Platypus (Ornithorhynchus anatinus) body size, condition and population structure in Tasmanian river catchments: variability and potential mucormycosis impacts

Abstract

Context Despite widespread interest in platypus (Ornithorhynchus anatinus) conservation, it is unclear how their fundamental morphometric and demographic characteristics differ over a range of scales. This hampers impact assessments and understanding of platypus ecology. Although the ulcerative fungal disease mucormycosis has infected platypuses in Tasmania for three decades, its population level impacts and conservation significance remain unknown. Aims This study examined morphometric and demographic patterns in Tasmanian platypuses to provide a basis for investigating impacts of mucormycosis or other anthropogenic disturbances. It also sought to identify important spatial scales of natural variability and the magnitude of seasonal variation in platypus body size, condition and population structure. The hypothesis of higher mucormycosis prevalence and mortality in adult males was also investigated. Methods Extensive live-trapping surveys were conducted from January 2008 to July 2009 in 75 streams and 18 river catchments across Tasmania including King Island. The sex, age, body size, tail volume index, health and moult condition of 195 individuals were assessed, and population age and sex structures characterised. Sampling focussed on assessing variability within and between river catchments and compared populations in river catchments with contrasting disease status. Key results Differences in platypus morphometrics within and between catchments and seasonal moulting patterns were detected. Adult males had higher fat stores than adult females, especially during winter. This study also provided the first evidence of population level consequences of disease in platypuses. The demographic group most commonly affected by mucormycosis was confirmed to be adult males. Differences in male age structure among catchments of varying disease status were consistent with the hypothesis of higher adult male mortality rates and turnover in currently affected catchments. Conclusions More than 25 years after mucormycosis was first detected in Tasmanian platypuses, the disease continues to play a low-level, ongoing role in affected populations. Implications The present study provides the first systematic multi-scale spatial investigation of platypus mucormycosis, which contributes to unravelling the epidemiology of the disease and detecting its impacts. By identifying the magnitude and important scales of morphometric and demographic differences in Tasmanian platypuses the study also assists researchers choose comparable demographic groups and spatial scales for meaningful comparisons in future impact studies.

Investigation Into Individual Health and Exposure to Infectious Agents of Platypuses (Ornithorhynchus Anatinus) in Two River Catchments in Northwest Tasmania

“…We found no evidence of mucormycosis in the study animals but found a new differential diagnosis for this disease. Platypuses are sexually dimorphic in body length and body mass and are generally larger in Tasmania (Grant and Temple-Smith 1983;Connolly and Obendorf 1998;Munks et al 1998;Bethge 2002;Koch et al 2006;Gust and Griffiths 2011). Our findings are consistent with these observations.…”

“…Mean body masses in our study were near the middle of these ranges and were very close to the mean values of 1.3 kg for females and 2.1 kg for males reported by Macgregor (2008) in the Inglis River Catchment. Gust and Griffiths (2011) reported mean body length values for platypuses from different river systems as 42-48 cm for females and 48-56 cm for males. Our results were close to the upper ends of these ranges and were higher than the values of 43 cm and 49 cm, respectively, reported by Macgregor (2008), also in the Inglis River Catchment.…”

“…Our findings are consistent with these observations. Studies of Tasmanian platypuses have shown mean mass in different river systems in the ranges 0.91-1.65 kg and 1.47-2.5 kg for females and males, respectively (Connolly and Obendorf 1998;Stewart 2001;Bethge 2002;Koch et al 2006;Gust and Griffiths 2011). Mean body masses in our study were near the middle of these ranges and were very close to the mean values of 1.3 kg for females and 2.1 kg for males reported by Macgregor (2008) in the Inglis River Catchment.…”

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

“…A clinal variation of weights and lengths in the platypuses, increasing from low to higher latitudes, is well documented 16 23 52 54 . Platypuses from north Queensland are the smallest (mean lengths and weights for adult females and males): 37.7 cm ± 3.1sd/737 g ± 86sd and 43.6 cm ± 3.2sd/1118 g ± 197sd, respectively 16 , while those in Tasmania are the largest: 44.9 cm ± 4.0sd/1232 g ± 23sd and 54.8 cm ± 4.0sd/2154 g ± 33sd 23 . Sizes of platypuses in our upper Shoalhaven River population, in the lower third of its north-south distribution (35.5°S), fitted the cline of weight and length measures ( Table 1 ).…”

“…The platypus is “of least concern”, under current IUCN red listing 20 , but were identified recently as ‘near threatened’, given localised declines and extinctions in populations, particularly in the state of Victoria 21 . Despite increasing understanding, many aspects of the species’ biology, including its population dynamics remain relatively poor, reliant on few long-term studies investigating densities, reproduction, age structure and survival 22 23 24 . There are generally low recapture rates 22 25 , making reliable estimates of population sizes difficult.…”