Life history and dynamics of a platypus (Ornithorhynchus anatinus) population: four decades of mark-recapture surveys

From banks to burrows: Habitat preferences and nesting behaviours of platypuses in the Snowy River

Platypuses are a unique freshwater mammal native to eastern Australia. They are semi-aquatic, predominantly nocturnal, and nest in burrows dug into the banks of waterbodies. Quantifying nesting burrow characteristics is challenging due to the species' cryptic nature. We radio-tagged 11 female platypuses during their breeding season (September to November) on the Snowy River, located their resting and nesting burrows by radiotracking, and assessed plasma triglyceride concentration as a biomarker of egg production. We quantified and tested for differences in height and distance from water of resting and nesting burrows, as well as for differences in both canopy and ground cover in the vicinity of resting and nesting burrows in comparison with background control sites in the area. Female platypuses displayed a strong selection for trees and shrubs, placing both their resting and nesting burrows within 5 m of these features. Compared with resting females, nesting females selected to dig nesting burrows higher above the river (nesting 1.98 m ± 0.27 SE vs. resting 1.15 m ± 0.10 SE) that were also further away from water (9.10 m ± 1.08 SE vs. 4.77 m ± 0.53 SE). Camera trap footage captured mice (Mus musculus) and black rats (Rattus rattus) entering two confirmed nesting tunnels on numerous occasions. During the first 3 weeks following the onset of nesting behaviour in two platypuses, rats entered the nesting tunnel a total of eight times and 31 times. Whether this is a previously unconsidered predator by invasive species remains to be evaluated. Synthesis: Riparian vegetation is a critical component of platypus habitat, providing stability for burrows, protection from predators, retaining high bank necessary to avoid inundation of burrows, and providing organic matter for nesting material and for abundant macroinvertebrate communities. Given ongoing declines and habitat degradation across their range, riparian habitat must be conserved and restored to promote breeding and population persistence.

Leptospirosis in the Platypus (Ornithorhynchus anatinus) in Australia: Who Is Infecting Whom?

The platypus (Ornithorhynchus anatinus) is an amphibious, egg-laying mammal of high conservation value that is found only in Australia. The zoonotic bacterium Leptospira interrogans serovar Hardjo was discovered in platypuses in prior studies, but little is known about its epidemiology. Samples in the Platypus Serum Bank were tested in 2023 and the results were combined with historical records. Antibodies against L. interrogans serovar Hardjo were found in 50% of 464 serum samples from 411 platypuses collected from 14 river basins in southeastern Australia between 1981 and 2012; prevalence remained high over three decades in the Shoalhaven River population. Seroprevalence increased with age, suggesting environmental exposure. Individual platypuses had persistent titres, some for six years. Seropositive females lactated, juveniles were recruited into the population, and there were no reports of clinical leptospirosis. Three necropsied platypuses were seropositive and had mild nephritis with leptospires in the renal tubules. The high seroprevalence, persistent titres, lack of disease, mild renal lesions, and renal colonisation suggest the platypus may be a maintenance host. Sympatric cattle had L. interrogans serovar Hardjo titres, but the spatial association with seropositive platypuses was statistically weak. Other mammalian wildlife species and sheep also have L. interrogans serovar Hardjo titres; therefore, a complex ecological network must be considered. A landscape-wide study is recommended to properly assess transmission pathways and confirm who is infecting whom.

Seasonal and geographic variation in packed cell volume and selected serum chemistry of platypuses

