Fukomys mechowii ( Peters, 1881 )

Caspar, Kai R, Burda, Hynek & Begall, Sabine, 2021, Fukomys mechowii (Rodentia: Bathyergidae), Mammalian Species 53 (1011), pp. 145-159 : 146-152

Fukomys mechowii ( Peters, 1881)

Giant Mole-rat

Georychus Mechowii Peters, 1881:133 . Type locality “Malange,” Angola.

G [eorychus]. Mechowi: Bocage, 1890:271 . Incorrect subsequent spelling of Georychus Mechowii Peters, 1881 .

Georychus Ansorgei Thomas, and Wroughton, 1905:175 . Type locality “Kukema R., Bihé. Alt. 5900 ft.,” Angola.

Georychus Mellandi Thomas, 1906:178 . Type locality “Mpika, N.E. Rhodesia. Alt. 5000 ft.,” Zambia.

C [ryptomys]. mechowi: Thomas, 1917:443 . Name combination.

Cryptomys blainei Hinton, 1921:372 . Type locality “Chisongwe, Luando River, (altitude 4000’),” Angola. Incorrectly spelled “ C. blaenei ” by Van Daele et al. (2013:185).

Cryptomys mechowii: Allen, 1939:429 . Name combination.

Cryptomys mechowi mechowi: Ellerman, Morrison-Scott, and Hayman, 1953:236 . Name combination.

Cryptomys mechowi mellandi: Ellerman, Morrison-Scott, and Hayman, 1953:236 . Name combination.

Georhychus mechowi: Ansell, 1978:67 . Incorrect subsequent spelling of Georychus Illiger, 1811:87 .

Georhychus mellandi: Ansell, 1978:67 . Incorrect subsequent spelling of Georychus Illiger, 1811:87 .

Coetomys mechowi: Ingram, Burda, and Honeycutt, 2004:1008 . Name combination. Incorrect subsequent spelling of epitheton derived from Georychus Mechowii Peters, 1881 .

Fukomys mechowii: Kock, Ingram, Frabotta, Honeycutt, and Burda, 2006:53 . First use of current name combination.

F [ukomys]. mechowi: Deuve et al., 2006:2 . Incorrect subsequent spelling of Fukomys mechowii Kock, Ingram, Frabotta, Honeycutt, and Burda 2006 .

CONTEXT AND CONTENT.Context as for genus. No subspecies are currently recognized. Georychus ansorgei , Georychus mellandi , and Cryptomys blainei are synonyms (but see “Nomenclatural Notes”).

NOMENCLATURAL NOTES. Various authors have incorrectly adopted the spelling mechowi for the epitheton of Fukomys mechowii , leading to the frequent occurrence of this term in the literature. The genus name comprises the term “fuko” which derives from Mfuko, a vernacular name for molerat in certain Zambian Bantu languages such as Bemba and Kaonde ( Ansell 1978; Kock et al. 2006), as well as mys which translates to “mouse” from ancient Greek. The latter is a common suffix of scientific names given to rodents. The epitheton mechowii honors the German explorer Friedrich Wilhelm Alexander von Mechow (1831–1904) who is especially known for his expeditions to Angola, where he collected the holotype of F. mechowii ( Peters 1881; Beolens et al. 2009). Common names are giant mole-rat, giant Zambian mole-rat, or Mechow’s mole-rat. It is not to be confused with the East African spalacid Tachyoryctes macrocephalus that is also frequently referred to as the giant mole-rat.

Molecular evidence suggests that populations of giant molerats that have traditionally been comprised under F. mechowii are actually paraphyletic. There is an “Eastern” ( Zambia and presumably eastern Democratic Republic of Congo) and a “Western” ( Angola, western Democratic Republic of Congo) clade of morphologically similar giant mole-rats (see “Genetics”). However, the Eastern lineage is more closely related to the small-bodied F. vandewoestijneae (Caroline’s mole-rat; previously: Salujinga haplotype and cytotype) from the Ikelenge region of Zambia and adjacent regions ( Van Daele et al. 2007; Visser et al. 2019; J. Krásová et al., in litt., University of South Bohemia, České Budějovice, Czech Republic, 24 May 2021). The name F. mechowii ( Peters, 1881) is attached to the Western group, as the holotype derives from Northern Angola ( Peters 1881). The oldest available name for the Eastern lineage is Georychus mellandi Thomas, 1906 . The name applied to these animals should therefore be Fukomys mellandi ( Thomas, 1906) .

