"How rapidly do kangaroos breed" is an important question, if we are to understand the nature of Australia.
This continent has lacked any terrestrial wild predator larger than a dog (https://en.wikipedia.org/wiki/Dingo), for thousands of years.
Carnivores depend on the productivity of their prey. Why did no carnivore specialise on kangaroos, in the way the cheetah (Acinonyx jubatus, https://www.inaturalist.org/taxa/41955-Acinonyx-jubatus) specialises on gazelles in Africa and Asia?
The answer lies partly in an understanding of the reproductive system of kangaroos.
It is true that an individual female can have three dependent offspring, in various stages of growth at the same moment. However, this is evidence of slow, rather than fast, growth. An overlap of this kind may amount to an insurance policy, allowing at least one offspring to survive under conditions that are generally unfavourable for growth.
How to compare reproductive rates?
Kangaroos can be compared with the most similar herbivores under similar climates on other continents. Body size is a prime factor. Reproduction is likely to be more rapid in small-bodied than in large-bodied species, and the corollary is that lifespan is shorter in the former than in the latter.
Adult female kangaroos have body masses of 25-35 kg. Terrestrial herbivores of similar body size, under similar climates, consist of various ruminants and one large rodent. Since males of kangaroos eventually grow to double the body mass of females in middle age (older than 10 years), other herbivores of up to 65 kg can also be considered comparable.
Reproductive rates can be measured as either the rate of increase of populations free of predators, or the rate at which an established population can be culled with no risk of extermination. The prime factors are how fast females grow to breeding age, and how many offspring they pack into a single litter. Periods need to be calculated from conception , not from birth, because pregnancy in kangaroos is extremely brief.
Populations of the red kangaroo (Macropus rufus, https://www.inaturalist.org/taxa/42885-Macropus-rufus and https://australian.museum/learn/animals/mammals/red-kangaroo/) can be culled at an average of up to 15% per year. An annual offtake rate of 3 out of every 20 in the population seems to be sustainable in the long term.
The corresponding figures for grey kangaroos (Macropus fuliginosus https://www.inaturalist.org/taxa/42881-Macropus-fuliginosus and Macropus giganteus https://www.inaturalist.org/taxa/42888-Macropus-giganteus) and the common wallaroo (Macropus robustus, https://www.inaturalist.org/taxa/42872-Macropus-robustus) seem to be 13% and 11%, respectively. These rates of reproduction are modest for mammals of this body size.
No match for ruminants:
Kangaroos are often compared to ruminants. However, the many spp. of ruminants maintain rapid reproduction and growth over a wide range of body sizes.
Species the size of kangaroos generally outbreed kangaroos two-to-one. Only the largest-bodied of ruminants, or those living on mountaintops or in the Arctic, reproduce as slowly as kangaroos do.
The domestic sheep (Ovis aries, https://www.inaturalist.org/taxa/121578-Ovis-aries) is very variable, according to breed and plane of nutrition. Sustainable offtake in sheep varies from 15% in the case of the merino breed that coexists with kangaroos, to 300% in the case of the most prolific breeds on other continents.
In the domestic sheep, growth from conception to sexual maturity takes at least 5 months, and usually 15 months, which is less than in the red kangaroo. Females of the domestic sheep - despite being the more massive - have a shorter lifespan than that of females of kangaroos.
Wool breeds allocate protein to fleece instead of milk, and females wean only one offspring each per year. Nevertheless, the merino breed (https://en.wikipedia.org/wiki/Merino) in Australia outbreeds the red kangaroo, relative to body mass.
Average reproductive rates are similar, but females of the ungulate (65 kg) grow to double the body mass of females of kangaroos. Therefore, even wool-producing breeds of sheep can produce food for meat-eaters more rapidly than can any of the four spp. of kangaroos mentioned so far.
Please note the following:
Kangaroos are renowned for nurturing up to 3 offspring simultaneously. However, the finnsheep does the same for up to 10 offspring (5 in the womb, 5 at the udder).
Thus, the most prolific breeds of sheep reproduce up to 20-fold more rapidly than kangaroos do, if comparisons are made on a long-term basis with correction for differences in body mass of females.
The domestic goat (Capra hircus, https://www.inaturalist.org/taxa/123070-Capra-hircus) reproduces several-fold more rapidly than the coexisting common wallaroo does. The feral goat normally bears twins in semi-arid Australia, whereas the common wallaroo has the slowest reproduction among kangaroos.
Relatives of camels breed as slowly as kangaroos:
The largest ruminants, giraffes (Giraffa spp., https://www.inaturalist.org/taxa/42157-Giraffa-camelopardalis), sustain an average offtake of 15% of the population per year, on cattle ranches in Africa where they are conserved and culled. Like kangaroos, giraffes bear a single newborn. This means that kangaroos breed no more rapidly than a ruminant 25-fold their body mass (females: 30 kg vs 800 kg; males: 65 kg vs 1300 kg).