Platypuses (Ornithorhynchus anatinus) inhabit the permanent rivers and creeks of eastern Australia, from north Queensland to Tasmania, but are experiencing multiple and synergistic anthropogenic threats. Baseline information of health is vital for effective monitoring of populations but is currently sparse for mainland platypuses. Focusing on seven hematology and serum chemistry metrics as indicators of health and nutrition (packed cell volume (PCV), total protein (TP), albumin, globulin, urea, creatinine, and triglycerides), we investigated their variation across the species' range and across seasons. We analyzed 249 unique samples collected from platypuses in three river catchments in New South Wales and Victoria. Health metrics significantly varied across the populations' range, with platypuses from the most northerly catchment, having lower PCV, and concentrations of albumin and triglycerides and higher levels of globulin, potentially reflecting geographic variation or thermal stress. The Snowy River showed significant seasonal patterns which varied between the sexes and coincided with differential reproductive stressors. Male creatinine and triglyceride levels were significantly lower than females, suggesting that reproduction is energetically more taxing on males. Age specific differences were also found, with juvenile PCV and TP levels significantly lower than adults. Additionally, the commonly used body condition index (tail volume index) was only negatively correlated with urea, and triglyceride levels. A meta-analysis of available literature revealed a significant latitudinal relationship with PCV, TP, albumin, and triglycerides but this was confounded by variation in sampling times and restraint methods. We expand understanding of mainland platypuses, providing reference intervals for PCV and six blood chemistry, while highlighting the importance of considering seasonal variation, to guide future assessments of individual and population condition.

Synergistic effects of a severe drought and fire on platypuses

The strong inter-dependence between terrestrial and freshwater ecosystems, mediated by the character of vegetation and landscapes, can have significant impacts to freshwater species. A changing climate towards hotter and drier climates is already increasing fire frequencies and severity around the world. The platypus (Ornithorhynchus anatinus) is an iconic freshwater Australia species, facing increasing threats since European colonisation and with a distribution which coincides with fire prone areas. While some evidence suggest platypuses are resilience to fires, the combination of severe wildfires and reduced water availability may significantly impact platypus populations. In this short communication we investigated the effects of fire on platypus populations in two rivers, following an extreme drought, comparing burnt and unburnt in adjacent river catchments, with similar habitat and geomorphology. Findings suggests significantly low platypus numbers in burned sites compared to those on the unburnt river, as well as to known densities across the species' range. Whether the fires directly impacted platypuses remains undetermined but the timing of the fires as well as an extreme drought likely impacted recruitment as we did not record any juveniles on both rivers. Platypuses are increasingly under threat from direct and indirect human developments across much of their range and increased frequency and severity of fires and droughts will further strain the viability of platypus populations, particularly in small streams more likely to dry out. Improving the resilience of platypus populations and their freshwater environments to both droughts and fires needs to become a priority.

Long-term movements and activity patterns of platypus on regulated rivers

The platypus is a semi-aquatic mammal, endemic to freshwater habitats of eastern Australia. There are gaps in the understanding of platypus movement behaviour within river systems, including spatial and temporal organization of individuals. We tracked movements of 12 platypuses on the regulated Snowy and Mitta Mitta Rivers for up to 12-months, the longest continuous tracking of platypus using acoustic telemetry. Platypuses remained relatively localized, occupying 0.73-8.45 km of river over 12 months, consistent with previous tracking studies over shorter periods. Males moved further than females, and larger males had higher cumulative movements, suggesting a possible relationship to metabolic requirements. Platypuses moved greater distances on the Mitta Mitta River, possibly associated with impacts of altered flow regimes to their macroinvertebrate diet. Increased movements and diurnal activity during winter were primarily driven by males, possibly attributable to breeding behaviours, rather than increased costs of winter foraging. Evidence for relatively small movements has implications for declining populations, given areas of localised declines are unlikely to be supplemented by migrating platypuses, especially when dispersal is restricted by dam walls. Understanding platypus movement behaviour is pertinent for their conservation, as water resource development and habitat modification continue to reduce connectivity between populations across their distribution.