Although we are aware of this issue, we choose to apply the name F. mechowii in a traditional sense to both Eastern and Western giant mole-rats here to not preempt a proper taxonomic revision of the group. The vast majority of data available on giant mole-rats corresponds to the Eastern lineage found in Zambia. To discern information referring to either the Western or the Eastern clade, we attempted to include the geographic provenance of the populations described whenever feasible. If not done so, data refer to the Eastern giant mole-rat clade from Zambia.

DIAGNOSIS

Fukomys mechowii is easily distinguished from sympatric mole-rat species based on its body size (females: 200–355 g; males: 250–995 g), which is by far the largest in the genus ( Kawalika and Burda 2007; Van Daele et al. 2013; see Caspar et al. 2021 for body size variation within Fukomys ). However, subadult individuals might be mistaken for the closely related F. bocagei (Bocage’s mole-rat) and F. vandewoestijneae that share a similar fur coloration and which may occur in sympatry with F. mechowii in parts of its range. Fukomys mechowii and F. vandewoestijneae typically lack the white head spot characteristic for many species of the smaller Fukomys mole-rats. Definite species assignment of immature specimens can therefore only be achieved based on karyotyping ( F.mechowii : 2n = 40— Macholán et al. 1993; all other species: 2n ≥ 42— Van Daele et al. 2004) and DNA sequence markers ( Van Daele et al. 2013).

GENERAL CHARACTERS

Fukomys mechowii is a large, robustly built African mole-rat. Adults display concolorous ochre-brown to golden pelage ( Fig. 1 ). Fur color changes markedly during ontogeny. The pelage of pups is typically dark gray, but slowly turns brown after weaning and eventually takes on an ochre hue when attaining sexual maturity; it may get paler with increasing age ( Kawalika and Burda 2007; Bennett and Burda 2013). A white head spot, typical for most other Fukomys mole-rats, is usually lacking in F. mechowii . Individuals (of both sexes) show only very rarely a small white spot on the forehead ( Kawalika and Burda 2007). The body hair is uniformly short (hair length: 7 mm at the trunk, 7.5 mm at the ventrum— Šumbera 2019). There can be prominent patches of dark rusty-brown coloration in the corners of the mouth in adult males ( Fig. 1 ; Peters 1881, only rarely distinctly expressed in females). These markings appear to be stained by wax-like glandular secretions and have been linked to reproductive status by some authors (e.g., Sichilima et al. 2008a), an association we have not observed in captivity. Apart from that, no sexual dichromatism is observable. Local mole-rat hunters in the Zambian Kasama region anecdotally describe exceptionally large, whitebellied or completely white giant mole-rats ( Kawalika 2004). So far, these reports remain unconfirmed.

Hands and feet lack fur and are instead dorsally covered and fringed by sparse pale hairs. Similarly, the short tail is naked except for white caudal bristles. Claws on the hind feet are dorsoventrally flattened and appear nail-like ( Kingdon 1974). The rhinarium, eyelids, and the immediate area surrounding the external ear opening are naked. As all bathyergids, F. mechowii lack pinnae. Females exhibit two pairs of pectoral and one pair of inguinal nipples ( Scharff et al. 1999).

