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Supplementary Text for "Resource partitioning among top predators
in a Miocene food web" by Domingo et al.
Pretreatment of tooth enamel
Enamel pretreatment followed the methods of Koch et al. [1]. The powder was first
treated with 2% NaOCl for 24 hours to remove organic matter. After rinsing the
samples five times with ultrapure water, enamel powder was soaked in a 1M acetic
acid-1M calcium acetate buffer solution for 24 hours to eliminate diagenetic carbonates.
Then after removing the supernatant, the samples were rinsed again five times with
ultrapure water, frozen and freeze-dried overnight.
After treatment, isotopic analyses were performed on a Thermo MAT253 dualinlet isotope-ratio mass spectrometer coupled to a Kiel IV Carbonate Device at the
Stable Isotope Laboratory of the Department of Earth and Environmental Sciences,
University of Michigan.
Motivation for the use of IsoSource
In the last decade, following the increasing use of stable isotope analyses in ecological
and paleoecological studies, several mathematical mixing-model analyses have been
developed. Mixing models permit quantitative assessment of the proportional
contribution of sources to a mixture (i.e., contribution of prey to a predator diet).
IsoError [2] and IsoSource [3] are two of the most widely used mixing models for
trophic analyses. Other models such as IsoConc [4] allow users to compensate for
differences in elemental concentrations among potential sources, or the more recent
MixSIR [5] or SIAR [6] utilize a Bayesian statistical framework. It is not within our
scope to describe all these models here, and we refer the reader to the recent review of
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mixing models by Phillips [7]. In view of the usefulness of IsoError and IsoSource in
previous paleoecological analyses [e.g., 8-11], we favored these two models in our
dietary analysis of the predators from Cerro de los Batallones.
In using IsoError, the data are subject to an analytical constraint: with n isotope
signatures, only n+1 sources can be determined. In our study, where we only have one
isotopic system (carbon isotopes), no more than two diet sources can be resolved [2].
Following Phillips et al. [12], there are different criteria for reducing the overabundance
of sources so that IsoError can be used. The first one involves combining sources with
statistically similar isotopic signatures. In our study, the ANOVA test for the herbivores
(electronic supplementary material, Table S3a) showed that all of them yielded similar
13C values, so a logical separation of end members was not feasible. In figure 2, some
herbivore taxa show affinity for open woodland whereas others would have lived in a
more closed habitat. However, they form a continuum in the woodland habitat spectrum
and, therefore, binning the taxa in two different end members is not possible.
Another way to reducing the number of sources is to lump together comparable
diet sources, e.g., combination of animal food sources, plant food sources, or marine
food sources (see [12] for examples). In our case, herbivores from a similar habitat, we
cannot form logical groupings.
A final criterion was applied in our study. Phillips et al. [12] indicated that a way
to simplify a mixing-model analysis is to delete some sources from consideration. The
deletion of possible food sources must be well reasoned. As indicated below, by
analogy with modern studies on the diet of top predators, the largest herbivores were not
included in the main dietary analysis because they should have contributed minimally to
the diet of all the three carnivorans. Still, the number of diet sources did not reduce to
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two and, therefore, it was not possible to perform IsoError in our dietary analysis of the
carnivorans from Cerro de los Batallones.
The advantage of IsoError over IsoSource is that it incorporates uncertainty
from multiple types of isotopic variation, including the sample sizes of the species
considered. Nevertheless, IsoSource is well suited to cope with more than n+1 diet
sources which is the case in our analysis. Besides, IsoSource has a requirement for
conservation of isotopic mass balance, so it can be used to find multiple combinations
of source proportions that are isotopically feasible solutions and at least place bounds on
possible diet proportions. Our decision to use IsoSource in the dietary behaviour of the
predators of Cerro de los Batallones was corroborated when we used the decision tree
designed by Phillips et al. [12] to discern the best way to proceed in mixing-model
analyses (see figure 4 in [12]).
Role of large herbivores in the diet of top predators
We surveyed the literature on the diet of different modern predators (mainly lion, tiger,
leopard and spotted hyena) with the aim of obtaining data about the role of
megaherbivores (> 1 ton in adult body weight) in their diets. In our study, the giraffe,
two rhinoceroses and a proboscidean are considered megaherbivores. Megaherbivores
rarely constitute a major dietary source for modern carnivorans and as adults are
practically immune to predation [13, 14].
