Australasian Science Magazine

Scanning electron micrographs of the penis from a fawn hopping mouse (left) and spinifex hopping mouse (right) showing the narrower shaft but much larger spines that enable the spinifex hopping mouse’s penis to lock into place during mating.

By Bill Breed

They look cute and even like to cuddle, but what do the small testes and spikey penis of the spinifex hopping mouse tell us about their ability to survive and thrive between periods of drought and flooding rain?

Many people will be aware that marsupials and monotremes have been present on the Australian landmass since the breakup of the southern supercontinent, Gondwana, 50 million or more years ago. Less well known is the fact that the first native rodents arrived in Australia from South-East Asia around six million years ago and now make up approximately a quarter of all Australia’s land mammal species.

There are two major groups of native rodents: the New Endemics and the Old Endemics. The New Endemics are composed of eight or so species of rats in the genus Rattus whose ancestors arrived in Australia around one million years ago. The Old Endemics first arrived around six million years ago and comprise a more diverse variety of around ten genera that exhibit a range of body forms and sizes, from the delicate mouse weighing 8 grams to the carnivorous water rat and giant white-tailed rat that weigh up to nearly 1 kg.

Old Endemic rodents occur in a range of habitats throughout Australia and include a group of bipedal species that are specialised for life in sandy regions. The most widespread of these species is the spinifex hopping mouse (Notomys alexis), which weighs around 30 grams. It ranges from south-western Queensland, across central Australia to the coast of Western Australia in the Shark Bay region.

Individuals of this species are quite commonly kept as pets in Australia and reproduce fairly readily in captivity. They have a highly unusual reproductive anatomy that is quite unlike that of related mice and rats except for most other species of hopping mice.

Hopping mice, like nearly all other Old Endemic rodents, have a relatively low reproductive rate compared with the house mouse, black rat and Norway rat, which were introduced into Australia by Europeans within the past few hundred years. These latter species, like the New Endemics, have a pregnancy length of around 20 days and litter size of up to 10 pups, which enables their numbers to build up to very high densities – as anyone who has witnessed a house mouse “plague” in Australia well knows.

By contrast, the spinifex hopping mouse has a considerably longer pregnancy of at least 32 days. These animals have only four nipples, which limits the size of their litter.

Despite this, spinifex hopping mice undergo dramatic boom-and-bust population cycles in the wild, with long periods of scarcity during dry times alternating with abundance during boom times. How does this species manage to achieve this and what can a study of the reproductive biology tell us about these animals?

Unusual Genitalia
The external morphology of adult male spinifex hopping mice reveals something strange. Unlike most rodents, there is no large scrotal sac housing the testes. Rather, the scrotal area shows only a tiny swelling and is covered by sparsely-haired, and sometimes pigmented, skin. On dissection it is not always easy to find the testes, even in sexually mature males, although careful examination will reveal them lying in either the very small scrotum or in the lower abdominal cavity.

In males that have recently sired young, the total weight of testes is no more than about 0.15% of their body mass. This contrasts with most other Old World rats and mice of similar body size (apart from other hopping mice species), which generally have a testis mass that is around 1–3% of their body mass.

When we submitted a paper on these findings to a scientific journal, we were told this must be an artefact of housing the animals in captivity. This resulted in a quick visit to the mammal collection at the South Australian Museum, where we dissected a few wild-caught adult males and obtained similar results. Thus we had to conclude that these tiny testes were, indeed, a characteristic feature of this species and not a laboratory artefact.

Subsequent observations of other Notomys species have shown very small testes in dusky hopping mice (N. fuscus), Mitchell’s hopping mice (N. mitchelli) and northern hopping mice (N. aquilo). Somewhat larger testes occur in the fawn hopping mouse (N. cervinus), no doubt reflecting the retention of the ancestral condition.

Further examination of sexually mature male spinifex hopping mice showed that, in addition to the extremely small testes, they have a suite of other highly divergent features of the reproductive tract (see box, p.34). Furthermore, it became evident that the penis of the spinifex hopping mouse was also very unusual.

Most rodents, including the Old Endemics, have a penis that is barrel-shaped and covered with very small spines. By contrast, the spinifex hopping mouse has a very narrow shaft with large spines protruding from its surface.

When put together, these findings show that the entire reproductive anatomy of adult male spinifex hopping mice differs markedly from other mice and rats, including those in the other genera of Australia’s Old Endemics.

Two obvious questions arose from these findings.

1. Have female hopping mice evolved divergent features of the reproductive tract?

2. Do these unusual male reproductive tract features relate to differences in the mating system and social behaviour of these animals?

While female spinifex hopping mice have a similar gestation length and litter size to most other Old Endemic Australian rodents, it became evident that the structure of the vagina and cervix is indeed dramatically different. Most mice and rats have a highly fibrous cervix within which there are small cervical canals that pass to each horn of the uterus. In adult female spinifex hopping mice, however, the cervix is barely distinguishable from the vagina. Furthermore, the vagina has a very narrow lumen and is surrounded by a thick coat of smooth muscle.

Mating Behaviour
Around the time that we were carrying out our studies on the reproductive anatomy of spinifex hopping mice, we also set up a video system to record the social and sexual behaviour of groups of these animals using a low light intensity camera. To enhance the chances of observing mating behaviour, we constructed a group of cages each with a central removable barrier, and placed females on one side and males on the other. A few hours before removing the barrier, we injected the females with gonadotrophins to bring about oestrus and ovulation. Their behaviour was then recorded. What did we observe?

