1. Animals that rely extensively on scavenging rather than hunting must exploit resources that are inherently patchy, dangerous, or subject to competition. Though it may be expected that scavenging species should therefore form opportunistic feeding habits in order to survive, a broad population diet may mask specialisation occurring at an individual level.

2. To test this, we used stable isotope analysis to analyse the degree of specialisation in the diet of the Tasmanian devil, one of few mammalian species to develop adaptations for scavenging.

3. We found that the majority of individuals were dietary specialists, indicating that they fed within a narrow trophic niche despite their varied diet as a population.

4. Even in competitive populations, only small individuals could be classified as true trophic generalists; larger animals in those populations were trophic specialists. In populations with reduced levels of competition, all individuals were capable of being trophic specialists.

5. Heavier individuals showed a greater degree of trophic specialisation, suggesting either that mass is an important driver of diet choice or that trophic specialisation is an efficient foraging strategy allowing greater mass gain.

6. Devils may be unique among scavenging mammals in the extent to which they can specialise their diets, having been released from the competitive pressure of larger carnivores.

Using bulk stable isotope analysis, we aimed first to test our hypothesis that individual Tasmanian devils are capable of dietary specialisation in both trophic level (detected via stable-nitrogen isotope values) and feeding area (detected via stable-carbon isotope values), despite feeding broadly as a population. Second, we tested whether the degree of specialisation varied with inherent characteristics (sex, age, and size) or environmental effects (site and intraspecific competition level).

Whisker sample collection

We sampled 71 individual devils between 1^st August and 4^th October 2018, captured overnight in custom-made PVC pipe traps, across seven study sites in north-western Tasmania. Sex, mass, and head width were recorded, and age was assessed based on the extent of canine eruption and other markers modified from Pemberton (1990). The longest posterior mystacial whisker (A - F) was collected by cutting as close to the skin as possible with scissors, and its position was recorded. Two study sites (Dip River and New Haven) were classified as having lowered intraspecific competition, following severe population decline in 2013 and 2014, respectively, after the introduction of Devil Facial Tumour Disease (DFTD) (C. Hughes, unpublished data). All other sites were classified as having normal competition.

Whisker preparation

Each whisker was measured to the nearest 0.5 mm and an estimated intradermal length was added to calculate its estimated total length (M. Attard, unpublished data). The amount of time each whisker represented was then modelled based on a discrete von Bertalanffy equation (von Bertalanffy 1957; Hall-Aspland et al. 2005; T.L. Rogers et al. 2016) so that the whiskers could be sectioned into lengths representing a period of a few days. Whiskers were cleaned to remove lipids and other debris by washing once in ultrapure water and twice in a 2:1 chloroform:methanol solution for 20 mins each. Three segments of each whisker weighing between 0.2 and 0.5 mg (mean = 0.32 ± 0.08 mg) were cut and placed in tin capsules for analysis. Each isotope segment (n = 213) represented approximately 2.7 ± 1.5 days of growth (and thus assimilated diet), falling between 19^th May and 1^st September 2018. A buffer section of approximately 11.3 ± 2.2 days of growth was also cut between each isotope segment, ensuring a greater degree of independence between samples. In total, the three analysed segments represented approximately one month of isotope data.

Potential food item collection and preparation

Potential Tasmanian devil food items were opportunistically collected, usually as roadkill, from study sites and the rural areas surrounding Smithton, Irishtown, Roger River, and Montagu between 2016 and 2019 and frozen at -20°C. A feather from each bird was collected and soaked once in ultrapure water and twice in a 2:1 chloroform:methanol solution for 20 minutes each. Two samples of the rachis weighing between 0.2 and 0.5 mg were cut and placed into tin capsules for analysis. A sample of muscle tissue (usually from the thigh or torso) was cut from each mammal and snake, rinsed thoroughly twice in ultrapure water, and left to air dry. The outer edges of the muscle sample were then cut using a scalpel to remove as much potentially contaminated tissue as possible. Invertebrates were cleaned by rinsing twice thoroughly in ultrapure water and analysed whole. Mammal, snake, and invertebrate samples were then placed in a freeze-dryer (Alpha 1-4 LSCbasic, CHRIST) for at least 12 hours and ground into a powder using an oscillating mill (MM 200, Retsch). Two samples of powder weighing between 0.2 and 0.5 mg from each individual were taken and placed into tin capsules for analysis.

Stable isotope analysis

All whisker and food item samples were combusted in an elemental analyser (Flash 2000 Organic Elemental Analyser, Thermo Scientific) and the nitrogen and carbon isotope ratios (δ¹⁵N and δ¹³C) were determined using a continuous flow isotope ratio mass spectrometer (Delta V Advantage, Thermo Scientific) at the Bioanalytical Mass Spectrometry Facility, University of New South Wales, Australia. Isotope ratios are expressed using standard delta notation as parts per thousand (‰) and corrected to atmospheric N₂ (Air) for δ¹⁵N and Vienna Pee Dee Belemnite (VPDB) for δ¹³C values (Bond and Hobson 2012). Instrument drift and measurement error were corrected using international standards USGS40 (δ¹⁵N_AIR = -4.52 ± 0.06‰; δ¹³C_VPDB-LSVEC = -26.39 ± 0.04‰) and USGS41a (δ¹⁵N_AIR = 47.55 ± 0.15‰; δ¹³C_VPDB-LSVEC = 36.55 ± 0.08‰) (Qi et al. 2003; Qi et al. 2016).

