Part I: Evolution, Taxonomy, and Domestication32
moderately elongated with main cusps that are more numerous
than in Potamochoerus. The third molar crowns are also moder-
ately heightened. Those features are related to the versatile her-
bivorous diet of Hylochoerus, comprising grasses and other kinds
of plants (e.g. d’Huart 1978).
In Phacochoerus, the occlusal surfaces of the molars are flat.
The narrow, elongated, and very high-crowned third molars are
composed of numerous cusps, all tightly packed together within
a cover of cement. Those features are related to the specialized
diet of Phacochoerus, comprising almost only grass (both aerial
and underground parts) (Vercammen & Mason 1993).
The morphologies and ecologies observed in extant African
suids are therefore used as a crude guide to reconstruct the diets
of extinct relatives. The third molars, frequently fossilized, are
especially useful to classify the extinct suids into omnivorous
morphotypes (similar to Potamochoerus) and herbivorous
morphotypes (similar to Hylochoerus and Phacochoerus). This
approach was the most commonly used before the develop-
ment of other proxies such as stable carbon isotopes (Cooke &
Wilkinson 1978; Harris & White 1979; Kullmer 1999).Stable Carbon Isotopes
Stable isotope biogeochemistry uses relative abundances of dif-
ferent isotopes of the same element preserved in the animal tis-
sues to reconstruct its ecology and environments (e.g. Cerling
et al. 2010). I give an overview of the use of stable carbon iso-
topes in paleoecology and its limitations before presenting the
results of the literature review.
Stable isotopes are non-radioactive atoms of the same chem-
ical element that differ by their number of neutrons. Carbon
has two stable isotopes, the ‘light’^12 C (six protons, six neutrons)
and the ‘heavy’^13 C (six protons, seven neutrons). Due to their
slightly different mass, light and heavy isotopes behave dif-
ferently in their geochemical cycles (isotopic fractionation).
During a change of physical state, one of the isotopes reacts less
than the other, which gives different isotopic ratios in the prod-
ucts. These phenomena of fractionation are used to track differ-
ent elements in the cycles.
The tiny quantities of the different stable isotopes are meas-
ured using an isotope-ratio mass spectrometer (IRMS) and
expressed as parts per thousand (‰) relative to a standard
defined internationally. δ^13 C is reported as a difference com-
pared to the international standard V-PDB (‘Vienna Pee Dee
Belemnite’) and is calculated as follows:δ^13 C = [(RSAMPLE/RSTANDARD − 1 )] * 1000
with R = ratio of heavy to light isotope.
Dental enamel, due to its strong mineralization and low
porosity, is less prone to chemical exchanges during diagenesis
(Wang & Cerling 1994), and it is therefore the favored target for
isotopic analyses of fossils. Enamel is not reworked during the
lifetime of one individual and environmental variations (cli-
mate, diet) are recorded during mineralization. Enamel miner-
alization is a complex, discontinuous process that takes place
in several stages and involves several successive mineralization
fronts running in different directions. Thus, no matter the size
of the enamel sample, the resulting isotopic ratio representsan average based on a time period (Balasse 2002). That can be
important when comparing data coming from different sam-
pling strategies.
δ^13 C recorded in animal enamel reflects that of the food
items that are mainly consumed during the dental growth. The
isotopic enrichment factor ε expresses the isotopic fractionation
and the difference between the two phases (enamel and food).
It is defined as follows (see Cerling & Harris 1999 and Passey
et al. 2005):εenamel-food = ((( 1000 + δ^13 Cenamel)/( 1000 + δ^13 Cfood)) − 1 ) * 1000.Knowing εenamel-food, it is possible to estimate δ^13 Cfood from
δ^13 Cenamel. That variable was estimated through experimenta-
tions. Most studies indicate a value of ε close to 14 ‰ for large
herbivores (Cerling & Harris 1999; Passey et al. 2005; Warinner
& Tuross 2009). The ε factor varies according to the digestive
physiology of the animal (Passey et al. 2005), the digestibility
of consumed food items (Codron et al. 2011), and even other
factors not well understood that could be linked to stress or the
well-being of the animal.
In suids, the results indicate ε values ranging from 12.3 ‰
and 13.3 ‰ for Passey et al. (2005) depending on the digestibil-
ity of the food items (respectively, pure C 3 versus pure C 4 ), and
between 14.4 ‰ and 14.6 ‰ for Warinner and Tuross (2009) for
a similar diet.
For that synthesis, I use as ε the average between the mini-
mal and maximal values from Passey et al. (2005) and Warinner
and Tuross (2009); that is, 13.3 ‰. Isotopic composition of
consumed food items can be estimated from that of enamel as
follows:δ^13 Cfood = 1000 + δ^13 Cenamel * ( 1000 /(εenamel-food + 1000 )).δ^13 C of terrestrial plants consumed by mammals depends
mostly on the photosynthesis type used (C 3 , C 4 , and CAM, or
Crassulacean Acid Metabolism). Those types of plants do not
fractionate the stable carbon isotopes of the atmospheric CO 2 in
the same way and their tissues display different δ^13 C values. C 3
plants display δ^13 C values ranging from –38 ‰ to –22 ‰ (modal
value –27 ‰), whereas C 4 plants display δ^13 C values ranging
from –19 ‰ to –9 ‰ (modal value –12.5 ‰). CAM plants display
intermediate δ^13 C values (Farquhar et al. 1989; Kohn 2010).
In modern tropical and subtropical Africa, in low-altitude
areas, all trees as well as most shrubs and bushes (dicotyledons)
are C 3 plants, whereas 90 per cent of grasses (monocotyledons)
are C 4 plants (Livingstone & Clayton 1980; Sage & Monson
1999). Those two types of plants correspond broadly to the
plants consumed by browsers and grazers. As δ^13 C values of C 3
and C 4 plants do not overlap, it is possible to distinguish brows-
ers from grazers in tropical environments from Africa based on
enamel δ^13 C values. This rationale is used in most studies deal-
ing with the paleoecology of Neogene and Quaternary African
mammals (see syntheses by Cerling et al. 2010, 2015).
At low altitudes in tropical environments, C 3 grasses are rare
and limited to humid and shady habitats. At higher altitudes, C 3
grasses are abundant. Above 1300 m, their abundance increases
and they are the only grasses present above 4000 m (Livingstone
& Clayton 1980). Similarly, in non-tropical African habitats,.00512:31:27