The group hunting strategy of lions allows them to prey on herbivores as large as
adult giraffe. Giraffes are considered to be the largest herbivore that a lion can take as
common prey and their body size is far from the weight of preferred prey species (350
kg) [15].
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There is considerable uncertainty in discerning the social behaviour of ancient
carnivorans. However, the demographic structure of the fossil assemblage of sabretoothed cats from Batallones-1 (dominated by prime adults and absence of cubs) and the
fact that only one modern felid, the lion, has the kind of behaviour that can be
considered social led Antón et al. [16] and Salesa et al. [17] to infer that Machairodus
aphanistus and Promegantereon ogygia had a solitary lifestyle.
The inferred body size of Promegantereon ogygia indicates that this felid could
not have fed systematically on large herbivores. In the case of Machairodus aphanistus,
we believe that the tiger, not the lion, is a better analog in terms of predator-prey
relationships. The tiger displays a solitary lifestyle. Its documented hunting behaviour
indicates a preference for herbivore species between 60 and 250 kg in weight, resulting
in a ratio of predator to preferred prey close to 1:1, similar to that of other solitary felids
[18].
Little can be said regarding the social behaviour of Magericyon anceps.
Amphicyonids features differ from those of living carnivores, so it is not possible to
establish a direct analog for them. Agustí & Antón [19] indicated that the characteristics
of Amphicyon giganteus (robust canines, stout carnassials and body anatomy), an early
to middle Miocene amphicyonid, suggest active hunting, and the agility of a predator
practicing the solitary stalk-pounce hunting style of modern felids. Magericyon anceps
shared the active hunting features of Amphicyon giganteus and this is why a solitary
style of life is a plausible interpretation. Sorkin [20] performed an ecomorphological
analysis on two amphicyonine genera, Amphicyon and Ischyrocyon and concluded that
they were probably solitary predators based on the short distal limb segments of the
fore- and hind-limb compared to the daphoenine Daphoenodon superbus. The same
differences are observed between the solitary tigers and the social lions. In any case,
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and as happens today with lions, a giraffe the size of the Batallones species (median
body size of 902 kg) should have been a minor diet source for Magericyon anceps.
We searched the literature for information regarding the contribution of very
large herbivores to the diets of modern predators under unusual circumstances, i.e.,
preying on sick, young or old individuals, feeding on their carrion. We found no
quantitative data in this regard. Only under atypical circumstances (drought periods),
has it been observed that lions incorporate elephant calves in fairly large quantities
(23% of their kills), as in the case studied by Loveridge et al. [21] in Hwange National
Park (Zimbabwe). However, medium-sized prey still dominate the diet of the lion, a
strategy that may optimize food intake in relation to energy expended in hunting.
Literature reporting systematic consumption of megaherbivore calves by carnivorans is
anecdotal. Thus, feeding on calves of large herbivores should be considered the
exception and not the rule.
Information about the amount of megaherbivore carrion in predator diets is
rarely reported. This kind of information is totally absent in the studies analyzing the
diet of modern felids because scavenging is not a common dietary practice for these
animals. Quantitative data are only occasionally provided in the case of carnivorans
with clear scavenging behaviour such as modern hyaenas. Cooper et al. [22], in their
study of a clan of Crocuta crocuta in Masai Mara National Reserve (Kenya), reported
that megaherbivore carrion makes up 3.5% of the total dietary biomass for the group.
Magericyon anceps was a hypercarnivore with hunting capabilities but was also
able to scavenge [17]. Since it belonged to the extinct Amphicyonidae, there is no
modern counterpart for obtaining an estimate of carrion (and less so megaherbivore
carrion) consumed. Carrion is a relatively unreliable food resource for large carnivores
because its availability is governed by unpredictable environmental conditions such as
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the incidence of drought or diseases within herbivore populations. Thus, the amount of
carrion incorporated by a carnivore at a particular time period and location should not
be used to infer the percentage of carrion incorporated by the same carnivore at another
time and location. This information is even more difficult to extrapolate to species from
a different geological time period.
In summary, the general behaviour of modern large mammalian predators
suggests that the carnivorans of Batallones would have not regularly fed on
megaherbivores either by scavenging or subduing adult, young, old or sick individuals.
Incorporation of these animals (in our study, the giraffe, the two rhinoceroses and the
proboscidean) in the diets of predators should be considered incidental and not a
common practice. For this reason, we did not include these large herbivores in the main
dietary analysis but in a separate analysis, we evaluated whether it was feasible, based
on their 13C values, that they contributed, under particular conditions, to the predator
diets.
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