DOI: 10.5061/dryad.vhhmgqnvh
title: Effects of intraspecific competition and body mass on diet specialisation in a mammalian scavenger

description: 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.
description: 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). <em>Whisker sample
 collection</em> We sampled 71 individual
 devils between 1<sup>st</sup> August and
 4<sup>th</sup> 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.
 <em>Whisker preparation</em> 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<sup>th</sup> May and 1<sup>st</sup> 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.
 <em>Potential food item collection and
 preparation</em> 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. <em>Stable isotope
 analysis</em> 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
 (δ<sup>15</sup>N and δ<sup>13</sup>C) 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 N<sub>2</sub> (Air) for
 δ<sup>15</sup>N and Vienna Pee Dee Belemnite (VPDB) for
 δ<sup>13</sup>C values (Bond and Hobson
 2012). Instrument drift and measurement error were corrected
 using international standards USGS40
 (δ<sup>15</sup>N<sub>AIR</sub> = -4.52 ± 0.06‰;
 δ<sup>13</sup>C<sub>VPDB-LSVEC</sub> = -26.39 ±
 0.04‰) and USGS41a (δ<sup>15</sup>N<sub>AIR</sub>
 = 47.55 ± 0.15‰;
 δ<sup>13</sup>C<sub>VPDB-LSVEC</sub> = 36.55 ±
 0.08‰) (Qi et al. 2003; Qi
 et al. 2016).
 <em>Specialisation</em> Nitrogen
 and carbon specialisation indices (NSI and CSI) were calculated for each
 devil to describe the variance in δ<sup>15</sup>N and
 δ<sup>13</sup>C 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 &gt; 0.5), while those restricted to less than 20% of the total niche width were classified as specialists (SI &lt; 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.
description: <em>devil_isotope_segments.csv</em>
 <strong>segment_id - </strong>Unique identification
 name for each isotope segment.
 <strong>segment_date -</strong> Date which the
 isotope segment is estimated to represent (dd/mm/yyyy).
 <strong>segment_season -</strong> Season which the
 isotope segment is estimated to represent.
 <strong>date_collected -</strong> Date when the
 whisker was collected from the animal (dd/mm/yyyy).
 <strong>site -</strong> Field site at which the
 animal was caught. A map of all field sites is included as Supplementary
 Figure 1 in the main article. <strong>competition
 - </strong>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. <strong>animal_id -
 </strong>Animal microchip number
 <strong>name - </strong>Animal personal
 name <strong>sex - </strong>Animal sex
 (male or female) <strong>age -
 </strong>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.
 <strong>age_grouped - </strong>Age group of the
 animal, where all one-year-olds are classified as yearlings and all
 individuals aged 2-4 are adults. <strong>mass
 - </strong>Animal mass in kg.
 <strong>head_width - </strong>Animal head width in
 mm.  <strong>whisker_location
 - </strong>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. 
 <strong>est_intradermal_length - </strong>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).  <strong>extradermal_length
 - </strong>Measured extradermal length in mm of the whisker
 collected from the animal.
 <strong>est_total_length - </strong>Estimated total
 length in mm of the whisker (intradermal length + extradermal
 length) <strong>segment_length
 - </strong>Length in mm of the segment cut for stable isotope
 analysis.  <strong>segment_mass
 - </strong>Mass in mg of the segment cut for stable isotope
 analysis. <strong>segment_time
 - </strong>Time in days that the segment cut for stable isotope
 analysis is estimated to represent.
 <strong>unused_buffer_length - </strong>Length in mm
 of the buffer section cut between each isotope segment to ensure
 independence. <strong>total_section_length
 - </strong>Length in mm of the isotope segment + buffer
 section. <strong>total_section_time
 - </strong>Time in days that the isotope segment + buffer section is
 estimated to represent. <strong>delta_n
 - </strong><sup>15</sup>N:<sup>14</sup>N
 value (δ<sup>15</sup>N). <strong>%n -
 </strong>percent of nitrogen in the isotope sample.
 <strong>delta_c
 - </strong><sup>13</sup>C:<sup>12</sup>C
 value (δ<sup>13</sup>C). <strong>%c
 - </strong>percent of carbon in the isotope sample.
 <strong>c_n_ratio - </strong>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).
 <strong>mean_delta_n - </strong>The
 mean δ<sup>15</sup>N value for the three isotope segments
 taken from each individual. <strong>mean_delta_c
 - </strong>The mean δ<sup>13</sup>C value for the three
 isotope segments taken from each individual.
 <strong>var_inw_n - </strong>The variance
 in δ<sup>15</sup>N values for the three isotope segments taken
 from each individuals. Individual niche width (INW).
 <strong>var_inw_c - </strong>The variance
 in δ<sup>13</sup>C values for the three isotope segments taken
 from each individuals. Individual niche width (INW)
 <strong>var_binw_71_n - </strong>The variance
 in δ<sup>15</sup>N values for all 71 individuals.
 Between-individual niche width (BINW).
 <strong>var_binw_71_c - </strong>The variance
 in δ<sup>13</sup>C values for all 71 individuals.
 Between-individual niche width (BINW)
 <strong>spec_index_n - </strong>The individual
 nitrogen specialisation index (SI) value where     <strong>spec_index_c - </strong>The individual carbon specialisation index (SI) value where     <strong>classification_n - </strong>Classification of the individual as a nitrogen specialist, intermediate, or generalist depending on their specialisation index value where individuals with values &lt; 0.2 are specialists, those with values between 0.2 and 0.5 are intermediates and those with values &gt; 0.5 are generalists. <strong>classification_c - </strong>Classification of the individual as a carbon specialist, intermediate, or generalist depending on their specialisation index value where individuals with values &lt; 0.2 are specialists, those with values between 0.2 and 0.5 are intermediates and those with values &gt; 0.5 are generalists.   <em>food_item_isotope_segments.csv</em> <strong>segment_id - </strong>Unique identification name for each isotope segment. <strong>date_collected -</strong> Date when the food item was collected (dd/mm/yyyy). <strong>location -</strong> 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. <strong>individual_id - </strong>Unique identification name for the food item. <strong>class - </strong>Animal class the food item belongs to. <strong>species - </strong>Animal species (scientific name) the food item belongs to. <strong>common name - </strong>Animal species (common name) the food item belongs to. <strong>tissue_type - </strong>Type of tissue that was sampled for isotope analysis. Either whole animal, muscle tissue, or feather. <strong>tissue_location - </strong>Location on the animal where the sample was collected from. <strong>segment_mass - </strong>Mass in mg of the segment collected for stable isotope analysis. <strong>delta_n - </strong><sup>15</sup>N:<sup>14</sup>N value (δ<sup>15</sup>N). <strong>%n - </strong>percent of nitrogen in the isotope sample. <strong>delta_c - </strong><sup>13</sup>C:<sup>12</sup>C value (δ<sup>13</sup>C). <strong>%c - </strong>percent of carbon in the isotope sample. <strong>c_n_ratio - </strong>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). <strong>mean_delta_n - </strong>The mean δ<sup>15</sup>N value for the two isotope segments taken from each individual. <strong>mean_delta_c - </strong>The mean δ<sup>13</sup>C value for the two isotope segments taken from each individual.