Fukomys mechowii is the largest species of the genus Fukomys and shows pronounced sexual size dimorphism ( Burda and Kawalika 1993) which develops shortly after the onset of sexual maturity ( Scharff et al. 1999; Chimimba et al. 2010). The degree of body size dimorphism is among the most extreme among the Bathyergidae and rodents in general ( Caspar et al. 2021). Average body size as approximated by body mass varies substantially between populations. In a sample of individuals from several Zambian localities in the Copperbelt Province, the mass (g; range and sample size n presented in parentheses) of adult males was found to be 345.3 ± 95.4 SD (250–560; n = 15), whereas that of adult females was 252 ± 34 SD (200– 295; n = 18—Scharff et al. 2001). Higher average body masses were found in specimens from the Chingola area in the Zambian Copperbelt province (males: 570.7 ± 20.7 SE (220–995; n = 79); females: 391.3 ± 11.7 SE (240–650; n = 76— Sichilima et al. 2008b)). Fukomys mechowii from Chibale and Ndola-Chichele ( Zambia) is reported to be smaller than specimens from other areas ( Kawalika and Burda 2007), with a body mass of typically less than 300 g in males (Scharff et al. 2001). However, this population was found not to be distinct from other F. mechowii based on sequence data derived from the mitochondrial cytochrome- b gene ( Van Daele et al. 2007). Individuals from Mount Moko in Angola have been noted to be the largest representatives of the species ( Bennett and Burda 2013), but no data on body mass have been published and no molecular or karyological data on this population are available. In general, exceptionally large males of F. mechowii can reach weights of 700–800 g ( Kawalika 2004; Kawalika and Burda 2007; but see Sichilima et al. 2008b for reporting on wild males weighing up to 995 g).

Further standard body measurements (mm; mean with SD and range presented in parentheses) are available for 10 male and 10 female specimens of F. mechowii , respectively, deriving from the Copperbelt Province in Zambia ( Bennett and Burda 2013): head–body length—190 (22, 156–262), 165 (18, 135–205); tail length—27.3 (2.3, 23–31), 27.8 (3.8, 23–33.7); length of hind foot—35.3 (2.0, 30.6–37.8), 32.2 (1.0, 31–34); pinna length— not applicable.

The skull of F. mechowii ( Fig. 2 ), apart from its size, shows the morphology typical of the genus and resembles that of other tooth-digging bathyergids (e.g., McIntosh and Cox 2016; see “Form and Function”). It displays reduced bony orbits, hystricognathy ( Gomes Rodrigues et al. 2016) and ( Van Daele et al. 2008). The infraorbital foramen is elliptical and thin-walled ( Kawalika and Burda 2007). Sexual dimorphism in skull morphology is pronounced ( Chimimba et al. 2010).

Mean skull measurements (mm; ranges presented in parentheses) of five males and five females from the Copperbelt Province in Zambia ( Bennett and Burda 2013), respectively, were: greatest skull length—52.0 (45.6–59.2), 42.2 (34.0–49.7); greatest skull width—46.7 (40.3–53.2), 33.2 (28.6–37.0); length of toothrow (P4–M3)—9.1 (7.9–10.2), 7.8 (6.9–9.2). More detailed cranial measurements (mm, approximated to 0.1 mm; mean with SD and range presented in parentheses), some selectively presented below, are available for a large sample of F. mechowii (n = 47; sexes and ages combined) deriving from various locations throughout its range in Angola, Democratic Republic of Congo, and Zambia ( Van Daele et al. 2013): greatest skull length—47.75 (5.1, 38.6–61.4); skull width at processus mastoideus—24.48 (3.4, 20.1–30.6); diastema length—17.0 (2.4, 12.3–23.7); palatilar length—31.4 (3.7, 12.3–23.7); length of bulla—12.48 (1.0, 10.3–14.7); width of bulla—9.2 (0.9, 7.4– 11.0); width of zygomatic plate—11.7 (2.1, 7.8–18.3); maximum zygomatic width—37.6 (4.9, 28.7–50.2); length of infraorbital foramen—5.8 (1.2, 3.6–9.1); rostral width—11.9 (1.8, 8.7– 16.1); length of mandible—38.1 (5.0, 29.4–52.0); distance between coronoid and processus angularis—23.8 (3.6, 17.9–32.0). Further craniological measurements from 265 specimens from Chingola, Zambia, were provided by Chimimba et al. (2010) and grouped in respect to ontogeny.

