AARDVARK

Biology



Morphology

Aardvarks are heavy-set mammals, weighing between 30 and 60 kg. They have a distinctive arch-shaped profile and are tallest at the middle of the back. Other distinguishing characteristics include an elongated head and snout, robust legs, and a chunky, tapering tail. The end of the nose is slightly swollen with large nostrils and is incredibly soft to the touch. The exaggerated external features of the nose are, unsurprisingly, directly related to the highly developed sense of smell that plays a crucial role in the lives of aardvarks. Aardvarks find all their food by smell.

Another very well developed sense is hearing. Aardvark ears are long and tubular and are able to detect the slightest noise that might indicate approaching danger. They do not appear to be important in finding food because of their shape and the fact that they are never directed towards the ground where food is to be found. Eyesight, in contrast, appears to be a very poorly developed sense. Eyes are not only small, but remarkably unresponsive to any kind of movement. Anatomical studies confirm weak eyesight with no nocturnal adaptations.


(Brehms Tierleben, Small Edition, 1927)

Aardvarks have strong legs and big nails for digging and even have some webbing between their toes to facilitate this. They have thick skin but are sparsely furred. It seems that the fur thickness may vary geographically, with animals in southern Africa having quite thick fur on the legs, thighs and flanks, while aardvarks of rainforests are almost bald. These variations need confirmation. The fur itself generally looks grey or black but, because aardvarks spend a lot of time underground or covered in dirt, they can take on the colour of the soil. Except for some differences in colour reported by some authors, but not confirmed further, there is no sexual dimorphism in aardvarks. Both males and females have a scent gland in their groin that looks like a scrotum, which produces a strong musk-like odour to signal the presence of the animal (perhaps for territory marking).

Anatomy

Lateral view of a skull of O. afer (collection of the University of Poitiers). Modified after Lehmann, 2004, PhD Dissertation.

The aardvarks' skull is long, roughly tubular, particularly elongated on the snout, and widest at the level of the jugals. The nasal bones are triangular in shape, broaden caudally but are never fused. The lacrimal presents a rostral development and the lacrimal foramen is situated on the edge of the orbit. The frontal bones bulge dorsally in front of the orbit as a result of the highly developed nasal chamber (see below). In Tubulidentata, a part of the parietal bone joins the alisphenoid so that frontal and squamosal are not in contact. There is no sagittal crest and only faint temporal ones. In juvenile aardvarks an interparietal bone is present between the parietal and the occipital bones, but it fuses with the other cranial bones by adulthood. The zygomatic arch is complete but slender. The orbit is separated from the temporal fossa by a postorbital process. The palate is long and narrow; it presents two post-palatine foramina and ends caudally in a strong post-palatine torus. The tympanic bone is annular, and there is no auditory bulla. The mastoid portion of the petrosal is well exposed on the lateral and occipital side of the skull, a condition that differs from the amastoidy seen for instance in the Paenungulata. he horizontal ramus of the mandible is slender but swells at the level of the molars. The mandibular symphysis is not often fused. On the ascending ramus, the coronoid process is long and projects over the condylar process, which presents a rather flat articulation surface. As a counterpart, the mandibular fossa on the cranium is also rather flat, a condition that limits the jaw movements. A microwear analysis revealed that the main mastication direction in O. afer is rostro-caudal (Lehmann, in prep.).
The most amazing anatomical structure found in aardvarks is certainly their dentition. That aardvark do possess teeth is already a surprise, but the microstructure of these is even more unique. The tubulidentate teeth have no enamel and grow conti nuously from open roots. Each tooth consists of thousand of vertical tubes of dentine, hence the order name (tubuli-dentata). A digitation of the pulp runs inside each tubes until closure of the tube upon the surface. The premolars are peg-like whereas the molars are bicolumnar (8-shaped in occlusal view) with longitudinal grooves on each side. As first shown by Thomas (1890), the dentition is heterodont and diphyodont, with a highly variable permanent dental formula of I 0/0 C 0/0 P 2-4(usually 3)/2-4(usually 2) M 3/3. In contrast with numerous mammals, the M3 of the Tubulidentata erupt at an early stage of the ontogenesis and are thus not a good criterion for age determination (Lehmann, submitted).

The vertebral formula of the extant Aardvark is C 7, T 13, L 8, S 6, Ca 25-28. The high number of thoraco-lumbar vertebrae (21 in aardvark) has been suggested as an Afrotherian synapomorphy by Sanchez-Villagra et al. (2007). Aardvarks possess a large pelvis characterized by a dorso-caudal extension of the iliac bone. Their sacrum does not enter into contact with the ischium, and the pubic symphysis is unreduced in comparison with other digging mammals (MacPhee 1994). As discussed below, the humerus shows adaptations related to the fossorial way of life of the aardvark, like for instance the very broad distal epiphysis, the presence of a deltoid tuberosity, and the well-developed deltoid crest. The proximal epiphysis of the femur bears a condyle-like caput femoris but it is, like in most of the fossorial mammals, not separated from the diaphysis by a distinct neck (Reed, 1951). The greater trochanter is projected proximally at the level or slightly above the femoral head. The femoral shaft present a laterally projecting third trochanter and a specific pectineal tubercle (for the Mm. pectineus and iliacus), also called the fourth trochanter by some authors (e.g., MacPhee 1994). Noticeably, the proximal epiphyses of the tibia and fibula are fused, and the diaphysis of the tibia is bent. In the carpus of O. afer, the scaphoid and the centrum are fused (Clark & Sonntag 1926). Moreover, the pollex has disappeared, so that the hand has only four digits. Conversely, the hind foot still possesses five toes. In the tarsus, the talus does not articulate with the cuboid, a condition known as “serial tarsus” or taxeopody, like in hyracoids and proboscideans (see Rasmussen et al., 1990). The talus of the aardvark is rather primitive with a retained astragalar foramen and a ball-like distal articulation surface supported by a distinct neck.