Other large ruminants err to one side or the other of the value of 15%. For example, Bison bison (https://www.inaturalist.org/taxa/42408-Bison-bison) has one offspring per birth, breeds in only 2 of every 3 years, and sustains an average offtake of 10%. The moose (Alces alces, https://www.inaturalist.org/taxa/522193-Alces-alces) often has twins, and sustains an average offtake of more than 20%.
Camels, unlike giraffes, deer, and other true ruminants, reproduce relatively slowly. The vicugna (Vicugna vicugna, https://www.inaturalist.org/taxa/42236-Vicugna-vicugna) is the smallest living member of the camel family, and approaches kangaroos in body mass as well as its diet of grass. Whereas the domestic sheep is pregnant for 5 months, the like-size vicugna is pregnant for 11 months. The sustainable rate of offtake for the vicugna is likely to be less than 10% per year.
The vicugna is restricted to the Andes in South America, at double the altitude of the Australian Alps (https://en.wikipedia.org/wiki/Australian_Alps). It is less prolific than kangaroos, probably because its food supply is always limited.
Prolific ruminants of arid climates:
The springbok (Antidorcas marsupialis, https://www.inaturalist.org/taxa/42283-Antidorcas-marsupialis) is a ruminant of similar body mass and diet to the red kangaroo, restricted to dry climates in southern Africa, and likewise culled on sheep pastures.
Please note the following:
- Although the springbok bears only 1 offspring per birth, this species of gazelle can sustain double the offtake rate of the kangaroo (about 30% per year).
- Females of the springbok usually mature sexually in less than half the time taken by females of the red kangaroo: 13 months vs 31 months after conception.
- Under good conditions, females of the springbok can reach sexual maturity less than 1 year after being conceived, a record unmatched by any kangaroo.
- The springbok can bear 2 offspring per year after good rainfall, because it is pregnant for less than 6 months, and the mother is prepared to mate shortly after giving birth.
- The only gazelle commonly bearing twins is a relatively small-bodied species, Gazella subgutturosa (https://www.inaturalist.org/taxa/568727-Gazella-subgutturosa), which is able to survive the cold winters of Asia.
It is true that the red kangaroo can reproduce rapidly after rainfall, with each mother having up to 3 offspring in various stages of development: 1 freely suckling, 1 attached inside the pouch, and 1 in the womb. However, this does not compensate for the relatively slow growth of these offspring. Even in the best of seasons, the red kangaroo mother can wean no more than 3 offspring in 2 consecutive years, compared to 4 in the case of the springbok.
Herbivores in the extremely arid Sahara Desert are also prolific. Two spp. of gazelles just manage to survive in the central Sahara, after decades of persecution with sophisticated firearms and all-terrain vehicles.
Gazella leptoceros (https://en.wikipedia.org/wiki/Rhim_gazelle), similar in body mass to kangaroos, has always been restricted to sandy desert in North Africa. Remnant populations of the cheetah have been recorded in the Sahara (https://en.wikipedia.org/wiki/Northeast_African_cheetah#:~:text=Once%20existing%20in%20Egypt%2C%20the,blindfolded%2C%20and%20kept%20on%20leashes.).
The cheetah is likely to be particularly dependent on the fecundity of its prey in desert, where all herbivores are naturally rare. This felid, twice as massive as the extinct thylacine (Thylacinus cynocephalus, https://en.wikipedia.org/wiki/Thylacine) of Australia, may have survived to this day in a habitat more barren than the heart of Australia.
Giant rodents and grazing apes:
The capybara (Hydrochoerus hydrochaeris, https://www.inaturalist.org/taxa/74442-Hydrochoerus-hydrochaeris) is the largest rodent on Earth, similar in body mass to kangaroos. It is culled on cattle ranches in South America, with a sustainable offtake of about 35%. The capybara reproduces far more rapidly than kangaroos do, because it bears 1-8 (usually 2-6) offspring per litter.
The gelada (Theropithecus gelada, https://www.inaturalist.org/taxa/43530-Theropithecus-gelada) of the Ethiopian Highlands is the only living primate that relies on green grass for food, and it inhabits treeless grassland. This is the most strictly herbivorous and terrestrial of all monkeys, and is therefore comparable with kangaroos.
Primates in general breed slowly, and the gelada is no exception. It reproduces below par for kangaroos, because females take twice as long to reach sexual maturity, and give birth only once every two years. Like the vicugna, the gelada is adapted to altitudes beyond the ranges of most comparable ruminants.
Why are kangaroos not prolific?
Marsupials should be capable of rapid reproduction, because the small size of newborns potentially allows many offspring to be packed into each litter. For example, 56 maggot-size individuals emerged from one birth of the North American opossum (Didelphis virginiana, https://www.inaturalist.org/taxa/42652-Didelphis-virginiana).
