Australia: The Land Where Time Began
Spinifex Hopping-mouse (Notomys alexis) 'Alexandra Downs southern mouse' (Dargawarra - to the Aborigines)
This common, widespread small mammal has 3 subspecies:
Upper parts fawn or chestnut, greyer on muzzle and between the eye and ear. Over the back and rump black guard hairs are prominent. The underparts, as well as the lower jaw and cheeks are whitish. The long pale pink tail, that is darker above, has a terminal tuft of sparse silvery hairs. There is a throat pouch on both sexes with a central area that is bare, behind which is a low fleshy ridge.
They are nocturnal and gregarious, groups sheltering in deep burrow systems with several vertical entrance holes that lack a spoil heap. At night they can be seen bounding across open spaces with the body and tail almost horizontal. They are opportunistic breeders, breeding when conditions are suitable. They are omnivorous, eating seeds, most parts of plants as well as arthropods.
There are several similar species, such as the Dusky Hopping Mouse, in which both sexes have distinctive throat patches that are surrounded by a fleshy ridge that has stiff white hairs that point inwards.
The Fawn Hopping Mouse inhabits gibber plains. There is no throat pouch or chest gland on the females, the male has a bare raised glandular area between the forelegs.
Both sexes of the Mitchell's Hopping Mouse lack a throat pouch and chest gland, and a broad shiny band from throat to chest that is white. The elongated hindfeet and legs are not present on the Pseudomys, and the have post-interdigital pads on both hindfeet. Their ears are shorter tail, that is bicoloured and has no obvious terminal tuft (Source 2).
This mouse breeds up to prodigious numbers after the rare heavy, prolonged wet spells that occasionally flood the arid lands leading to a flush of vegetation. Huge numbers of them invade areas they are not usually found in, then as suddenly as they appeared they are gone once the dry conditions return and the food supply is back to its normal state.
This tiny mouse has the distinction of having the most efficient kidneys and the most concentrated urine of any mammal ever recorded in the entire world. It has another extraordinary adaptation to desert life, when the temperature in its burrow reaches above 38o C, the normal temperature of the mouse, it is unable to lose heat by sweating because the air in the burrow is near saturation, it simply becomes hyperthermic, raising its body temperature above the ambient temperature, so that the ambient temperature seems cooler to it. The burrows may reach a depth of more than 1 m beneath the surface, and typically burrow systems consist of a nest chamber that is large and horizontal, that is lined with small twigs, leaves and other plant material, with several vertical shafts leading to the surface, with no loose sand around the entrance. When they emerge from their burrows at dusk, the move on all fours at slow speed.
It is believed their diet is quite variable, and according to Finlayson, who states 'at permanent camps in the Everard Hills where vegetables were grown near the soaks it became for a time in 1932 a nuisance owing to its depredations on the young shoots of cucumbers and beets' and that 'according to the blacks the large round woody seeds of the quandong (Eucarya acuminata) ... are also eaten ... the seed case is neatly drilled on 1 side only with a small hole and the contents extracted. The author suggests that the Spinifex Hopping Mouse is clearly omnivorous, eating various plant parts and invertebrates, depending on their the relative abundance and availability of the food types.
Reproduction is believed to occurs mostly in spring, though it can take place at any time of the year. The litter size is usually 3-4, but can be up to 6, the 6 young being reared on the 4 teats. When the adults are foraging the young are left in the nest, and when the young stray from the nest they are retrieved by both parents. The usual length of pregnancy is about 32 days, though this can be extended, as the unattached embryos remain in the uterus longer if the previous litter are still suckling. At about 60 days sexual maturity is reached.
An unusual, if not unique, feature among rodents, is the minute size of the testes of the Spinifex Hopping Mouse that are about 0.2 % of body weight, about 1/10 the size of those of other mammals of similar body mass. The relatively few sperm that are produced are highly variable, and the only accessory sex gland of this animal that are not extremely small are the ventral prostates, with the result that after mating there is no hard vaginal plug formed, as in most other murids. The also have a pampiniform plexus. There are also large spines on the thin shaft of the penis, which is also unusual, and there is a narrow lumen and thick vaginal muscular coat. These morphological features are related to locking during copulation, at which time the pair often struggle violently, the female sometimes biting the male, and the mating pair often turn back to back before disengaging.
In the natural environment the social behaviour of this animal is unknown at the time of writing. It was suggested by early observers that the Spinifex Hopping Mouse possibly travelled in groups, but more recent observations suggest foraging is solitary, with unusual abundance of food resulting in group foraging.
The reproductive anatomy of the male indicates they may be monogamous, though it has been found that in captivity several individuals of both sexes cohabit, with multiple insemination of the females sometimes occurring. See Source 3.
It also has a number of more normal adaptations to the desert conditions, such as being active at night, and not requiring to drink, being able to live on a diet of dry seeds and still not needing extra water.
Granivory in the Australian arid zone is a viable survival strategy only for vagile (able to move around in an environment) animals such as birds and social insects like ants, that can survive on reduced food supplies for extended periods and store seeds in a granary. The Australian arid zone is different from other desert areas around the world where granivory among rodents is a successful food option. Most of the few desert rodents of Australia have a tendency to some degree of omnivory. The erratic climate of the arid zone, where seed supplies vary considerably from year to year, makes granivory a difficult way of life in the long term.
The Spinifex Hopping Mouse is found over much of the arid zone in central and western Australia. It was said to be found mainly on loamy mulga flats (Finlayson, 1940). In a large area of Triodia a few miles east of Mt Connor, that had responded with luxuriant growth following local rain. A mat of severed stalks, that the seeds had been removed from, were found around the base of almost every clump, and the tracks of Notomys formed a reticulated pattern on the sand. More recent observations have shown that the Spinifex Hopping mouse is characteristic of spinifex-covered sandflats and sandhills of this area that had been stabilised, as well as much of central Australia.
During dry times it is suggested to probably be restricted mostly to sandy areas, moving into other habitats as the population increases after good rains.
Experiments were carried with male hopping mice and lab mice in which the animals were exposed to temperatures of either 37-38o C or 23-24o C. The results showed that the testes of lab mice were significantly reduced in size, but those of the spinifex hopping mice were not. The number of apoptotic spermatagonia and primary spermatocytes in the seminiferous tubules of the heat-treated mice of both species were increased (P<0.05). Increased vacuolation in the seminiferous epithelium of heat-treated mice was displayed in preliminary observations, and in at least 3 animals with reduced spermatid numbers, degenerating semininferous tubules were present. The authors concluded that germ-cell survival during spermatogenesis is reduced in hopping mice, suggesting that in this species adaptation to arid environments doesn't appear to have resulted to any marked reduction in the germ cells' sensitivity to degeneration when exposed to high environmental temperatures (Source 4).
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|