biology & population ecology
Only a few aspects of kangaroo biology and how populations are regulated are discussed here. Additional reading for the NSW Scientific Committee should include Kangaroos: their ecology and management in the sheep rangelands of Australia (Caughley et al 1987) and Kangaroos: biology of the largest marsupials (Dawson 1995).
2.1 Time to Sexual Maturity
A summary of the data (average in months) for each species is provided below (from Dawson 1995 Table 6.1).
Eastern Grey Kangaroos
In captive populations under optimal conditions, Eastern Grey Kangaroo males have been recorded producing sperm at 2 years of age; in wild populations in arid conditions, maturity is delayed in males to 4 years. Males in the wild do not usually breed until c. 5 years of age; alpha male status (significant contribution to breeding) is attained between 10 and 12 years of age at Fowlers Gap Station, and only 5% or so of animals survive to this age.
Eastern Grey Kangaroos in the wild also start to breed at a much later age than in yard studies. Several yard studies indicate that first breeding may start at about 2 years old, but in detailed field observations at Wallaby Creek in Northern NSW, Robyn Stuart-Dick found that first conception did not occur until 3-3.5 years.
Western Grey Kangaroo
Female Western Greys are given by Caughley et al (1982) as “sexually mature” at 18 months, however he cites Pople & Catling (1974) who gave 24.6 months to “first offspring”.
Red Kangaroos (Dawson 1995)
Breeding of Red Kangaroos in captivity has been recorded from 15-20 months, though in the wild, Frith & Sharman reported that under good conditions …some females were mature at 17 months, but some 4-year-old animals were not yet mature. In another study made during drought at Cunnamulla, QLD, no females were sexually mature before 20-24 months, and many females 4.5-5 years of age had not been sexually active.
Bilton & Croft (2004) describes successful rearing of young to independence is not usual until later years of age. Their first joey is the most likely to die, kangaroos learn how to mother, and have higher rates of rearing success in their later years, to whit:
Female kangaroos are generally most reproductively successful between the ages of 6-15 years…
2.2 Population Movement & Home Ranges
For the purposes of the following discussion and calculations, migration / immigration / emigration are not considered to be significant contributors to population “growth” and “crash” data. Whilst “localised movement of Red Kangaroos, especially in response to patchy rainfall during dry periods, is a reality” (Croft 2004), over large areas these movements are likely to cancel each other out, particularly over time. In addition all species have been shown to be site attached (described by having various "home ranges"). Priddell et al (1988)for example gives the home range of Red Kangaroos as being “less than 10km²” and “seldom moving more than 10km”; indeed Priddell (1998) and Denny (1980) both recorded a high degree of homing (rather than random wandering around after translocation), even to distances of 70km.
2.3 Seasonal conditions
Seasonal conditions are described everywhere as the main factor determining growth rates of kangaroo populations. Age of breeding tends to be younger during good seasons, and the proportion of young which survive is higher during good seasons in semi-arid regions.
2.5 Non-breeding animals
Sub-adults and juveniles comprise between 10-36% of a “Wandoo Woodland population” in Western Australia (Arnold et al 1991), and females rarely breed after c. 12 years.
2.5 Breeding animals
Assume that an age distribution curve provides that 65% of females in a population are of breeding age (c. 3.5 – 12 years old), a conservative estimate provides that in the vicinity of 80% of female Eastern Grey Kangaroos of breeding age will be carrying young (Priestley reported in 2009 that 80% (32 of 40) and 98% (56 of 57) of Eastern Grey does shot at Mount Panorama had young; “91% of adult females over 24kg had pouch young annually” according to Arnold et al 1991; Hacker et al (2004) provides only 66% on average; 94% of adult females shot in Tidbinbilla Nature Reserve in the ACT in 1997 were carrying pouch young (ACT Kangaroo Advisory Committee 1997).
Dawson (1995) and his colleagues reported that only 50% of Euros visiting a waterhole during a dry summer were carrying pouch young, compared to 86% of Red Kangaroos. They surmised that the Euro’s reproduction was more sensitive to poor conditions than was the Red Kangaroo.
Once breeding, Western Grey Kangaroo does produce one joey annually. By 12 years of age few females continued to produce offspring (Arnold et al 1991); this is presumed for the purposes of this nomination to be similar for Eastern Grey Kangaroos.
2.6 Reproduction rates
Red Kangaroos and Wallaroos are given as breeding “continuously”; this is taken to mean these species are theoretically capable of producing one pouch young (based on first emergence) every 185 days and 200 days respectively. This can be interpreted as roughly 2 young per annum for these species (under optimal conditions), by caring for at any one time one blastocyst in embryonic diapause (in some species), one pouch young, and one young at foot (weaning in Red Kangaroos at around 360 days and in Wallaroos at approximately 375 days).
Note that holding a blastocyst in embryonic diapause does not give a major reproductive advantage – a blastocyst is comprised of only 70-100 cells, and is only 0.25mm in diameter (Dawson 1995). With rearing of young to full emergence taking c. 200-360 days, skipping one oestrus cycle and a little bit of embryonic development will not dramatically improve reproductive rates in the macropodidae.
