Sensitivity of The Stripe-Faced Dunnart, Sminthopsis Macroura (Gould 1845), To The Phenyl Pyrazole Insecticide, Fipronil, Toxicological Signs And Implications For Pesticide Risk Assessments In Australia

A lack of toxicity data quantifying responses of Australian native mammals to agricultural pesticides prompted an investigation into the sensitivity of the stripe-faced dunnart, Sminthopsis macroura (Gould 1845) to the insecticide, fipronil (5-amino-3-cyano-1-(2,6-dichloro-4-trifluoromethylphenyl)-4-trifluoromethylsulfinyl pyrazole, CAS No. 120068-37-3). Using the Up-And-Down method for determining acute oral toxicity in mammals, derived by the Organisation for Economic Cooperation and Development (OECD), median lethal dose estimates of 990 mg kg− 1 (95% confidence interval (CI) = 580.7–4770.0 mg kg− 1) and 270.4 mg kg− 1 (95% CI = 0.0 - >20000.0 mg kg− 1) were resolved for male and female S. macroura respectively. The difference between median lethal dose estimates for males and females may have been influenced by the increased age of two female dunnarts. Further modelling of female responses to fipronil doses used the following assumptions: (a) death at 2000 mg kg− 1, (b) survival at 500 mg kg− 1 and (c) a differential response (both survival and death) at 990 mg kg− 1. This modelling revealed median lethal dose estimates for female S. macroura of 669.1 mg kg− 1 (95% CI = 550–990 mg kg− 1; assuming death at 990 mg kg− 1) and 990 mg kg− 1 (95% CI = 544.7–1470 mg kg− 1; assuming survival at 990 mg kg− 1). These median lethal dose estimates are 3–10-fold higher than the only available LD50 value for a similarly sized eutherian mammal, Mus musculus (L. 1758; 94 mg kg− 1) and that available for Rattus norvegicus (Birkenhout 1769; 97 mg kg− 1). Implications for pesticide risk assessments in Australia are discussed.

A baseline terrestrial vertebrate fauna survey of Pullen Pullen; a significant conservation reserve in south-west Queensland

Arid and desert landscapes are characterised as boom-bust systems, and sometimes its fauna can remain cryptic or hidden. This short paper reports the results of baseline monitoring of the terrestrial fauna in the Bush Heritage Pullen Pullen reserve in south-west Queensland, most notable for the presence of the night parrot Pezoporus occidentalis. We conducted two fauna surveys over 22 sites representing Spinifex grasslands, and Mitchell and Chenopod grassland habitats in 2018 and 2019, using pitfall, funnel and Elliott traps, and active searching. A total of eight mammal and 28 reptile species comprising 730 records were documented, as well as eight additional incidental species. Frequently recorded species were Ctentous lateralis, Ctenotus pantherinus, Gehyra versicolor, Tachyglossus aculeatus, Sminthopsis macroura, Pseudomys desertor. Fourteen of the species recorded were highly variable in abundance from one survey to the next. The fauna of the Spinifex grasslands, and Mitchell and Chenopod grassland sites was compositionally very different, and the extent of ground cover or Triodia in the surrounding landscape predicted the abundance of some mammals (i.e. Pseudomys desertor) and reptiles ( Delma nasuta, Ctenotus pulchellus, Cyclodomorphus melanops). These data provide a baseline for future monitoring on this important reserve, and an opportunity to enhance or increase the survey scope via additional sites and a future more adaptive management and question-driven approach.