The dentition is typical for the genus Fukomys . Each jaw quadrant carries one incisor, one premolar, and three molars (e.g., Van Daele et al. 2013). Incisors are ungrooved with roots extending posteriorly beyond the molar alveoli ( Honeycutt et al. 1991). The single premolar is larger than the molars, which decrease in size from anterior to posterior ( Honeycutt et al. 1991). Molars are highly simplified and appear as enamel cylinders enclosing a dentine core. Traces of reentrant folds may be observable on the labial molar surface ( Honeycutt et al. 1991).

hystricomorphy. However, the small infraorbital foramen, as in all mole-rats of the Cryptomys and Fukomys genera, is only penetrated by a rudimentary extension of the zygomaticus muscle

DISTRIBUTION

Fukomys mechowii is restricted to inland central Africa south of the equator. Its distribution extends from northern Angola and the southern Democratic Republic of Congo into northern Zambia ( Fig. 3 ; Maree and Faulkes 2016). However, because detailed surveys are lacking, it is unknown whether the range is continuous ( Kawalika and Burda 2007). Profound genetic differences between Eastern ( Zambia, presumably eastern Democratic Republic of Congo) and Western ( Angola, western Democratic Republic of Congo) lineages of giant mole-rats (see “Nomenclatural Notes” and “Genetics”) could indicate a disjunct range. Additional records from Malawi and Tanzania are doubtful ( Ansell and Dowsett 1988; Maree and Faulkes 2016). In the southeast, the Zambian Muchinga Escarpment, Mulungushi River, Lukanga swamps, and Kafue River have been suggested to restrict its distribution, whereas in the southwest the range borders at the arid Kalahari basin ( Ansell 1978; Kawalika and Burda 2007). The Congo River and the equatorial rainforest belt in the northwest can be assumed to act as dispersal barriers as well. Its northernmost confirmed record is on the Batéké Plateau, close to Kinshasa in the Democratic Republic of Congo (PalataKabudi et al. 2005). The range of F. mechowii extends into western Angola but does not reach the coastal areas ( Van Daele et al. 2013; Maree and Faulkes 2016).

As suggested by its wide distribution, F. mechowii is adaptable and tolerates a broad range of habitats and varying seasonality. It is typically found in bushland and mesic savannahs but also occurs in forested areas, marshes (dambo), and in diverse cultivated lands (plantations, fields, orchards— Kawalika and Burda 2007). Fukomys mechowii has been recovered from various soil types, ranging from ones with a stony, lateritic consistency to pure sand or clay soils ( Ansell and Dowsett 1988; Bennett and Burda 2013).

FOSSIL RECORD

African mole-rats have long been considered to be an ancient group of hystricomorph rodents. Initial molecular studies supported this view by placing their emergence into the early Eocene, about 45 Ma ( Faulkes et al. 2004; Ingram et al. 2004). However, lately a more recent time of origin for the clade in the early Miocene, about 21 Ma, has been suggested ( Bryja et al. 2018). The earliest fossils of bathyergids also date to the early Miocene and are assigned to genera which cannot be securely placed in respect to crown-group taxa ( Winkler et al. 2010). Current fossil evidence suggests that the family was restricted to sub-Saharan Africa since its emergence. The social, sister genera, Cryptomys and Fukomys (common mole-rats), represent the most recent and most successful radiation of bathyergids. Given the topology of the bathyergid family tree and distribution patterns of extant genera, the Cryptomys – Fukomys clade was suggested to have originated in Africa south to the Kalahari Desert ( Faulkes et al. 2004). The palaeo-Zambezi River could have acted as a barrier separating ancestral populations of these genera ( Ingram et al. 2004). Conflicting molecular data suggest the clade originated either in the middle Miocene, 11–17 Ma ( Faulkes et al. 2004; Ingram et al. 2004), or in the latest Miocene, 6 Ma ( Bryja et al. 2018). The oldest known fossils of small bathyergids akin to modern Cryptomys and Fukomys date to the early Pliocene of South Africa ( Winkler et al. 2010). Problematically, an assignment of fossils to either genus is not possible, because both are currently distinguished only by means of ethology, and on the karyological and molecular level ( Kock et al. 2006; Monadjem et al. 2015). Although extant species of Cryptomys are restricted to South Africa and southern Zimbabwe, Fukomys colonized diverse habitats of varying humidity, primarily west to the African Rift Valley, as far north as into the northern tropical savannahs ( Faulkes et al. 2010a). However, fossil material that could potentially be assigned to Fukomys (but was originally classified as Cryptomys ) is sparse ( Monadjem et al. 2015), not unambiguously referable to extant species, and does not elucidate phylogeographic patterns within the genus.