The most detailed anatomical description on O. afer so far, was given in a monograph of Orycteropus afer, divided in a series of three papers (Sonntag 1925, Sonntag & Woolard 1925, Clark & Sonntag 1926). More details about its dentition can be found for instance in Anthony (1934) and Heuvelmans (1939). Further references can be found in our “Articles” section or in Shoshani et al. (1988).

         Skeleton of O. afer. Sketch after specimen AZ 391 from Transvaal Museum, Pretoria
(modified after Lehmann et al., 2006)

Aardvarks are well known for their prodigious digging activities. Aardvarks use their forelegs for digging and enter the category of the “scratch diggers” like the armadillo, the badger, and the pangolin (Hildebrand, 1985). Accordingly, the musculature of humerus, radius, and ulna is highly developed and the joints are strengthened. Likewise, a strong vertical crest on the distal epiphysis of the metacarpal bones helps to prevent dislocation during the digging effort. The radius of O. afer is always more than 25% shorter than the humerus (Lehmann, 2004). Such “brachial index” characterises fossorial mammals according to MacPhee (1994). While the forelimbs and the robust nails are directly involved in the excavation effort, the hindlimbs provide a firm base while digging, and are used to shovel soil backwards along passages and out of the burrows. The sturdy tail provides additional support while digging.

Muscle insertion surfaces on a left humerus of O. afer. Modified after Thewissen & Badoux, 1986.Notice the large insertion surface of the M. brachialis (20) and the developed lateral crest for the M. deltoideus (18 & 19).


Distribution

The extant aardvark is restricted to Africa south of the Sahara, being found almost anywhere where suitable food and soils are available. They are only completely absent from true deserts, and are even found in semi-arid areas and rainforests. The extent of their distribution in the latter is not well documented.


Assumed repartition area of O. afer (dashed lines). Circles represent the supposed sub-species. Modified after Lehmann, 2006.



Habitat and Population Density

Present day aardvarks are found in most habitats, including all savannah types (e.g. grasslands and woodlands), semi-arid areas (e.g. Karoo, South Africa) and also some rainforests (see Pagès, 1970). The extent of their occurrence in forests is unknown at present because of the difficulties of working in such habitat and, therefore, few studies have been conducted. Aardvarks are found at elevations of 3200 m in the Bale Mountains of Ethiopia (Yalden et al. 1996).

Within a habitat type, aardvarks are found mostly in flat or gently sloping areas that are not too rocky; characteristics that facilitate their burrowing and feeding habits. However, they are not totally excluded from steep areas if there are sufficient prey or if they separate parts of a home range. Seasonal floodplains may also be used if sufficient prey are available, but these are abandoned during times of high water. General habitat requirements are access to food and shelter, i.e. sufficient ant and termite numbers and a soil type that allows the excavation of burrows.

Aardvark densities are very difficult to estimate because of the difficulties of studying/counting them, but they appear to be low everywhere. It is not a simple matter of counting burrows and extrapolating this with aardvark numbers because aardvarks change burrows frequently and because one home range may contain hundreds of abandoned burrows. In the Karoo, South Africa, where long term studies have been conducted, aardvark densities are approximately eight animals per thousand hectares (Taylor & Skinner, 2003). Home ranges vary between 200 and 400 ha. Aardvarks are probably territorial, but no studies on this issue have been undertaken yet.


General Behaviour

Aardvarks are predominantly nocturnal and are active every night. In summer they normally exit their burrows soon after dusk and are active all night, returning to their burrows before dawn. In winter they become active earlier and in cold areas such as the Karoo sometimes emerge in the afternoon (Taylor & Skinner, 2003). They then often return to their burrows before midnight at ambient temperatures of approximately 2ºC. Nearer the equator, activity periods are more consistent. While above ground they spend their time foraging, engaging in few other behaviours. The small size of their prey requires them to consume hundreds of thousands of ants and termites per night, and this necessitates them spending all their time foraging to satisfy their energy requirements.