Daily changes in food availability, but not long-term unpredictability, determine daily torpor-bout occurrences and frequency in stripe-faced dunnarts (Sminthopsis macroura)

Daily torpor, a short-term reduction in body temperature and metabolism, is an energy-saving strategy that has been interpreted as an adaptation to unpredictable resource availability. However, the effect of food-supply variability on torpor, separately from consistent food restriction, remains largely unexamined. In this study, we investigated the effect of unpredictable food availability on torpor in stripe-faced dunnarts (Sminthopsis macroura). After a control period of ad libitum feeding, dunnarts were offered 65% of their average daily ad libitum intake over 31 days, either as a constant restriction (i.e. as equal amount of food offered each day) or as an unpredictable schedule of feed offered, varied daily as 0%, 30%, 60%, 100% or 130% of ad libitum. Both feeding groups had increased torpor-bout occurrences (as a proportion of all dunnarts on a given day) and torpor-bout frequency (average number of bouts each day) when on a restricted diet compared with ad libitum feeding, but torpor frequency did not differ between the consistently restricted and unpredictably restricted groups. Most importantly, torpor occurrence and daily bout frequency by the unpredictably restricted group appeared to change in direct association with the amount of food offered on each day; torpor frequency was higher on days of low food availability. Our data do not support the interpretation that torpor is a response to unpredictable food availability per se, but rather that torpor allowed a rapid adjustment of energy expenditure to manage daily fluctuations in food availability.

Torpor and hypothermia: reversed hysteresis of metabolic rate and body temperature

Regulated torpor and unregulated hypothermia are both characterized by substantially reduced body temperature (Tb) and metabolic rate (MR), but they differ physiologically. Although the remarkable, medically interesting adaptations accompanying torpor (e.g., tolerance for cold and ischemia, absence of reperfusion injury, and disuse atrophy) often do not apply to hypothermia in homeothermic species such as humans, the terms “torpor” and “hypothermia” are often used interchangeably in the literature. To determine how these states differ functionally and to provide a reliable diagnostic tool for differentiating between these two physiologically distinct states, we examined the interrelations between Tb and MR in a mammal ( Sminthopsis macroura) undergoing a bout of torpor with those of the hypothermic response of a similar-sized juvenile rat ( Rattus norvegicus). Our data show that under similar thermal conditions, 1) cooling rates differ substantially (approximately fivefold) between the two states; 2) minimum MR is approximately sevenfold higher during hypothermia than during torpor despite a similar Tb; 3) rapid, endogenously fuelled rewarming occurs in torpor but not hypothermia; and 4) the hysteresis between Tb and MR during warming and cooling proceeds in opposite directions in torpor and hypothermia. We thus demonstrate clear diagnostic physiological differences between these two states that can be used experimentally to confirm whether torpor or hypothermia has occurred. Furthermore, the data can clarify the results of studies investigating the ability of physiological or pharmacological agents to induce torpor. Consequently, we recommend using the terms “torpor” and “hypothermia” in ways that are consistent with the underlying regulatory differences between these two physiological states.

Comparative thermoregulatory physiology of two dunnarts, Sminthopsis macroura and Sminthopsis ooldea (Marsupialia : Dasyuridae)

Metabolic rate and evaporative water loss (EWL) were measured to quantify the thermoregulatory patterns of two dasyurids, the stripe-faced dunnart (Sminthopsis macroura) and the Ooldea dunnart (S. ooldea) during acute exposure to Ta between 10 and 35°C. S. macroura maintained consistent Tb across the Ta range, whereas S. ooldea was more thermolabile. The metabolic rate of both species decreased from Ta = 10°C to BMR at Ta = 30°C. Mass-adjusted BMR at Ta = 30°C was the same for the two species, but there was no common regression of metabolic rate below the thermoneutral zone (TNZ). There was no significant difference between the species in allometrically corrected EWL at Ta = 30°C. Total EWL increased significantly at Ta = 10 and 35°C compared with the TNZ for S. macroura, but was consistent across the Ta range for S. ooldea. At any Ta below the TNZ, S. macroura required more energy per gram of body mass than S. ooldea, and had a higher EWL at the lower critical Ta. By being thermolabile S. ooldea reduced its energetic requirements and water loss at low Ta. The more constant thermoregulatory strategy of S. macroura may allow it to exploit a broad climatic envelope, albeit at the cost of higher energetic and water requirements. Since S. ooldea does not expend as much energy and water on thermoregulation this may be a response to the very low productivity, ‘hyperarid’ conditions of its central Australian distribution.