Stable isotope analyses
Workers were dried in an oven at 60˚C for 24-48 hours and then stored in
airtight vials prior to processing. The abdomens of all ants were
removed prior to weighing to avoid the effects of stomach contents on
isotopic signatures (Tillberg et al., 2006). To achieve appropriate
weights for each sample, five to ten workers per sample were pooled and
chopped in glass vials to fine homogeneous powders using small scissors
(Fig 1). Approximately 0.400 mg of each sample was weighed into tin
capsules (Costech Analytical Technologies Inc., Valencia, CA, USA) using
a microbalance (Mettler Toledo, Columbus, OH, USA). All samples were
analyzed at the Texas A&M University Stable Isotopes for Biosphere
Science Laboratory (https://sibs.tamu.edu/) using a Delta V Advantage
Isotope Ratio Mass Spectrometer coupled with a Costech Elemental
Analyzer and Thermo ConFlo IV Universal Interface (Thermo Fisher
Scientific, Waltham, MA, USA). All baseline samples (collected before
mounds were fed the tracer) were analyzed before any post-feeding
samples to ensure that natural abundance values were not influenced by
memory effects from the high levels of 15N in spiked
samples. Nitrogen isotope ratios are presented in \(\delta\) notation:
δ15N (‰) = [(Rsample –
Rstandard)/Rstandard] x
103
where Rsample is the15N/14N ratio of the sample and
Rstandard is the15N/14N ratio of the atmospheric N
standard (Mariotti, 1983; Coplen, 2011). Precision was 0.1‰.
To verify that our isotope tracer methods could detect resource sharing
over 30-m distances, we conducted a proof of concept experiment using a
population of tawny crazy ants (Nylanderia fulva ) near College
Station, Texas. This species forms a single supercolony throughout its
invaded range in North America, in which workers regularly share
collected resources with each other and occupy transitory nests (Eyer et
al., 2018; Wang et al., 2016). Our results from this experiment
confirmed that the isotope tracer is highly successful at detecting
sharing in unicolonial populations at distances that were relevant for
our study (i.e., up to 28.4m from the treated area; Kjeldgaard et al.,unpublished data ).