Also embryonic diapause is rare in Eastern Grey Kangaroos, and is unknown in Western Grey Kangaroos (Dawson 1995).
From Pople & Grigg 1999, Red Kangaroo “calculated” growth rates are given as 0.67 per annum, rather than the “theoretical maximum of 0.25 for a population with stable age distribution and balanced sex ratio”. “Numerical response models” “have suggested maximum growth rates of increase for Red Kangaroos as 0.33-0.58 in western NSW, and 0.38-0.92 in South Australia”. Caughley et al 1984 provides a maximum annual rate of increase of 45% per annum in Red Kangaroos, however under “average annual rainfall conditions”, populations more usually increased at a rate of 30-35% per annum.A comparison with feral goats: from Watts (1984):
The combination of an early initial breeding age, short gestation, postpartum oestrus, high breeding rate and twinning allows goat populations to achieve annual growth rates of 10-35%
Population growth rates for goats (under optimal conditions) are given more recently as being 0.5 (DEWHA 2008). Cleon & Kimberling (2010) provide that female goats are sexually mature at 8-10 months, gestation is 150 days, there is frequent twinning and even triplets, and the possibility of 2 reproductive cycles a year. Weaning (and effective independence) is at 8-12 weeks (though they still hang around with “mum” for a while), and M/F sex ratios are 0.44 (Tadeuz et al 2003). The Molonglo Catchment Group (undated) provides that feral goat populations can double every 1.6 years.
Another comparison: Sows can breed from the age of 7-12 months, they can produce 2 litters of 5-10 piglets in a period of 12-15 months. Feral pigs are given as having potential for populations to increase at a rate of 86% per annum (Invasive Animals CRA 2011):
This high potential reproduction rate is closer to that of rabbits than other large herbivores…
Interestingly, juvenile mortality is given for the first year of life as being similar between many mammal species, first discussed by Caughley (1967). Jezierski (1977) and Briedermann (1990) refine juvenile mortality for the wild boar (50 - 75%), however in Queensland mortality in juvenile feral pigs can be as low as 10-15% during good conditions, or up to 100% during drought (McGaw & Mitchell 1998). Arnold (1991) provides 73% mortality in the Western Grey Kangaroo; the ACT Kangaroo Advisory Committee (1997) suggests mortality in juvenile Eastern Grey Kangaroo in the ACT in “over-populated” parks and reserves was “high” (though no quantitative work was undertaken); up to 100% mortality has been reported in Western Grey and Red Kangaroo juveniles, depending on conditions (Shepherd 1987).
How do Pople and Grigg justify that Red Kangaroos, at one young per year (to weaning), are capable of reproducing at 67% per annum, or even up to 92% per annum, even exceeding the pig's capacity to reproduce, with litters of up to 10 piglets up to twice a year? This is clearly absurd, and suggests a highly biased pro-harvest stance on the part of “researchers” who suggest such high reproductive rates. A paper describing the macropods as having a biologically impossible reproduction rate creates an impression of kangaroos as a super-organism, perhaps so that the “harvest” seems more reasonable. Who does McLeod, the Department of Primary Industries Vertebrate Pest “researcher” refer to, when he suggests the harvest is humane and sustainable at 17% per annum? Pople and Grigg.
Western Grey Kangaroos are given as having twins in 0.36% of births by Norbury 1987, which is “consistent with the very low occurrence of twins in the macropodid marsupials observed by Inns (1980)”. Nonetheless after good rain (and floods) in 2010, Ray Borda (President of the Kangaroo Industry Association of Australia (KIAA) – the peak lobby group promoting the kangaroo meat industry) came out with the preposterous claim that “…every doe is reported to have twins, the [population] growth rate of all kangaroos, with feed and water around, is twice as fast as it would normally be” (ABC Rural 13/12/2010).
In 2011 John Kelly (CEO of the KIAA) was again claiming populations were “increasing dramatically” (interviewed by ABC’s Radio National PM 5.8.2011) in response to better conditions, in an attempt to put pressure on the Australian Government to assist with re-opening the Russian meat market (the Federal Government is working on it). Steven Tully (AgForce) suggests the population will“explode on the back of the big wet”, and that Queenslands kangaroo population of 14M in 2010 will “double by 2013” (that would be a 100% increase in 2 years, or 0.5 per annum).
Pople and Griggs (1999) “calculated” (Red Kangaroo) maximum population growth rate of 0.67 represents either wild optimism, a significant discrepancy (that somehow kangaroos can out-breed goats), or an as-yet unobserved super-ability within the macropodidae to reproduce rapidly and successfully under exceptional conditions. If we consider the high juvenile mortality rates given for all of the macropods, “calculated” population growth rates to 0.92 are farcical; further these sort of “calculations” are irrelevant when discussing populations in the wild.
From Arnold et al (1991), after shooting 105 of 200 Western Grey Kangaroos making up the Wandoo Woodland population, within 6 years the population had recovered to 158 animals. This represents an average increase of 10.5 animals per annum, the growth rate averaged only 9% (or 0.09).
