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The platypus (Ornithorhynchus anatinus) is one of the world’s most evolutionarily distinct mammals, one of five extant species of egg-laying mammals, and the only living species within the family Ornithorhynchidae. Modern platypuses are endemic to eastern mainland Australia, Tasmania, and adjacent King Island, with a small introduced population on Kangaroo Island, South Australia, and are widely distributed in permanent river systems from tropical to alpine environments. Accumulating knowledge and technological advancements have provided insights into many aspects of its evolutionary history and biology but have also raised concern about significant knowledge gaps surrounding distribution, population sizes, and trends. The platypus’ distribution coincides with many of Australia’s major threatening processes, including highly regulated and disrupted rivers, intensive habitat destruction, and fragmentation, and they were extensively hunted for their fur until the early 20th century. Emerging evidence of local population declines and extinctions identifies that ecological thresholds have been crossed in some populations and, if threats are not addressed, the species will continue to decline. In 2016, the IUCN Red Listing for the platypus was elevated to “Near Threatened,” but the platypus remains unlisted on threatened species schedules of any Australian state, apart from South Australia, or nationally. In this synthesis, we review the evolutionary history, genetics, biology, and ecology of this extraordinary mammal and highlight prevailing threats. We also outline future research directions and challenges that need to be met to help conserve the species.

The platypus (Ornithorhynchus anatinus) is an evolutionarily distinct mammal, endemic to Australian freshwaters. Many aspects of its ecology and life-history, including detailed understanding of movements, are poorly known, hampered by its cryptic and mainly nocturnal habits and small numbers. We effectively trialled intraperitoneal implanted acoustic transmitters in nine platypuses in the Severn River (NSW), Australia, as a potential approach for studying movements in this challenging species. We tracked platypus movements over six months, at fine and broad spatial scales, using an array of acoustic sensors. Over six months (March-August 2016), four of five adult platypuses (two females\three males) maintained localized movements (average monthly maximums 0.37 km ± 0.03 sd), while one adult, one sub-adult, and one juvenile (males) moved further: average monthly maxima 1.2 km ± 2.0 sd, 0.9 km ± 0.6 sd, 4.5 km ± 5.9 sd, respectively. The longest recorded movement was by a male adult, covering 11.1 km in three days and travelling a maximum distance of about 13 km between records. Only one implanted animal was not detected immediately after release, indicative of transmission failure rather than an adverse event. High cumulative daily movements (daily 1.9 km ± 0.8 sd) indicated high metabolic requirements, with implications for previous estimates of platypus abundances and carrying capacities, essential for effective conservation. This novel approach offers new avenues to investigate relating to mating, nesting, and intraspecific competition behaviours and their temporal and spatial variation.

The platypus is an egg-laying mammal which, alongside the echidna, occupies a unique place in the mammalian phylogenetic tree. Despite widespread interest in its unusual biology, little is known about its population structure or recent evolutionary history. To provide new insights into the dispersal and demographic history of this iconic species, we sequenced the genomes of 57 platypuses from across the whole species range in eastern mainland Australia and Tasmania. Using a highly-improved reference genome, we called over 6.7M SNPs, providing an informative genetic data set for population analyses. Our results show very strong population structure in the platypus, with our sampling locations corresponding to discrete groupings between which there is no evidence for recent gene flow. Genome-wide data allowed us to establish that 28 of the 57 sampled individuals had at least a third-degree relative amongst other samples from the same river, often taken at different times. Taking advantage of a sampled family quartet, we estimated thede novomutation rate in the platypus at 7.0×10−9/bp/generation (95% CI 4.1×10−9− 1.2×10−8/bp/generation). We estimated effective population sizes of ancestral populations and haplotype sharing between current groupings, and found evidence for bottlenecks and long-term population decline in multiple regions, and early divergence between populations in different regions. This study demonstrates the power of whole-genome sequencing for studying natural populations of an evolutionarily important species.

Monotremes (platypus and echidna) are the descendants of the oldest ancestor of all extant mammals distinguished from other mammals by mode of reproduction. Monotremes lay eggs following a short gestation period and after an even briefer incubation period, altricial hatchlings are nourished over a long lactation period with milk secreted by nipple-less mammary patches located on the female’s abdomen. Milk is the sole source of nutrition and immune protection for the developing young until weaning. Using transcriptome and mass spectrometry analysis of milk cells and milk proteins, respectively, a novel Monotreme Lactation Protein (MLP) was identified as a major secreted protein in milk. We show that platypus and short-beaked echidna MLP genes show significant homology and are unique to monotremes. The MLP transcript was shown to be expressed in a variety of tissues; however, highest expression was observed in milk cells and was expressed constitutively from early to late lactation. Analysis of recombinant MLP showed that it is an N-linked glycosylated protein and biophysical studies predicted that MLP is an amphipathic, α-helical protein, a typical feature of antimicrobial proteins. Functional analysis revealed MLP antibacterial activity against both opportunistic pathogenic Staphylococcus aureus and commensal Enterococcus faecalis bacteria but showed no effect on Escherichia coli, Pseudomonas aeruginosa, Staphylococcus epidermidis, and Salmonella enterica. Our data suggest that MLP is an evolutionarily ancient component of milk-mediated innate immunity absent in other mammals. We propose that MLP evolved specifically in the monotreme lineage supporting the evolution of lactation in these species to provide bacterial protection, at a time when mammals lacked nipples.