Burrow use by juvenile platypuses (Ornithorhynchus anatinus) in their natal home range

We determined patterns of burrow use by juvenile platypuses (Ornithorhynchus anatinus) in their natal home range, and evaluated associations between burrows and vegetation. Between March 2015 and March 2017, we captured seven juvenile platypuses along a 3-km stretch of Badger Creek, Victoria and fitted them with radiotransmitters. We recorded the locations of animals in their burrows daily while transmitters were attached (range: 14–132 days). Juveniles used 74 different burrows, with each using 11 ± 2 burrows. Overall, 65% of burrows (48) were used once, 22% (16) were used between 2 and 9 times (moderate-use), and 13% (10) were used frequently (> 10 times). No juveniles dispersed during the monitoring period (14–132 days). Although some association was observed between burrow use and particular vegetation communities, vegetation was not a strong factor driving site selection of burrows. Use of multiple burrows may allow juveniles to avoid competition with conspecifics, reduce exposure to ectoparasites, and develop shelter-seeking behavior. Juvenile platypuses remained in their natal home range, where conditions are likely to be good because they supported recent breeding, while completing their growth and development prior to dispersal.

The platypus: evolutionary history, biology, and an uncertain future

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.

Insights into Platypus Population Structure and History from Whole-Genome Sequencing

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 6.7 M 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 among other samples from the same river, often taken at different times. Taking advantage of a sampled family quartet, we estimated the de novo mutation 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.

Use of implanted acoustic tags to assess platypus movement behaviour across spatial and temporal scales

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.

Female control of reproductive behaviour in the platypus (Ornithorhynchus anatinus), with notes on female competition for mating

Opportunities for studying platypus courtship and mating behaviours in the wild are limited due to the nocturnal and cryptic nature of this species. We report on platypus courtship and mating behaviour from a successful breeding program at Healesville Sanctuary, Victoria, in which platypuses were held as either breeding pairs or trios over seven years. Behaviour was recorded daily on infrared cameras resulting in over 80,000 h of footage that was analysed for activity periods, and courtship and mating behaviours including non-contact and contact courtship, mating and avoidance. Our aims were to describe and quantify courtship and mating interactions between males and females, and to determine if either sex controlled the initiation and continuation of the behaviours. From our observations, we describe a new courtship behaviour, non-contact courtship, which constituted the majority of all mating season interactions between males and females. The time between first and last appearance of a courtship and mating behaviour was 41.0 ± 6.6 days, with the females showing behavioural receptivity for 29.6 ± 5.1 days. Female platypuses used three evasive strategies in relation to approaches by males: avoidance, flight and resistance. Females controlled the duration of 79% of encounters using resistance. For the first time, two females were seen competing with each other over access to the male platypus in their enclosure and for nesting material. Time investment in courtship and mating behaviours was a poor indicator of receptivity and breeding success, and we suggest that breeding failure is more likely to be associated with failure of fertilisation, nest building, embryonic development or incubation. We describe how female platypuses demonstrate evasiveness and control of courtship and mating behaviours, and the importance of providing these opportunities in captivity to promote successful breeding.

Effect of flow on platypus (Ornithorhynchus anatinus) reproduction and related population processes in the upper Shoalhaven River

The platypus (Ornithorhynchus anatinus) occupies a wide range of aquatic habitats, feeding mainly on benthic macroinvertebrates. In this study, we investigated how flow affects platypus reproduction in the unregulated upper Shoalhaven River in rural New South Wales. In a mainly dry period, the population occurred at relatively high density (12.4 animals km–1) and was strongly female-biased (84% of resident animals); mean annual loss and recruitment of resident females were respectively estimated to be 12% and 14%. Percentage lactation was 17–71% (n = 23 years), and annual reproductive success (defined as the mean number of juveniles captured per adult/subadult female from February to April) varied from 0 to 1.5 juveniles female–1 (n = 21 years). A significant positive linear relationship was evident between percentage lactation and antecedent discharge in the five months before breeding (March–July) and a positive curvilinear relationship was evident between percentage lactation and mean litter size. Conversely, reproductive success was compromised by high poststorm discharge in the period when juveniles are confined to a nesting burrow, especially from late November to early January. The relationships identified in our study between flow and reproduction also appear to apply to platypus populations occupying urban streams in Victoria, suggesting that they may be widely relevant to this species.