At first there was much hopping around the cage but, after a fairly short time, the whole group would settle down and often huddle together regardless of the age or sex of the individuals. Mounting of a female by one or more males would sometimes take place, and occasionally this would result in mating, at which time the pair would often fall on their sides and remain locked together for up to 8 minutes.

After they separated we performed vaginal washes and viewed the aspirates using a microscope to determine if sperm were present. From our video recordings we observed that in some cases while a pair was locked together, the female, who is generally a little larger than the male, would drag the male around the cage and occasionally would turn around and try to bite him, causing him to disengage. In general, it appeared that the female was able to determine the length of time the pair was locked together and consequently whether or not insemination took place.

Thus it seems that the highly spinous penis of the male and the thick muscular wall surrounding the narrow vaginal lumen of the female spinifex hopping mouse have co-evolved to facilitate locking during mating. Furthermore, it also would appear that, since the female is clearly the larger and more aggressive sex, even though she has lower levels of testosterone in her blood, she is able to determine whether or not a particular male fertilises her recently ovulated eggs and sires her offspring.

That testosterone makes males bigger and more aggressive clearly does not hold true for hopping mice. Perhaps, in this species, cryptic female choice occurs with the behaviour of the female greatly reducing the chances of more than one male inseminating her during the same oestrus. This may be at least one reason why such small testes have evolved in this species.

During the resultant pregnancy, if one occurs, the male will huddle together with the female and, once the pups are born, he will exhibit paternal care by retrieving the pups as frequently as the female if they wander from the nest.

Boom and Bust
Why do these animals have such an unusual reproductive anatomy? Spinifex hopping mice live in the highly unpredictable environment of arid Australia, where prolonged dry periods alternate with occasional periods of abundant resources after rain. They share this environment with small insectivorous marsupials from the family Dasyuridae, such as the fat-tailed dunnart. These latter species exhibit seasonal reproductive activity coinciding with increased abundance of insects during spring. Although hopping mice readily eat seeds, they are generally omnivorous and eat insects as well as a variety of plant material.

It was originally thought that spinifex hopping mice exhibited an “opportunistic” reproductive strategy, whereby reproduction is restricted to good seasons and no breeding occurs throughout the long, dry periods. However, in our periodic sampling of animals in central Australia, we have found that reproductive activity tends to also occur mainly in spring and early summer, with some suggestive evidence that, even under dry conditions, reproduction may be attempted at this time of year.

The relatively small testes of adult males indicate that these animals invest less energy and resources in sperm production than most other mice and rats of similar body size. This reduced energy expenditure on sperm production presumably enables males to remain fertile even when limited resources are available.

Clearly far more observations are needed, but it is possible that spinifex hopping mice, like the small dasyurids, exhibit some seasonality of reproduction that is extended to other times of the year when good conditions prevail. This prolonged period of reproduction, and no doubt high survival rate of pups due to abundant resources, probably enables the population to build up to high densities during these good times.

A study is now required to find out whether similar reproductive biology and behaviour has evolved in rodents in other deserts of the world. We do know however that, here in Australia, other Old Endemic rodents in the other genera such as the plains mouse (Pseudomys australis) and desert mouse (Pseudomys desertor) do not exhibit this highly derived reproductive biology. Perhaps these latter species evolved in the more predictable southern regions of the country, where droughts tend to be less prolonged, and then subsequently extended their ranges into the arid zone.

Box: How the Male Reproductive Tract Differs
In rodents, after sperm are produced in the testes they pass along a few small efferent ducts into a long coiled tube called the epididymis. As they slowly make their way along this tube they become motile and develop the potential to fertilise the eggs, with storage of sperm occurring in the tail of the epididymis.

During mating, the surrounding duct muscle coat contracts and the sperm are rapidly passed from the tail of the epididymis along a highly muscular duct, the vas deferens, into the first part of the urethra, the prostatic urethra. Here, secretions from several ducts that, in most mice and rats, include the seminal vesicles and various lobes of the prostate, are mixed with the sperm. This results in the production of semen, which is rapidly passed into the penile urethra and then deposited in the vagina of the female at ejaculation. The secretions from the seminal vesicles and part of the prostate, the coagulating gland, form a vaginal plug of material that helps the sperm pass rapidly through the highly fibrous cervix.

In the spinifex hopping mouse, several regions of the male reproductive tract differ markedly from those of most other mice and rats. Firstly, the testes are very small. The epididymis is also much shorter. Consequently, there is very limited room available for sperm storage and far fewer sperm are present in this region than in most other mice and rats, including house mice, even though hopping mice are about twice the size of these animals.

Unusually, in hopping mice, as the vas deferens passes up towards the urethra, the muscle layer gets thinner rather than thicker and the lumen expands with the result that this region of the tract acts as an accessory sperm storage site. Furthermore, the arrangement and size of the accessory sex glands differs, with the seminal vesicles and coagulating glands being minute whereas another part of the prostate, the ventral prostate, is very large. We do not know the full significance of this very different arrangement and relative sizes of the accessory sex glands but clearly, unlike in most mice and rats, no large plug of accessory sex gland secretion is deposited in the vagina during mating. The co-evolved structure of the cervix, which is not a large fibrous structure in hopping mice, apparently does not need a large plug of accessory sex gland secretion to facilitate sperm passage through it, so the male seminal vesicles and coagulating glands have become markedly reduced in size compared with other rodents.