FORM AND FUNCTION

Like all African mole-rats, Fukomys mechowii displays a strictly subterranean lifestyle and shows striking anatomical adaptations to life underground. Similarly, its physiology is adapted to the peculiar conditions of its underground habitat which poses specific demands on respiration, thermoregulation, and sensory systems ( Bennett et al. 1994; Begall et al. 2018). The body of all Fukomys mole-rats is cylindrical, their extremities and tail are short, testes lay abdominal, pinnae are missing, and their hearing range is restricted to comparatively low frequencies (see below). Combined, these traits represent an adaptive syndrome to subterranean life, though they might separately occur in epigeic mammals as well. The skin of bathyergids is only loosely attached to the underlying musculature by connective tissue. Thus, it can be widely shifted along the rump, reducing friction on the body when moving in narrow underground tunnels. The velvety, evenly short fur brushes in either direction and facilitates locomotion in the burrow system (e.g., Kingdon 1974). Well-innervated guard hairs, probably enhancing tactile sensing, are found all over the body ( Krehbiehl 2010). The density of guard hairs is especially high at the tail allowing the animal tactile control when moving backwards.

When burrowing, soil is removed from the surrounding substrate by the incisors. Accordingly, F. mechowii , as most bathyergids, employs a chisel-tooth-mediated mode of digging (McIntosh and Cox 2016). As typical for the family, hairy lip folds close medially behind the procumbent incisors, even at full gape. Thereby, dirt is prevented from being ingested during the digging process ( Kingdon 1974). To avoid obstruction of the airways, the nostrils can be sealed as well (compare Banke et al. 2001). Loosened soil is pushed below the rump by the forelimbs and is dispensed posteriorly by the hindlimbs ( Kingdon 1974; Morlok 1983). To allow for efficient shoveling, both fore- and hind feet are laterally widened by a sesamoid bone (prepollex and prehallux, respectively— Schmitt et al. 2009; Prochel et al. 2013). Furthermore, the os lunatum and os scaphoideum of the carpus are fused, which might increase the stability of the palm when digging ( Prochel et al. 2013). Whereas the forefeet display pointed claws, the ones of the hind feet are dorsoventrally flattened and appear nail-like, assisting in effectively dispensing loosened soil ( Kingdon 1974). The shoulder girdle is translocated cranially, compared to epigeic rodents, and exhibits ossified mesoscapular segments, situated between the shoulder blades and clavicles. Both characters are associated with a burrowing lifestyle and are frequently observed in different subterranean mammal clades ( Morlok 1983). The angulus caudalis of the shoulder blade is elongated to enlarge the attachment site for the teres major muscle, a forelimb retractor that facilitates digging ( Morlok 1983). However, most important for burrowing are the adaptations found in the skull and dentition.

Compared to its body size, the skull of F. mechowii is notably large and its jaws are elongated, permitting a wide gape (McIntosh and Cox 2016). Similar to other chisel-tooth-digging rodents, the skull is deep and broad with widened zygomatic arches for supporting the substantially hypertrophied jaw musculature ( Gomes Rodrigues et al. 2016; Cox et al. 2020). Especially the relative size of the temporalis muscle is increased, as it allows to sustain high bite forces at the wide gapes (approaching 70°) which typically occur during tooth-digging (McIntosh and Cox 2016; Van Wassenbergh et al. 2017). Relative to body mass, Fukomys mole-rats display the second most massive jaw musculature of all subterranean rodents, being surpassed only by the naked mole-rat ( Heterocephalus glaber — Morlok 1983). Fukomys mechowii shows the strongest relative bite force of all mammals studied so far (P. A. A. G. Van Daele, in litt., Ghent University, Ghent, Belgium, 1 May 2010). Adding to this, evidence from finite element analyses of virtual skull models point to a high mechanical efficiency of biting in F. mechowii which is comparatively well maintained at wide gapes ( Cox et al. 2020). The mandible is highly mobile due to an unfused mandibular symphysis and enlarged, flattened glenoid fossae ( Gomes Rodrigues et al. 2016). The size of the brain (approximately 2.15 g) and the number of neurons in specific brain regions of F. mechowii conform to rodent scaling rules ( Kverková et al. 2018).

