Chapter 3: Diet and ecology of extant and fossil wild pigs33
and in higher latitudes in other continents, C 4 grasses are very
rare. In that case, the browser/grazer dichotomy is not detect-
able using stable carbon isotope biogeochemistry.
Additional factors also complicate the use of that C 3 browser/
C 4 grazer dichotomy in paleoecology, even in low-altitude
tropical environments in Africa.
- CAM plants, with their intermediate δ^13 C values, are usu-
ally neglected in paleoecology as they form a negligible
part of the vegetal biomass targeted by African herbivores.
They mostly include xerophytic plants (cactus-like),
succulent plants from arid habitats, and epiphytic plants
(liana-like) from closed forested environments (Sage &
Monson 1999). Those plants are, however, consumed by
suids, for example by common warthogs in dry environ-
ments (Vercammen & Mason 1993) or giant forest hogs in
closed environments. - Sedges (Cyperaceae) are plants usually inhabiting humid
environments, notably numerous kinds of reeds. They use
varied photosynthesis types. Sedges are consumed abun-
dantly by suids (Meijaard et al. 2011). - Aquatic plants display extremely variable δ^13 C values that
rather depend on physical and chemical characteristics
of the aquatic habitat occupied: pH, quantity of dissolved
organic matter, and speed of flow (Sage & Monson 1999).
All extant suids, and notably Potamochoerus, are known to
consume at least some aquatic plants (Meijaard
et al. 2011). - δ^13 C values of plants also show seasonal variations. C 3
plants are strongly influenced by seasonal environmental
variations (water stress, light intensity/brightness) and their
δ^13 C display seasonal variations, usually around 2 ‰ to 3 ‰
(Smedley et al. 1991). Seasonal variations of C 4 plants seem
to be weaker, less than 2 ‰ (Codron et al. 2005). - δ^13 C values also vary within plants among the different
organs. Non-photosynthetic tissues of C 3 plants (ligneous
stems, roots, seeds, and fruits) display higher δ^13 C values
than those of leaves (Cernusak et al. 2009). The difference
between δ^13 C values of roots and leaves was estimated to
be around 1–3 ‰ for several C 3 plants. Similar values are
estimated for the difference between seeds and leaves.
Intraplant variation in C 4 plants is weaker, lower than
1 ‰ (Cernusak et al. 2009). Extant omnivorous suids
consume a lot of roots and fruits of C 3 plants (Meijaard
et al. 2011). - All extant suids consume at least some animal matter
(Meijaard et al. 2011). Animal tissues display very differ-
ent δ^13 C values depending on the organs consumed (up
to 6 ‰ between two organs according to Gearing 1991).
Consumption of carrion by suids can therefore introduce
one supplementary source of variation of δ^13 C values of
their enamel. - It is also necessary to take into account the relative propor-
tions of C 3 and C 4 herbaceous plants within the herbaceous
stratum of the Neogene and Quaternary environments.
Although the modern situation is relatively simple, with
C 4 grasses strongly dominating the C 3 herbaceous plants in
low-altitude tropical environments, the situation was very
likely much more complicated in the past. The timing and
rhythm of the transition from C 3 -dominated grasslands to
C 4 -dominated grasslands are totally unknown. C 4 grasses
are clearly abundant since the late Miocene (e.g. Cerling
et al. 2010; Uno et al. 2011). However, it is possible that
C 4 grasses were not that dominant as today, with a more
mosaic pattern (C 3 and C 4 herbaceous plants) during the
Pliocene and the beginning of Pleistocene (Bonnefille
2010). For example, based on studies of phytoliths,
Rossouw and Scott (2011) showed that the herbaceous
stratum at Laetoli during the late Pliocene was composed
of a combination of C 3 and C 4 herbaceous plants. Similarly,
several studies of paleosols δ^13 C indicate that relatively low
values corresponding to C 3 -rich environments (around
–8 ‰) were common up to 2.7 Ma whereas grass pollens
were abundant (Bonnefille 2010). That would be congruent
with a greater relative abundance of C 3 plants in the herba-
ceous stratum during the Pliocene.
When interpreting stable carbon isotopic data from fos-
sils, it is common to use the dichotomy C 3 browser/C 4 grazer
observed in modern low-altitude tropical environments. This
dichotomy was likely much more blurred during the Neogene
and Quaternary. The tendency by paleontologists to interpret
the percentage of C 3 plants in the diet of fossil mammals as
representing non-herbaceous plants could therefore lead to an
underestimation of the proportion of herbaceous plants con-
sumed (e.g. Harris & Cerling 2002; Cerling et al. 2015).
By adding the value of isotopic enrichment factor εenamel-food
(13.3 ‰) to the modal values of the δ^13 C of C 3 (–27 ‰) and C 4
plants (–12.5 ‰), the threshold values of enamel δ^13 C corre-
sponding to diets composed of 100 per cent of C 3 plants and 100
per cent of C 4 plants are obtained. I consider that a suid feeding
exclusively on C 3 foods has δ^13 C values lower than –13.7 ‰ and
that a suid feeding exclusively on C 4 foods has δ^13 C values higher
than 0.8 ‰.
As the atmospheric δ^13 C has greatly increased since 1850
due to anthropogenic activity (the ‘fossil fuel effect’), it is nec-
essary to apply a correction of 1.4 ‰ when comparing δ^13 C of
fossil specimens to those of extant specimens (Francey et al.
1999). The 100 per cent C 3 and 100 per cent C 4 thresholds of δ^13 C
enamel of fossil suids are, respectively, –12.3 ‰ and +2.2 ‰. In
every figure that combines extant and extinct specimens, δ^13 C
values of extant specimens are shifted by 1.4 ‰ so that they can
be directly compared to those of fossil specimens.
Given the uncertainties linked to the isotopic enrichment
factor between enamel and diet and to the simplified model using
the C 3 browser/C 4 grazer dichotomy, estimations of paleodiets
must be treated with great caution. Although global patterns are
interpretable in terms of differences between C 4 grazers and C 3
plant eaters, it is more difficult to conclude about the nature of
the consumed C 3 plants.
Diagenesis is one of the biggest potential problems when
applying stable isotope biogeochemistry to fossils (Wang &
Cerling 1994). Numerous methodologies were recently devel-
oped to evaluate, or even correct, the effect of diagenesis on
isotopic compositions of fossilized enamel, but they are rarely
applied (e.g. Zazzo et al. 2004)..00512:31:27