The paper is public.
https://www.science.org/doi/epdf/10.1126/sciadv.add9752
Wild chimpanzee behavior suggests that a savanna- mosaic
habitat did not support the emergence of hominin
terrestrial bipedalism
Bipedalism, a defining feature of the human lineage, is
thought to have evolved as forests retreated in the late
Miocene-Pliocene. Chimpanzees living in analogous
habitats to early hominins offer a unique opportunity
to investigate the ecological drivers of bipedalism that
cannot be addressed via the fossil record alone. We
investigated positional behavior and terrestriality in a
savanna-mosaic community of chimpanzees (P an troglodytes
schweinfurthii) in the Issa Valley, Tanzania as the first
test in a living ape of the hypothesis that wooded,
savanna habitats were a catalyst for terrestrial
bipedalism. Contrary to widely accepted hypotheses of
increased terrestriality selecting for habitual bipedalism,
results indicate that trees remained an essential
component of the hominin adaptive niche, with bipedalism
evolving in an arboreal context, likely driven by foraging
strategy.
"However, to date, locomotor studies have focused only on
forest-dwelling chimpanzees (Fig. 1) (36–38), overlooking
critical comparative data about how behaviors, including
bipedalism and degree of terrestriality, vary across
habitats."
"Issa Valley in western Tanzania is characterized as a
savanna-mosaic (14, 35, 39) similar to the
paleoenvironments reconstructed for the early hominins
..."
"Issa Valley in western Tanzania is characterized as a
savanna-mosaic (14, 35, 39) similar to the
paleoenvironments reconstructed for the early hominins
Orrorin, Ardipithecus ramidus, and Australopithecus
afarensis (9–13) and hosts a recently habituated (2018)
chimpanzee community (P. t. schweinfurthii). The area
is a mosaic of miombo woodland with strips of evergreen
riparian forest (classed as open and closed vegetation,
respectively; Fig. 1). Thus, Issa chimpanzees are well
situated for testing the savanna effect on chimpanzee
positional behavior, not only through comparison to
forest-dwelling communities but also by comparing how
individuals adjust their positional behavior across
vegetation types within a savanna-mosaic habitat. We
quantified locomotor and postural behaviors (table S1)
in Issa chimpanzees for 15 consecutive months within
the open miombo woodland and closed riparian forest to
characterize chimpanzee positional behavior in a
savanna-mosaic habitat. Combined with further
comparison to chimpanzees living in forest habitats
(facilitated by similar behavioral data collection
methods across studies), including at Taï (36, 40),
Kibale (37), Bwindi (38), Mahale, and Gombe (36, 41)
(Fig. 1) , we test the hypothesis that an open habitat
will induce greater terrestriality and terrestrial
bipedalism. Our findings offer a unique opportunity
to examine whether these positional behavioral changes
offer support to the hypothesis that a shift from
forest to a more open, savanna-mosaic habitat in the
late Miocene-Pliocene was a catalyst for the emergence
and evolution of bipedalism in early hominins."
"We obtained 13,743 instantaneous observations of
positional bouts from 13 adults (6 females and 7 males),
including a total of 2847 observations of locomotor
bouts (table S2)."
"In keeping with values reported from forest chimpanzee
communities, bipedalism remained a rare (<1% of all
positional behaviors) and primarily arboreal (>80%)
behavior in the Issa community (table S7). However,
despite no apparent effect of habitat (or vegetation)
type on overall frequency or terrestriality of
bipedalism, bipedal locomotion was four times more
frequent at Issa (25%) than at Mahale and Gombe (6%)
(42), and even more so compared with Bwindi chimpanzees
(0.6%; Fig. 4B) (38)."
"Variation in Issa chimpanzee positional behavior
indicates that terrestriality and bipedalism do not
increase within more open habitats and instead offers
support for hominin bipedalism evolving within an
arboreal context."
"Issa chimpanzees remained highly arboreal and did not
use more bipedalism in open vegetation (Figs. 3 and 4A).
Instead, they used more (arboreal) locomotor bipedalism
than forest-dwelling chimpanzees (Fig. 4B), lending
support to bipedal locomotion emerging and evolving as
an arboreal adaptation in early hominins (30, 53)."
"Across the savanna-mosaic habitat of Issa, the
chimpanzees use predominantly two vegetation types:
riparian forest and miombo woodland. The miombo woodland
has half the tree density compared with that of the
forest such that each habitat at Issa can be considered
closed (forest) and open (woodland; Fig. 1 and table S4). "
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