As typical for tooth-digging rodents, F. mechowii shows pronounced upper incisor procumbency, which, besides its direct relevance for digging, reduces friction on the nasal region ( Agrawal 1967). However, contrary to some claims (e.g., Landry 1957) procumbency in the Fukomys genus is not notably expressed in comparison to most other bathyergids (McIntosh and Cox 2016). The roots of both the upper and lower incisors in F. mechowii are remarkably deep and extend caudally beyond the respective molar rows, a character exclusively shared by tooth-digging bathyergids among rodents ( Ellerman et al. 1953). The extreme depth of the tooth alveoli could serve to increase mechanical resilience and facilitate force dissipation in the skull ( Landry 1957).

The anatomy of the gastrointestinal tract of F. mechowii shows no peculiarities and is very similar to the gastrointestinal tract of other bathyergids ( Kotzé et al. 2010). The stomach is simple and it amounts to about 9.6% of the total gastrointestinal length. The length of the small intestine relative to the total alimentary tract (47.2%) is roughly as long as the combined length of colon and cecum (43%). The cecum (with two taenia, and 13–16 haustra) is spirally coiled and, as in most mammals, lacks an appendix ( Kotzé et al. 2010). The ascending colon loops and is folded in the right abdominal cavity. The mean (± SD) total length of the gastrointestinal tract amounts to 1,268 ± 162 mm with a mean (± SD) total surface area of 46,829 ± 9,914 mm 2 ( Kotzé et al. 2010).

The hair density of the fur is much lower on the ventrum (2,800 hairs/mm 2) compared to the trunk (11,200 hairs/mm 2) and might help the animal dissipate heat effectively (Okrouhli et al. 2015; Šumbera 2019). In line with this, the mean (± SD) conductance of the skin of F. mechowii (0.09 ± 0.01 cm 3 O 2 × g−1 × h−1 × °C) is higher compared to surface living rodents ( Bennett et al. 1994). The thermoneutral zone of F. mechowii is reportedly between 29°C and 30°C ( Bennett et al. 1994). The mean (± SD) body temperature is on average 34 ± 0.53°C in the ambient temperature range 29–34°C, and the resting metabolic rate (0.6 ± 0.08 cm 3 O 2 × g−1 × h−1) at thermoneutrality is significantly lower than expected in respect to body mass ( Bennett et al. 1994). Digging metabolic rate of F. mechowii is 4.5 times higher than resting metabolic rate and independent from soil type (hard versus soft soil) or sex ( Zelová et al. 2010). Under laboratory testing conditions (temperature gradient: 16–37°C), F. mechowii prefers mean (± SD) ambient temperatures of 25.6 ± 1.2°C, thus much lower than its thermoneutral zone ( Begall et al. 2015). This might be due to a favorable volume to surface ratio in this large bathyergid.

The eyes of F. mechowii are tiny (equatorial diameter: 2.4–2.6 mm) but possess all structures typical for a mammalian eye (e.g., stratified retina, iris, lens) and permit dichromatic color vision ( Peichl et al. 2004). Apart from a well-developed musculus retractor bulbi, the extraocular musculature is reduced. The miniscule ocular dimensions likely impede the effective resolution of shapes or movement, while keeping sensitivity to changing light intensities. Avoidance of full-spectrum (white), blue, and green-yellow light but not UV or red light has been experimentally demonstrated ( Kott et al. 2010). The photoreceptor layer is dominated by rods that are fewer in number than in (nocturnal) epigeic rodents, but individual rods are larger ( Peichl et al. 2004). The proportion of cones (ca. 10%) among the total amount of photoreceptors is higher than in diurnal mammals. Most cones (ca. 90%) express S-opsins, which appears puzzling because blue light propagates much less efficiently in underground tunnels than light of longer wavelengths ( Peichl et al. 2004; Kott et al. 2014). However, it can be assumed that the high proportion of S-cones relates to an unusually low level of serum thyroxine (T4) as is the case in Ansell’s mole-rats ( F. anselli — Henning et al. 2018). If so, this exceptional opsin expression pattern might be a neutrally selected side effect of keeping the basal metabolic rate low ( Henning et al. 2018).

