The lesser hairyfooted dunnart (Sminthopsis youngsoni, Dasyuridae) is a generalist marsupial insectivore in arid Australia, but consumes wolf spiders (Lycosa spp., Lycosidae) disproportionately often relative to their availability. Here, we tested whether lycosids have relatively high energy or nutrient contents compared to other invertebrates, and hence whether these aspects of food quality can explain selective predation of lycosids by S.youngsoni. Energy, lipid and protein composition of representatives of 10 arthropod families that are eaten by S. youngsoni in the Simpson Desert were ascertained using microbomb calorimetry, chloroform-methanol extraction and Dumas combustion. Differences between invertebrate groups were assessed using separate analyses of variance (ANOVAs) and appropriate post-hoc tests. These analyses were performed using this data.
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Purpose
The lesser hairyfooted dunnart (Sminthopsis youngsoni) is a common generalist insectivore in arid Australia that consumes wolf spiders (Lycosa spp.) disproportionately often relative to their availability. This study aimed to uncover the underlying mechanisms that drive this observed pattern of selective predation.
Lineage
Determining energy and nutrient content of arthropods : [1] Collection of invertebrates. Study specimens were collected at Main Camp, Ethabuka Reserve in the north-eastern Simpson Desert, Queensland during five field trips in April, July and October 2016, and May and October 2017. Specimens from several arthropod Orders Araneae (including Lycosidae), Blattodea, Coleoptera, Orthoptera, and Scorpiones were collected from the Main Camp site for compositional analysis. Only arthropods known to be eaten by dunnarts were sampled. Arthropods were live-captured in vertebrate pitfall traps set on 16 permanent trapping grids, or through opportunistic diurnal searches to capture grasshoppers and katydids (Caelifera and Tettigoniidae, respectively), using a hat as a net. Hand-sampling was also undertaken over several nights covering an area of ~3 ha around Main Camp, using a spotlight (Fenix TK35) to locate eyeshine, and capture nocturnal arthropods such as arachnids (Lycosidae, Miturgidae). Captured arthropods were immediately placed into individual plastic vials or snap-lock bags to minimise water loss. [2] Preparation for energy and nutrient analyses Given the absence of any detailed identification key for Simpson Desert arthropods, collected specimens were identified to Order or Family (Coleoptera and Araneae). As lycosids were the focus of the study, two distinct morphospecies were also identified based on size (leg-span) and colouration. At the University of Sydney specimens were weighed to an accuracy of 0.0001 g before being dried for 72 h in an oven set to 60 C. Following this, they were weighed a final time before being ground into a powder using a mortar and pestle to get samples as homogeneous as possible. [3] Determining energy content A Phillipson Oxygen Microbomb Calorimeter was used to determine energy content of arthropods. Calorimetric assays require pellets weighing 0.01-0.02 g, so ~0.015 g (mean SE; 0.016 0.0003 g) of dried sample was combusted in 100 % oxygen. This process was repeated twice per sample and averaged for increased accuracy. A benzoic ash standard was used to calibrate the calorimeter every 10th sample and, after combustion, pellets were re-weighed to obtain the ash content and the known energy density of benzoic acid (i.e., 26.393 kJ g-1), was used to calculate the energy content of each arthropod sample in kJ per gram. [4] Nutrient content Total lipids were extracted using a chloroform-methanol-water (1:1:1, by volume) mixture. In summary, this method involved homogenising the tissue in the chloroform-methanol-water mixture before centrifuging and separating the resulting pellet. This process was repeated a second time and the lower chloroform phase was left to evaporate over 24-72 h before being re-weighed. Total lipid content was calculated gravimetrically as the difference in mass before and after the extraction method was applied. This process was repeated three times for each sample, and these triplicates were averaged to obtain a single value per sample.Total arthropod protein content was ascertained via the Dumas combustion technique using a LECO FP628 machine. Samples weighing 0.2 g were placed in the analyser and combusted to produce carcass nitrogen content. Crude protein was then calculated by multiplying the nitrogen value by a standard factor of 6.25.
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