Specialisation

Nitrogen and carbon specialisation indices (NSI and CSI) were calculated for each devil to describe the variance in δ¹⁵N and δ¹³C values over a one-month period (Roughgarden 1972). The degree of specialisation was calculated using the equation:

where SI is the specialisation index; INW is the individual niche width, the variance of isotopic values along the whisker; and BINW is the between-individual niche width, the total variance of isotopic values within the sampled population. BINW was calculated for the total population across all seven study sites as these sites are not discrete, with individuals frequently traveling between neighbouring sites (C. Hughes, unpublished data). Individuals that occupied over 50% of the total niche width (TNW = INW + BINW) were classified as nitrogen or carbon generalists (SI > 0.5), while those restricted to less than 20% of the total niche width were classified as specialists (SI < 0.2), based upon conventions introduced in the southern elephant seal (Hückstädt et al. 2011). Individuals with a specialisation index between 0.2 and 0.5 were classified as intermediates.

devil_isotope_segments.csv

segment_id - Unique identification name for each isotope segment.

segment_date - Date which the isotope segment is estimated to represent (dd/mm/yyyy).

segment_season - Season which the isotope segment is estimated to represent.

date_collected - Date when the whisker was collected from the animal (dd/mm/yyyy).

site - Field site at which the animal was caught. A map of all field sites is included as Supplementary Figure 1 in the main article.

competition - Classification of the level of intraspecific competition experienced by animals at the site they were captured. Low competition refers to those site that have undergone dramatic population decline due to the arrival of Devil Facial Tumour Disease. Normal competition refers to all other sites.

animal_id - Animal microchip number

name - Animal personal name

sex - Animal sex (male or female)

age - Estimated animal age in years at time of capture. All animals are estimated to have been born on May 1st to reflect the Tasmanian devil breeding season. Age was assessed based on the extent of canine eruption and other markers.

age_grouped - Age group of the animal, where all one-year-olds are classified as yearlings and all individuals aged 2-4 are adults.

mass - Animal mass in kg.

head_width - Animal head width in mm. 

whisker_location - Location of the collected whisker on the face of the animal. A photo indicating these positions in included in the main article as Supplementary Figure 2. 

est_intradermal_length - Estimated intradermal length in mm of the whisker that was left embedded during collection. Intradermal length is calculated uniquely for each whisker position based on previous data collected by Marie Attard (unpublished). 

extradermal_length - Measured extradermal length in mm of the whisker collected from the animal.

est_total_length - Estimated total length in mm of the whisker (intradermal length + extradermal length)

segment_length - Length in mm of the segment cut for stable isotope analysis. 

segment_mass - Mass in mg of the segment cut for stable isotope analysis.

segment_time - Time in days that the segment cut for stable isotope analysis is estimated to represent.

unused_buffer_length - Length in mm of the buffer section cut between each isotope segment to ensure independence.

total_section_length - Length in mm of the isotope segment + buffer section.

total_section_time - Time in days that the isotope segment + buffer section is estimated to represent.

delta_n - ¹⁵N:¹⁴N value (δ¹⁵N).

%n - percent of nitrogen in the isotope sample.

delta_c - ¹³C:¹²C value (δ¹³C).

%c - percent of carbon in the isotope sample.

c_n_ratio - C:N value. Calculated by dividing the percent of carbon by the percent of nitrogen and multiplying by 14/12 (the atomic weights of nitrogen and carbon).

mean_delta_n - The mean δ¹⁵N value for the three isotope segments taken from each individual.

mean_delta_c - The mean δ¹³C value for the three isotope segments taken from each individual.

var_inw_n - The variance in δ¹⁵N values for the three isotope segments taken from each individuals. Individual niche width (INW).

var_inw_c - The variance in δ¹³C values for the three isotope segments taken from each individuals. Individual niche width (INW)

var_binw_71_n - The variance in δ¹⁵N values for all 71 individuals. Between-individual niche width (BINW).

var_binw_71_c - The variance in δ¹³C values for all 71 individuals. Between-individual niche width (BINW)

spec_index_n - The individual nitrogen specialisation index (SI) value where    

spec_index_c - The individual carbon specialisation index (SI) value where    

classification_n - Classification of the individual as a nitrogen specialist, intermediate, or generalist depending on their specialisation index value where individuals with values < 0.2 are specialists, those with values between 0.2 and 0.5 are intermediates and those with values > 0.5 are generalists.

classification_c - Classification of the individual as a carbon specialist, intermediate, or generalist depending on their specialisation index value where individuals with values < 0.2 are specialists, those with values between 0.2 and 0.5 are intermediates and those with values > 0.5 are generalists.

food_item_isotope_segments.csv

segment_id - Unique identification name for each isotope segment.

date_collected - Date when the food item was collected (dd/mm/yyyy).

location - General location of where the food item was collected. Where the food item was collected from a field site, the name of the site is used. A map of all field sites is included as Supplementary Figure 1 in the main article.

individual_id - Unique identification name for the food item.

class - Animal class the food item belongs to.

species - Animal species (scientific name) the food item belongs to.

common name - Animal species (common name) the food item belongs to.

tissue_type - Type of tissue that was sampled for isotope analysis. Either whole animal, muscle tissue, or feather.

tissue_location - Location on the animal where the sample was collected from.

segment_mass - Mass in mg of the segment collected for stable isotope analysis.

delta_n - ¹⁵N:¹⁴N value (δ¹⁵N).

%n - percent of nitrogen in the isotope sample.

delta_c - ¹³C:¹²C value (δ¹³C).

%c - percent of carbon in the isotope sample.

c_n_ratio - C:N value. Calculated by dividing the percent of carbon by the percent of nitrogen and multiplying by 14/12 (the atomic weights of nitrogen and carbon).

mean_delta_n - The mean δ¹⁵N value for the two isotope segments taken from each individual.

mean_delta_c - The mean δ¹³C value for the two isotope segments taken from each individual.