Hearing in F. mechowii is restricted to relatively low frequencies up to 4 kHz and high levels of sound pressure as has been shown by studying auditory brain-stem responses ( Gerhardt et al. 2017). The species therefore displays one of the lowest high-frequency cutoffs among mammals. The highest hearing sensitivity is present at 1–1.5 kHz, where the hearing threshold is still located at about 30 dB sound pressure level ( Gerhardt et al. 2017). It was shown that F. mechowii , similar to other bathyergids, is sensitive to magnetic stimuli under controlled laboratory conditions (Oliveriusová et al. 2012).

ONTOGENY AND REPRODUCTION

Ontogeny. —Newborn Fukomys mechowii weigh on average 19.6 g (range 12.6–27.7 g — Scharff et al. 1999) and are 5–6 cm long ( Bennett and Aguilar 1995). Body mass of neonates is not correlated with litter size, but it is a good predictor of further survival ( Burda and Kawalika 1993; Scharff et al. 1999). Neonates are colored dark pink and are sparsely covered by thin whitish hairs. Their eyes and auditory meatus are closed but their incisors already pierce the lips ( Bennett and Aguilar 1995). Postpartal growth during the first 180 days after birth is slow with no significantly different mean growth rates between males (0.62 g /day) and females (0.61 g /day— Scharff et al. 1999). Eyes and ear meatus might be open by 6 days after birth ( Bennett and Aguilar 1995) but can remain closed until the newborns are up to 3–4 weeks old ( Burda and Kawalika 1993). Dark-gray pelage starts to develop within the first week, and at the age of 5–6 weeks pelage color changes from gray to brown ( Bennett and Aguilar 1995; Scharff et al. 1999). Pups are typically nursed for at least 3 months ( Burda and Kawalika 1993; Scharff et al. 1999); however, weaning might occur after as soon as 5–6 weeks ( Bennett and Aguilar 1995). Suckling bouts typically extend over more than 20 min ( Bennett and Aguilar 1995). Pups start to consume solids (including feces from family members) at the age of 2 weeks and commence to actively explore the environment at 7–10 days after birth ( Scharff et al. 1999). At that time, they also begin to spar with each other. Sparring with adults occurs at 2 weeks after birth ( Scharff et al. 1999).

Fukomys mechowii reaches sexual maturity at about 12 months, but pair formation in captivity is suggested to take place at 18 months of age or later ( Begall et al. 2018). The maximum life span of the species in captivity is> 26 years ( Begall et al. 2021; a female captured as an adult in June 1995 died in July 2020 at the University of Duisburg-Essen), data on free-living animals are not available. The median age reached by captive reproductive animals is 13.4 years (4,882 days) while that of nonreproductive ones is 8.3 years (3,012 days); a pattern that fits the bimodal aging pattern also found in congeneric species ( Dammann et al. 2011; Begall et al. 2021). There is no significant difference in life expectancy between males and females although the age difference between the oldest documented female (> 26 years) and male (16.4 years) is striking ( Begall et al. 2021).

Several genes related to the hypothalamic–pituitary–adrenal stress axis seem to be downregulated in breeding F. mechowii compared to age-matched nonreproductive siblings, and might provide an explanation for the bimodal aging pattern ( Sahm et al. 2021). Indeed, hair cortisol levels are significantly higher in nonreproductive animals compared to breeders in captivity. Captive nonbreeding F. mechowii that do not live in family groups together with their parents show markedly lower cortisol levels than individuals that remained in intact families and can approach reproductive animals in longevity ( Begall et al. 2021). In general, stability of gene expression in F. mechowii is remarkably high across various tissues when compared to laboratory Norway rats ( Rattus norvegicus ) of similar body mass, providing a proximate mechanism for comparatively slow aging in this species (Sahm et al. 2018).