Dispersal patterns can have a major impact on the dynamics and viability of populations, and understanding these patterns is crucial to the conservation and management of a species. In this study, patterns of sex-biased dispersal and waterway/overland dispersal are investigated in the endemic Australian platypus, Ornithorhynchus anatinus, a semi-aquatic monotreme. Analyses of over 750 individuals from south-eastern Australia at 13 microsatellite loci and two mitochondrial genes, cytochrome b and cytochrome oxidase subunit II, provide genetic insight into dispersal patterns. For the first time, platypuses of western Victoria are shown to be genetically distinct from other populations of the mainland. Despite distinct morphological differentiation either side of the Great Dividing Range, populations remain genetically similar between coastal and inland areas suggesting gene flow is likely to occur across these ranges. Landscape genetic analyses indicate variability in dispersal patterns between Victorian and Tasmanian platypuses with a greater avoidance of overland travel indicated in Victoria compared to Tasmania. Females appear to remain within their natal area or return to breed, maintaining greater genetic structure in maternally inherited mitochondrial DNA in comparison to nuclear DNA and sharing genetic similarity within a short river distance (i.e. ≤1.4 km). The results of this study provide a valuable spatial framework for the management of wild platypus populations within south-eastern Australia and a baseline for future monitoring of populations that are likely to be impacted by environmental and anthropogenic change.

Knowledge of the life-history and population dynamics of Australia’s iconic and evolutionarily distinct platypus (Ornithorhynchus anatinus) remains poor. We marked-recaptured 812 unique platypuses (total 1,622 captures), over four decades (1973–2014) in the Shoalhaven River, Australia. Strong sex-age differences were observed in life-history, including morphology and longevity. Apparent survival of adult females (Φ = 0.76) were higher than adult males (Φ = 0.57), as in juveniles: females Φ = 0.27, males Φ = 0.13. Females were highly likely to remain in the same pool (adult: P = 0.85, juvenile: P = 0.88), while residency rates were lower for males (adult: P = 0.74, juvenile: P = 0.46). We combined survival, movement and life-histories to develop population viability models and test the impact of a range of life-history parameters. While using estimated apparent survival produced unviable populations (mean population growth rate r = −0.23, extinction within 20 years), considering residency rates to adjust survival estimates, indicated more stable populations (r = 0.004, p = 0.04 of 100-year extinction). Further sensitivity analyses highlighted adult female survival and overall success of dispersal as most affecting viability. Findings provide robust life-history and viability estimates for a difficult study species. These could support developing large-scale population dynamics models required to underpin a much needed national risk assessment for the platypus, already declining in parts of its current distribution.

The field biology of the platypus,Ornithorhynchus anatinus, was first studied by a number of expatriate biologists who visited the Australian colonies to collect specimens in the 1800s. Their work was followed in the early to mid–1900s by a group of resident natural historians and later by an increasing number of academic biologists. All of these workers contributed significantly to the current understanding of the field biology of this unique Australian species. The platypus occupies much the same general distribution as it did prior to European occupation of Australia, except for its loss from the state of South Australia. However, local changes and fragmentation of distribution due to human modification of its habitat are documented. The species currently inhabits eastern Australia from around Cooktown in the north to Tasmania in the south. Although not found in the west–flowing rivers of northern Queensland, it inhabits the upper reaches of rivers flowing to the west and north of the dividing ranges in the south of the state and in New South Wales and Victoria. Its current and historical abundance, however, is less well known and it has probably declined in numbers, although still being considered as common over most of its current range. The species was extensively hunted for its fur until around this turn of this century. The platypus is mostly nocturnal in its foraging activities, being predominantly an opportunistic carnivore of benthic invertebrates. The species is homeothermic, maintaining its low body temperature (32°C), even while foraging for hours in water below 5°C. Its major habitat requirements include both riverine and riparian features which maintain a supply of benthic prey species and consolidated banks into which resting and nesting burrows can be excavated. The species exhibits a single breeding season, with mating occurring in late winter or spring and young first emerging into the water after 3—4 months of nurture by the lactating females in the nesting burrows. Natural history observations, mark and recapture studies and preliminary investigations of population genetics indicate the possibility of resident and transient members of populations and suggest a polygynous mating system. Recent field studies have largely confirmed and extended the work of the early biologists and natural historians.

