Does the Cape mole-rat (Georychus capensis) have aposematic colouration?

Why is the Cape mole-rat (Georychus capensis, https://en.wikipedia.org/wiki/Cape_mole-rat and https://academic.oup.com/mspecies/article/doi/10.1644/799.1/2600528?login=false and https://www.researchgate.net/publication/229245809_Georychus_capensis and https://animaldiversity.org/accounts/Georychus_capensis/) the only mole-rat on Earth with colouration so bold that the whole animal stands out from its environment?

BACKGROUND

It is well-known that as many as seven different families of rodents, around the world, have converged evolutionarily - to varying degrees - on the body-form of mole-rats.

These occur on all six continents except Australia - with four of the relevant families (Bathyergidae, Rhizomyidae, Heterocephalidae, and Spalacidae) present within Africa.

Furthermore, it is unsurprising that most of the species of mole-rats (including gophers, root-rats, tuco-tucos, coruros, etc.) have plain, dull colouration, given that the animals live in dark tunnels or depend on refuge underground, tend to have small eyes, and communicate mainly by sound and touch (https://link.springer.com/article/10.1007/BF00190397).

However, something remains unexplained.

This is that one species in the family Bathyergidae, namely Georychus capensis,

has bold colouration on the head, in most individuals. This consists of a pattern of dark/pale contrast (https://www.inaturalist.org/observations/159097477 and https://www.inaturalist.org/observations/150594110), and
is colour-polymorphic, with some individuals being plain.

Georychus capensis is the only specialised subterranean mammal on Earth that is adapted to nutrient-poor soils, and it may be the only one adapted to vegetation that is not only fire-prone but also sclerophyllous. Furthermore, it may be the only rodent specialised on a diet of corms, mainly of Iridaceae. This diet is likely to be carbohydrate-rich but nutrient-poorer than diets based mainly on greens and stem-tubers.

https://cdnsciencepub.com/doi/10.1139/cjz-2017-0016
https://cdnsciencepub.com/doi/abs/10.1139/cjz-2017-0016

However, it is not obvious how these ecological circumstances explain its odd colouration.

So, what is it about the habitat and niche of this species that might make whole-body conspicuousness adaptive?

There are three possible adaptive explanations.

EXTRAORDINARY EXPOSURE?

Firstly, it is possible that G. capensis is more active above the ground surface than expected for a small-eyed mole-rat, specialised on a diet of tubers. This would expose it to predation, against which its only defence is the protruding incisors common to mole-rats generally.

The pattern seen in most individuals of G. capensis may thus constitute warning colouration against carnivores. The accessory function of colour-polymorphism might be to add confusion of identity to the warning.

SOCIALITY?

Secondly, it is possible that G. capensisis is more social than other comparable rodents. If so, the colouration might serve social communication.

DEMOGRAPHIC LIABILITY?

Thirdly, it is possible that G. capensis breeds and grows particularly slowly for a mole-rat. If so, this might lead to demographic victimisation, in which the species, although not particularly targeted by predators, risks failure to replace losses to predation. If so, this might make for a life history strategy in which there is more of a premium on anti-predator defence than there is in other relevant taxa.

All mole-rats and analogous rodents are well-known to have slow metabolism, which makes sense given the restricted supply of oxygen underground. However, it is possible that rates of growth and reproduction vary considerably among families and genera, based on the nutritional regime of the habitats.

Let us examine the evidence for and against these hypotheses.

EXTRAORDINARY EXPOSURE

SOCIALITY

This explanation seems unlikely, because G. capensis is regarded as solitary, and has eyeballs smaller than those of e.g. tuco-tucos, pocket gophers, and coruros.

The only species of mole-rat, apart from G. capensis, that possesses noticeable colouration on or near the head is one of the four spp. of Fukomys, namely Fukomys damarensis (https://www.inaturalist.org/taxa/446619-Fukomys-damarensis). This functions plausibly as a social signal, because this species is one of the few eusocial vertebrates (https://books.google.com.au/books?hl=en&lr=&id=P70wCwAAQBAJ&oi=fnd&pg=PA338&dq=Convergence+in+molerats&ots=d-ivf6NMve&sig=l8vSiZQILaX9AO4na8p_mA40XaY#v=onepage&q=Convergence%20in%20molerats&f=false), and the precise pattern varies among individuals.

DEMOGRAPHIC LIABILITY

Does G. capensis have unusually slow reproduction and growth?

I have found nomevidence for this. Kingdon (2015, page 223) states that this is "a prolific species, known to have produced as many as 10 young in a single litter."

(https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-7998.1988.tb04989.x and https://www.jstor.org/stable/4217593 and https://www.journals.uchicago.edu/doi/abs/10.1086/physzool.60.1.30158638 and https://www.sciencedirect.com/science/article/abs/pii/0031938496000765 and https://www.ajol.info/index.php/az/article/view/152935).

Aposematism is based on anti-predator defence, and makes most sense where the defensive capacity is not obvious.

In the case of G. capensis, the defensive capacity seems particularly obvious (https://www.inaturalist.org/observations/133218093).

Since most mole-rats have similarly protrusive incisors, what is not obvious is how G. capensis is better-defended than the many spp. with plain colouration.

The following gives an indication of

the remarkable phylogenetic diversity of mole-rats and convergent forms of rodents, and
the uniformly dull, unremarkable colouration of most spp.

These patterns are clear, notwithstanding that

the arrangement of families and genera is so complex as to be bewildering, and
many of the spp. remain obscure, with no clear photographs on the Web, let alone in iNaturalist.

The salient point is that, among this plethora of convergent forms, the Cape mole-rat is extremely unusual in its colouration.

BATHYERGUS
https://www.inaturalist.org/observations?place_id=any&taxon_id=43757&view=species

CRYPTOMYS
https://www.inaturalist.org/observations?place_id=any&taxon_id=43747&view=species

FUKOMYS
https://www.inaturalist.org/observations?place_id=any&taxon_id=446616&view=species

HELIOPHOBIUS
https://upload.wikimedia.org/wikipedia/commons/9/98/Silvery_mole-rat.png and https://en.wikipedia.org/wiki/Silvery_mole-rat

HETEROCEPHALUS
https://www.inaturalist.org/taxa/43746-Heterocephalus-glaber

TACHYORYCTES
https://www.inaturalist.org/taxa/44989-Tachyoryctes-splendens

SPALAX
https://www.inaturalist.org/observations?place_id=any&taxon_id=852243&view=species

NANNOSPALAX
https://www.inaturalist.org/observations?place_id=any&taxon_id=852242&view=species

EOSPALAX
https://upload.wikimedia.org/wikipedia/commons/4/46/Eospalax_baileyi_%2811%29.jpg

MYOSPALAX
https://www.inaturalist.org/observations?place_id=any&taxon_id=44939&view=species

CANNOMYS
https://www.inaturalist.org/taxa/45728-Cannomys-badius