Reproduction. — Fukomys mechowii lives in monogamous families where typically only one pair (sometimes termed “king” and “queen”) is reproductively active, resulting in a strong reproductive skew ( Šumbera et al. 2012). Offspring remain with the parents into adulthood, in some cases, possibly lifelong, and support them in raising further young. Due to the reproductive division of labor, cooperative care for the young and strong philopatry of offspring, F. mechowii and all its congeners have been referred to as eusocial mammals ( Burda and Kawalika 1993; Kock et al. 2006; Kverkovà et al. 2018; the definition and applicability of the term are discussed by Burda et al. 2000). This social system relies on individual recognition combined with incest avoidance (see “Behavior”). Breeding occurs throughout the year (Scharff et al. 2001; Sichilima et al. 2008b).

The proportion of juveniles (body mass less than 50 g) among the total number of trapped F. mechowii in the field is, at 8%, very low compared to other social rodents (Scharff et al. 2001; Kawalika and Burda 2007). Under laboratory conditions, the breeding pair shows lifelong sexual monogamy, and incestuous matings between siblings or parents and offspring are exceedingly rare ( Kawalika and Burda 2007). During the 25-year breeding history of the species at the University of Duisburg-Essen, only one incestuous mating between father and daughter was observed and it occurred after the original queen had died ( Begall et al. 2021). In most groups of F. mechowii captured in the field, there is also only one female found to be reproductively active ( Wallace and Bennett 1998; Scharff et al. 2001; Sichilima et al. 2008b; Šumbera et al. 2012). Although they do not reproduce, nonbreeding F. mechowii of either sex are not sterile. Pituitary sexual hormone parameters do not differ between breeders and nonbreeders, suggesting behavioral instead of physiological regulations of reproduction (Bennett et al. 2000; see “Reproductive behavior”). Although observations on the reproductive physiology of nonbreeding male F. mechowii are sparse, its constitution might mirror the one of the better studied congeneric F. anselli . Testes of reproductive F. anselli males are significantly larger than those of nonreproductive males; however, nonreproductive individuals also produce viable sperm ( Garcia Montero et al. 2016). Information on whether F. mechowii females are induced or spontaneous ovulators is ambiguous ( Faulkes et al. 2010b). However, because F. anselli ( Willingstorfer et al. 1998) displays induced ovulation, it can be considered likely for F. mechowii as well. Females possess a uterus duplex, as typical for most rodent taxa.

The male’s penis shows no ornamentation ( Faulkes et al. 2010b) and exhibits a baculum. Females display an elongate flattened os clitoridis (baubellum) that approaches the baculum in size ( Thomas 1917).

Gestation lasts on average (mean ± SD) 112 ± 9 days with a minimum interbirth interval of 89 days indicating that the females experience postpartum estrus ( Scharff et al. 1999). The mean litter size is 2.1 ± 1.1 (range 1–5) and increases with the number of births experienced by one female ( Scharff et al. 1999). Very rarely, six pups are born in one litter ( Matschei and Bäthe 2012).

Figures

Fig. 1.—Group of Fukomys mechowii photographed in captivity at the Department of General Zoology, University of Duisburg-Essen, Essen, Germany. The animals display the typical huddling behavior that aids in conserving body heat. The animal in the upper center is an adult male, the onebelow an adult female.Note the cylindrical bodyshape,microphthalmy, and procumbent incisors.The male is paler than the female.At least in captive F.mechowii, this dimorphic color pattern has been repeatedlyobserved. Photograph used with permission of the photographer, Ulrich Hellinger.

Fig. 2.—Dorsal, ventral, and lateral views of skull and lateral view mandible of an adult male Fukomys mechowii from the research collection of the Department of General Zoology, University of Duisburg-Essen, Essen, German. Greatest length of skull is 51.8 mm. Photographs by KRC.

Fig. 3.—Geographic distribution of Fukomys mechowii. Names of countries from which the species has been reported are annotated. The range boundaries are largely conjectural and modified afterVan Daele et al. (2013) and Maree and Faulkes (2016).