This marsupial has a short natural lifespan for a mammal of its size (3 kg), dying of old age at 3 years even if it has survived predators and winter snow. Females wean 9-12 offspring in the second hear of life, and largely rely in this one summer's maternal effort for the propagation of the species. Therefore, the consistent restriction of kangaroos to 1 offspring per birth, and their extended lifespan (more than 20 years), are not merely the result of their genetic heritage as marsupials.
Drought does not seem to be the limiting factor. The original numbers of kangaroos were modest, even after a series of years with more than average rainfall.
The tropical north of Australia has copious rainfall and perennial rivers. However:
In southern Australia, the red kangaroo of the semi-arid interior outreproduces the grey kangaroos of coastal woodlands, if long-term averages are compared. However, the fecundity of the red kangaroo remains modest, even where bore water is provided.
Originally, kangaroos were remarkably scarce in the treeless mitchell grassland (dominated by Astrebla, a grass restricted to Australia) that covers an area the size of Britain (450000 square kilometres) in the northern half of Australia (https://en.wikipedia.org/wiki/Mitchell_Grass_Downs and https://www.agric.wa.gov.au/rangelands/mitchell-grass-alluvial-plain-pastures-pilbara-western-australia#:~:text=Curly%20Mitchell%20grass%20(Astrebla%20lappacea,)%20of%2010%20to%2025%25.).
Mitchell grassland is as extensive as the Highveld (https://en.wikipedia.org/wiki/Highveld) in South Africa, and is the closest equivalent in Australia to prairies and steppes on other continents. However, it lacked the expected herds of herbivores.
Research by Alan Newsome (https://www.eoas.info/biogs/P005736b.htm), together with traditional aboriginal knowledge, revealed that kangaroos were not migratory, and failed to graze mitchell grassland even after rain, before the advent of domestic livestock.
Kangaroos are adapted to nutrient-poverty:
Australia is the nutrient-poorest continent. Virtually all of its soils are poor in macronutrients (e.g. phosphorus) or micronutrients (e.g. cobalt), or unbalanced in their combinations of nutrients.
On other continents, herbivores with slow reproduction are marginalised to the most challenging environments. In Australia, they are prevalent in the form of kangaroos, owing to continent-wide nutrient-poverty, compounded by intense wildfire.
Nutrient-poverty, and a lack of succulent plants other than halophytes, explains why kangaroos have a limited ability to exploit arid environments.
All spp. of kangaroos are absent from parts of the Simpson and Great Sandy Deserts (maps by Graeme Caughley, https://www.goodreads.com/book/show/6819726-kangaroos and https://en.wikipedia.org/wiki/Graeme_Caughley and https://www.science.org.au/fellowship/fellows/biographical-memoirs/graeme-james-caughley-1937-1994). Even the spp. of the arid interior need drinking water or shade.
The red kangaroo depends mainly on alluvial woodlands, which retain a few pools evening droughts. The common wallaroo depends on boulder outcrops far from water, which provide shade even at noon.
By contrast, the springbok is extremely adapted for drought, and resides year-round in the Namib Desert, which is more arid (average less than 100 mm of rainfall per year) than the Simpson and Great Sandy Deserts. Although unlikely to reproduce in severe drought, adults of the springbok need neither drinking water nor shade to survive until the next episode of rain. The springbok seldom drinks even when water is available, although it visits pans (salinas, https://journals.co.za/doi/abs/10.10520/AJA03794369_3442) to eat succulent plants and to eat nutrient-rich earth.
Adaptation to nutrient-poverty would explain the paradox of limited reproductive rates of kangaroos, and their absence from large areas. Even the clay soils of mitchell grassland seem too poor to support the succulent plants capable of sustaining herbivores in drought. Unlike ruminants, kangaroos are unknown to eat earth as a nutritional supplement. The possible reason for this that such supplements are unavailable on the ancient, deeply weathered continent of Australia.
Whatever the reason, the contrast between semi-arid Australia and southern Africa was extreme. Although red kangaroo and springbok both prefer palatable, small grasses such as Enneapogon (a genus indigenous to both continents, https://www.inaturalist.org/observations?place_id=any&taxon_id=72116&view=species), the springbok was prolific to a degree unmatched in Australia.
Colonists of South Africa observed irruptions of tens of millions of individuals walking shoulder-to-shoulder, sweeping sheep before them, and denuding the vegetation over areas exceeding 100 km by 10 km.
Sceptical readers will realise that I have understated these 'springbok treks', on referring to eye-witness accounts (see J D Skinner and G N Louw, 1996, Transvaal Museum Monographs, no. 10, https://www.biblio.com/book/springbok-antidorcas-marsupialis-zimmermann-1780-transvaal/d/938429382). The springbok multiplied its populations rapidly, despite being prey to at least 4 spp. of carnivores larger than the dingo, and many other predators.
Irruptions were repeatedly recorded, most recently in 1896 (https://www.tandfonline.com/doi/abs/10.1080/00359199309520276?journalCode=ttrs20 and https://www.jstor.org/stable/29734323).
