult. By contrast, newborn rock-wallabies can be described as mere embryos, weighing only about one gram when transferred to the pouch. Considering that usually three share the womb in rock hyraxes whereas only one is born at a time in the wallabies, the result is a roughly 600-fold difference in the total mass of offspring delivered per birth.
Rock hyraxes not only have precocial newborns, but also synchronise the birth season, so that infants attract predators for a minimum proportion of each year.
Although rock hyraxes are not particularly protective of their suckling offspring, rock-wallabies – assuming that they behave like related genera of marsupials – renege to an extraordinary degree on parental care by mammalian standards. Mothers of kangaroos and wallabies tend to jettison their young when chased. This is done by the distressed mother releasing the sphincter of the pouch instead of tightening it. The jettisoned juvenile may be overlooked by the predator, surviving its rough tumble of maternal betrayal. However, it is consistently rejected by its mother - barred from the pouch, and abandoned to die of neglect anyway.
According to my interpretation, this amounts to something more than non-gestation: a kind of anti-gestation in which young are actually sacrificed in attempted appeasement of a predator that threatens the mother. This tactic may make sense where threats from predators are relatively infrequent. However, were rock hyraxes to ‘spend a baby’ each time they were stressed by predators, the species might not survive.
One way to verify that African mammals about the size of rock hyraxes are shaped mainly by predation is to examine the one lineage of hopping mammals that does occur in subSaharan Africa. Do these mammals have particularly secure environments, or compensate for their gaits in other ways, or both?
The purely African springhares (Pedetes capensis and P. surdaster, https://www.inaturalist.org/observations?taxon_id=45928) provide such a test: they resemble hyraxes and small wallabies in body masses (3 kg), and are as committed to fleeing on their hind legs as the wallabies are (https://www.inaturalist.org/observations/88590056). And indeed we find that the fast-growing springhares have a reproductive mode extremely different from that of rock-wallabies, supporting the idea that long gestation and extended periods of hiding the juveniles serves prey species in reproducing successfully in the face of predation.
The various differences between springhares and wallabies undermine any superficial semblance of evolutionary convergence in hopping locomotion. Springhares are sandhoppers rather than rockhoppers, which seldom stray far from their burrows on sandy plains. They hop over relatively short distances and only on flat ground, and emerge only at night, being more strictly nocturnal than rock-wallabies. While this avoids any possible threat from baboons, springhares are a particular target for many other predators.
As if in compensation for daring to hop in Africa, springhares show the following traits:
- extremely large eyes (https://www.amazinglife.bio/post/the-spring-hare-pedetes-capensis);
- extremely rapid growth of foetuses and juveniles;
- rapid growth from conception to sexual maturity (one year, which is half of the corresponding value for rock hyraxes);
- extremely large newborns (https://www.zooborns.com/zooborns/2014/08/loving-hands-for-springhare-baby.html) compared to rock hyraxes (and orders of magnitude larger than in rock-wallabies); and
- strict hiding of juveniles in the parental burrow until half the mother's body mass is attained.
These adaptations, which seem antithetical to those of rock-wallabies, hint that any mammal hopping bipedally on boulders would be exterminated by predators in Africa.
Comments
https://www.alamy.com/stock-photo-close-up-of-front-feet-kenyatree-hyrax-dendrohyrax-arboreus-25175179.html
https://www.tandfonline.com/doi/pdf/10.1080/00445096.1970.11447384
file:///C:/Users/Antoni%20Milewski/Downloads/Asher_et_al-2017-Journal_of_Vertebrate_Paleontology-AM.pdf
https://link.springer.com/chapter/10.1007/978-3-030-68398-6_11
The problem with some of your comparisons is that while comparing Hyraxes and Rock Wallabies is all and good, they also need to be compared to their relatives. So elephants/dungongs/aarvark/sengis also have foot pads, long gestation and other traits - some of which have nothing to do with being a lithophil. SImilarly, Rock Wallabies are just kangaroo/wallabies, and many of their features have nothing to do with lithophilly.
Any insights from fossil Hyraxes? Some of these were the size of hippos, and had nothing to do with rocks.
Another useful insight would be to compare Rock Hyraxes with Tree Hyaraxes: how many of the features you consider to be suitable for rocks are standard in Tree Hyraxes (Ditto Rock Wallabies with normal Wallabies)? Surely if you are interested in why convergence is not apparent, it is the adaptations to rocky habitats compared to the "norm" in the two groups that we should be comparing: i.e. what are the rock specific adapatations in Hyraxes versus rock specific adaptations in Wallabies, and which of these adaptations show convergence or divergence. The fact remains that these two lineages are still standard Hyraxes and standard Wallabies, suggests that living in rocks is not as "rigorous" as say a fossorial animal (e.g. moles, goldenmoles, marsupial moles) or water (seals, whales / otters, beavers, platypus).