During the day aardvarks use burrows to rest in. Although they typically excavate their own burrows, they more frequently renovate and utilise existing burrows when changing burrows. On average they use burrows for a week before moving on. Burrows normally have one entrance, but may sometimes have more. In the Karoo, research animals under observation always exited burrows from the same entrance they entered, implying they had single exit holes (Taylor & Skinner, 2003). In Uganda, 13 out of 18 excavated burrows had one entrance, two had two entrances, and two had three entrances (Melton, 1976). Burrows descend steeply before levelling out (at up to 3 m depth), may turn in any direction and extend over 10 m in length. They are just wide enough to allow the passage of an aardvark, with diameters of up to 450 mm. They often fork and terminate with an enlarged chamber. The buffered microclimate of deep burrows protects aardvarks from cold weather and rain.

Aardvarks are solitary and social interactions are uncommon. They communicate their presence mostly by scent marking soil using scent glands described above. When two animals meet (male/female), they spend a short time sniffing each other, especially around the base of the tail, before moving on separately. If sexual interest is shown, the interaction may last longer. During copulation, males hang on tightly to the females with their fore feet, their claws inflicting numerous scratches on the females' backs and flanks. Several attempted mountings may occur in a short space of time. Interactions between two adult males have not been observed.

Aardvarks do not vocalise, but instead indicate their presence by scent marking their home ranges with the scent gland. Aardvarks are fast runners, attaining speeds of up to 40 km/h and escape predation by entering burrows.


Feeding Habits

Aardvarks mostly eat ants and termites that they dig out of the ground. Ant genera that are known to be eaten include Aenictus, Alaopone, Anoplolepis, Camponotus, Crematogaster, Dorylus, Messor, Monomorium, Pheidole, Solenopsis, Tetramorium, and Typhlophone; while termites include Allodontermes, Basidentitermes, Cubitermes, Hodotermes, Macrotermes, Microhodotermes, Odontotermes, Pseudacanthotermes, and Trinervitermes. There are probably many other prey genera that are eaten in areas not yet investigated. Aardvarks are also known to eat the pupae of dung rolling beetles (Scarabaeidae) and may well eat other insects occasionally. It is unlikely, however, that these other prey are important for aardvark subsistence.


In the Karoo (a semi-arid area of South Africa), where detailed dietary studies have been undertaken, more ants are eaten than termites, with ants comprising about 80 % of the diet. At this study site, one ant species alone (Anoplolepis custodiens) comprises about 70% of the total number of prey eaten. The termite Trinervitermes trinervoides, which builds epigeal mounds, makes up about 20% of the diet, while the remaining 10% is filled by another ±15 ant/termite species. Seasonal changes in diet may occur but, to date, studies have shown different patterns. In Uganda, Aardvarks are reported to eat fewer termites in the dry season (Melton, 1976), but in the Karoo, aardvarks eat more termites in the dry season (which coincides with winter).

When active above ground, aardvarks spend almost all their time foraging, and feed from many (±200) ant and termite nests each night. Most feeds are short, lasting less than 2 min, but feeds from termitaries may last over 30 min. There are always ants or termites left active on the surface or in the nest at the end of a feed, probably because of diminishing returns. Also, prey becomes less accessible when on the surface because the aardvarks tongue is adapted to removing them from small tunnels.

Mechanical and chemical defences of ants and termites often play an important role in the feeding behaviour of myrmecophagous mammals. When the number of soldiers gets too high many animals are forced to stop feeding. This is not generally the case for Aardvarks; the defences of most ants and termites are ineffective on aardvarks.

In addition to insects, there are unconfirmed reports of aardvarks eating a geocarpic fruit (where the fruits form underground), the aardvark cucumber Cucumis humifructus (Meeuse, 1958, 1963). Seeds from this species have been found in faeces and the plant has been found growing at the entrance of burrows where droppings are often deposited. It has been suggested that aardvarks eat the fruit for its moisture content and in the process disperse the seeds that can only be germinated if they are first dug out of the ground. Aardvarks, however, fulfil their water requirements from ants and termites. An alternative hypothesis to the direct consumption of the fruit is that aardvarks ingest seeds while feeding on seed collecting ants, such as Messor capensis.


Reproduction

Reproduction is known mostly from captive animals with few observations coming from the wild (and most of these result from dissection of dead females to check for pregnancy). Mating periods may vary regionally. In southern Africa they appear to mate in early summer (October/November), with one observation also in May. Births in the wild have been reported from May to June in Ethiopia, early November in Uganda (Kingdon 1971), October and November in DR Congo (Shoshani et al. 1988), and May to August in southern Africa (Smithers 1983). Normally there is one young, but occasionally two are born. According to zoo records, the average gestation period is 35 weeks (range=33.5-37.0 at the Chicago Zoological Park). Babies are born alert and active, without hair, and weighing 1.8 kg (Chicago Zoological Park records). Zoo offsprings grow quickly, reaching 10 kg after seven weeks, and 40 kg after just seven months. Conversely, wild animals grow much more slowly.

The behaviour of mothers with young is not known in the wild. However, in captivity, it has been observed that young first follow their mothers out of nesting chambers at about 18 days. Moreover, females consume faeces and urine excreted by babies and have been observed to scent mark them. In the wild, it is not known how long young remain in the burrow or with their mother and dispersal of young animals is unknown.


IUCN/SSC Afrotheria Specialist Group