The population has now apparently “stabilised” at around the original 200 animals, and does not seem to fluctuate wildly, as populations in semi-arid zones are reported to do. In the case of the Wandoo Woodland population it “appeared to be relatively stable” after another 15 years.
Incredibly no-one seems to have done an equivalent study on Red Kangaroos, Wallaroos / Euros or Eastern Grey Kangaroos. The author has been unable to find observed population growth rates for these species, other than what can be inferred from the state-wide monitoring data (OEH 2010), which contains inconsistencies which render this data unreliable for these sort of calculations (see below).
Given comparable ages to sexual maturity between the species (for does), and assuming similar rates of juvenile mortality, a simplistic comparison of time-to-weaning (from Dawson 1995) has been used by this nomination to suggest an Eastern Grey Kangaroo population growth rate of 10%, a Red Kangaroo population growth rate of 13.5%, a Wallaroo population growth rate of 14%, and a Euro population growth rate of 12%.
Why only 10-14% form population growth rates? One of the main factors that influences population growth rate is juvenile mortality; Arnold et al 1991 provides that only 27% of (Western Grey) young survive their first year out of the pouch in the Wandoo Woodland population, and estimates that mortality in (Western Grey Kangaroo) adults is 5% per annum.Banks et al (2000) provides that 50% of emergent young are taken by foxes.Life expectancy is given as 9-18 years (Dawson 1995), and that “poor nutrition” is the principle factor causing death.Adult mortality in Wallaroos is given as 4.55-25.81% / annum (Clancy & Croft 1992).
2.8 M/F Sex Ratios
Eastern Grey Kangaroo sex ratio is given as “close to parity” (Coulson 1982 citing Kirkpatrick 1965 and Poole 1973).
Arnold et al (1991) studied Western Grey Kangaroos in WA, and provided a M/F ratio of 1:2 (ie 46 males to every 100 females).
The base chance of having a male or female young has been reported as depending on the “fitness and age of the dam” in Red Kangaroos (Bilton & Croft 2004); older female Red Kangaroos have been found to have higher proportions of male young.
ACT Parks and Conservation Service (1997) identified a M/F ratio of 1:2 in Eastern Grey Kangaroos at Tidbinbilla Nature Reserve.
Red Kangaroo M/F sex ratio is given as 0.46 (Pople & Grigg 1999).
Hacker et al (2004) provides M/F ratios for Western Grey Kangaroo, Eastern Grey Kangaroo and Red Kangaroo:
For the purposes of this nomination, pouch young sex ratios from Hacker et al (2004), being roughly parity (which was also concluded by Coulson, Kirkpatrick, Poole and Pople / Grigg above) are assumed to indicate the “natural” (pre-European) sex ratio in the various species of large macropod subject to this nomination.
This nomination also presumes roughly equivalent survivorship to adulthood between the sexes, however Dawson (1995) identifies that both Eastern and Western Grey Kangaroos seem to experience higher mortality in the males, with only 5% surviving to full maturity (c. 60kg) in the wild.
Shooting traditionally targeted male animals, and it is suggested that outlying M/F sex ratios above (Western Grey Kangaroo according to Arnold et al 1991, and Red Kangaroo (shot sample) according to Hacker et al 2004), may be artifacts of male bias in former culling / shooting programs.
Sex ratios are important in considering population growth rates – obviously if more young are female, a species is more able to recover from losses. However parity means that if a doe first conceives at 3 years of age, and has its first young to independence by 4.5 years of age (this assumes success in rearing, and ignores the very high reported juvenile mortality rates), 50% of the time it takes a minimum of 4.5 years for a doe to replace itself with a female offspring. If the doe has a male joey, it will be another 12 months before there is another 50/50 chance she will replace herself with another female.
2.9 Age Structure
From Driessen (1992), a paper on the effects of hunting and rainfall on populations of Bennetts Wallaby and Tasmanian Pademelon: …hunting had a direct effect on age structure of the two wallaby species
Given that the harvest targets the largest kangaroos, it is reasonable to assume that this likewise affects the age distribution of the large macropods. How do populations respond when the alpha males, and the mature females, with more experience and success at raising joeys, are shot?
Consider a hypothetical scenario:
If an Eastern Grey Kangaroo doe only raises its first joey to the pouch-emergent stage by 3.5 years of age, and finishes breeding at around 12 years of age, she is only likely to produce 8 young in her lifetime.
If 75% of joeys are likely to die, this leaves only 2 young to survive to adulthood. If the male female sex ratio is parity, only one surviving joey will be a doe, and on average the original female will only effectively replace herself once in her lifetime.
The author’s lecturer in population ecology, highly regarded wildlife researcher Dr Johannes Bauer, suggested that these sort of generalities with regard to reproduction are rarely replicated in the wild, and that application of averages (as in the hypothetical scenario above) is not necessarily valid (pers comm 2011). However this possible scenario must remain a matter of concern to the cautious manager.
Regardless of which, given the rates at which kangaroos are being destroyed, relatively “low” reproduction rates provide that it is no surprise that there are vast regions in NSW which seem to be almost completely devoid of kangaroos.