Still: am enjoying your insights, for which many thanks, but you do need to ramp up your null hypotheses to be a tad more rigorous.
@tonyrebelo
Many thanks for considering these comparisons so deeply.
Part of the problem is that what I'm attempting here is only rather approximately about evolutionary convergence in the strictest sense, i.e. in this case an in-situ modification of the organism specifically for the habitat in question. More precisely, it is about the similarities of the organisms compared, regardless of how they became similar.
The conceptual framework includes both evolutionary modification and recruitment. The recruitment of a given life-form into a given habitat would, to my mind, be just as affirmative of 'evolutionary convergence' (in the more encompassing sense) as the in-situ modification of some ancestor. What matters is the final similarity.
If a certain job is advertised, some applicants will be 'naturals', born for the role and so suited that all they have to do is show up for work. Others may be only approximately qualified, requiring much training and experience before they achieve the same competency as the 'natural'. However, both candidates may be eligible as long as competency is achieved. The 'pre-adaptation' of the 'natural' should not detract from our acknowledgement of his/her competency.
Perhaps I can illustrate these principles by means of a few extreme examples.
Falco peregrinus (https://www.inaturalist.org/taxa/4647-Falco-peregrinus) occurs in both one continent and another, in similar habitat and filling a similar niche. It filled these niches by means of the process of recruitment, with negligible in-situ evolution. But for the purposes of the kinds of intercontinental comparisons I make, I take this as full confirmation of evolutionary convergence. This is based partly on the axiom that all life-forms live at least partly in environments somewhat different from the ones they evolved in. Furthermore, the evolution of any given organism took millions of years, going through various stages. So there are few organisms that literally live where they evolved, and even fewer (perhaps a few cave-dwellers?) that live exclusively where they evolved.
Taking the opposite extreme: 'mole-rats' on several continents belong to several different families of rodents. Most are different enough in their own ways as to be only superficially similar, but in a few cases the similarities are close enough (https://en.wikipedia.org/wiki/Silvery_mole-rat and https://www.mindenpictures.com/stock-photo-eastern-pocket-gopher-geomys-sp-showing-teeth-which-are-used-to-naturephotography-image00136199.html) to be remarkable for members of different families. In this case the main component of the process has been evolutionary modification, with recruitment playing a minor role and one confined to a within-region scale. Nonetheless from my viewpoint the similarity is the point of interest.
Where similar environments, such as boulder outcrops, have life forms as different as rock hyraxes and rock-wallabies, the main point of interest to me is the overwhelming non-similarity. It matters little to me whether that non-similarity is owing to 10% failure of evolutionary modification + 90% failure of recruitment, or vice versa. The important point is that a surprising difference has arisen, that leads to two broad alternatives of interpretation. Either we can assume that the process is invalid (e.g. owing to phylogenetic constraints or geographical barriers), or we can assume that it is the actual environments that are, in some hidden or subtle way, not as comparable as they seemed in the first place. I tend to opt for the latter assumption, because in my experience that leads me to new ideas and perspectives.
In the case of Falco peregrinus, the recruitment component is the main part of the process. However, in all cases the process that led to a certain life form occupying a certain niche in a certain habitat at a certain location will have included some measure of both evolutionary modification and recruitment.
Um: OK - 2 broad alternatives:
. 1. phylogenetic inertia
. 2. habitats not comparable
But what about:
. 3. the habitats are comparable but the selection pressures are not strong. Dont expect convergence.
. 4. there are different solutions to the same problem - instead of one convergent morphology, there are several alternatives. Expect convergence, but to several different domains of attraction. (e.g. the dassie-klipspringer-lizard solutions) And of course, the possibility that one solution might preclude 2 others (e.g. hypothetically Rock Dassie & Klipspringer compatible vs Rock Wallaby incompatible with Rock Wombat)
. 5. the habitats are comparable, but the biological environment is different: so other overriding pressures are overriding the expected convergence (your argued this with the predators - baboons: which applies to Hyraxes, but not Klipspringers).
Ideas are a dime a dozen. Half a dozen impossible ideas before breakfast, etc.! But a real understanding requires fine tuning these new ideas into novel insightful perspectives.
Anyway: it is really entertaining and educational reading, so please dont stop.
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