Blood samples were collected from the bill sinus of nine free-living platypuses (Ornithorhynchus anatinus) within 12 min of capture of each and again after 1 to 12 hr, New South Wales, Australia, 1981 to 1988. In seven animals which were not anesthetized, there was a significant (P < 0.01) fall in lymphocyte count between the two samples. The reduction ranged from 10 to 58% of the initial lymphocyte count and caused a significant reduction in the total white cell count (P < 0.05). Both the neutrophil and the lymphocyte counts increased in two platypuses which were anesthetized with ether prior to collection of the second blood sample. We propose that the peripheral blood lymphocyte count is a simple means of monitoring the stress response of non-anesthetized, newly-captured platypuses and may be a useful adjunct to behavioral observation.

The use of small mesh size gill and drum nets in the inland fishery until around 1950 probably had a considerable impact on platypus populations in most New South Wales rivers. The prohibition of nets in the capture of introduced salmonid species by the Fisheries Act, 1902 and the current fishing regulations have provided a good deal of protection to the species in most water bodies of the State. However, it is probable that the present overlap of commercial (and illegal) fishing operations with the distribution of platypuses in the lower Murray and Murrumbidgee rivers, and their affluent streams, are suppressing platypus numbers. An experiment indicated that the legal drum nets used in the industry are less of a threat to platypuses than the use of gill nets and the illegal nets used by poachers. Banning of gill nets from the industry and stringent law enforcement would largely protect platypus populations in the inland rivers. The current regulations of the eel fishery in New South Wales should minimize...

Using implanted radiotransmitters, we monitored body temperatures in five platypuses ranging freely in the Thredbo River in Australia's southern alps between April and October 1988, where the water gets as cold as any that a platypus is likely to encounter. Activity pattern showed a distinct daily cycle. No evidence of hibernation or even brief periods of torpor was found, all individuals maintaining body temperatures close to 32-degrees-C throughout the winter (mean+/-s.d., 32.08 +/- 0.75-degrees-C, range 29.2-34.6-degrees-C, n = 2237). No differences were found between the means or the variances of body temperatures of animals during day-time rest in stream-bank burrows and those during night-time foraging in winter at temperatures as low as 1.0-degrees-C.

Although platypus spend short periods feeding in captivity, they do spend up to several hours at a time foraging for food in the wild. The $T_{b}$ of platypus netted during feeding periods in the water in various areas of New South Wales varied from a mean value of 32.1 C in the summer to 30.4 C in the winter. Struggling and the destruction of the fur insulation are thought to account for these temperatures being lower than those recorded in captive animals, especially in winter. The $T_{b}$ of the platypus in air over a temperature range of 5-30 C is between 32 and 33 C. Platypus which were confined to water or allowed to enter or leave it at will maintained $T_{b}$ of between 33 and 34 C. The slight elevation of $T_{b}$ from resting levels in air is attributed to muscular activity and the restriction of heat loss by changes in tissue insulation.

Feeding in the platypus Ornithorhynchus anatinus was investigated by identifying material found in the cheek pouches of animals trapped in nets in a study area on the upper Shoalhaven River, N.S.W. Benthic samples were taken in the rapids and pools of the river and the relative abundance of food items available was related to the food found in the platypus cheek pouches. The main food organisms found were insect larvae, notably Trichoptera, Diptera, Coleoptera, Ephemeroptera and Odonata. Subsidiary food items found included freshwater shrimp Paratya australiensis, bivalve mollusks (Sphaerium sp.) and one anuran. The main food items were generally taken in direct proportion to their numerical abundance in the river. Comparisons are made with the food selected by the brown trout Salmotrutta from the area. Estimates are made of the weights of invertebrate food necessary to maintain a platypus in various seasonal conditions.

The platypus, Ornithorhynchus anatinus had a minimum resting metabolism in air of 2.2 W·kg−0.75 at a $T_{a}$ of 25 C. In absolute terms, this metabolism was 8% less than that of marsupials in general, and 35% lower than that of eutherian mammals. The increase in metabolism in response to cold in air was 1.8 times the resting level at a $T_{a}$ of 5 C. However, the immersion in cold water raised this to 3.2 times the resting metabolism (7.0 W·kg−0.75 at a water temperature of 5 C). The resting metabolism of the platypus was approximately 32% higher than that of its terrestrial monotreme relative, the echidna (Tachyglossus aculeatus). Evaporative heat loss (EHL) by the platypus was low, being 0.62 to 1.36 W·kg⁻¹ over a temperature range from 5 to 30 C. The maximum measured water loss at 30 C (probably due to sweating), although lower than the level considered to constitute thermal sweating in large mammals, possibly plays a significant part in cooling the platypus during moderate heat stress and exercise. A...