https://www.frontiersin.org/articles/10.3389/fevo.2024.1321115/full
Front. Ecol. Evol., 12 March 2024

The influence of multiple variables on bipedal
context in wild chimpanzees: implications for
the evolution of bipedality in hominins

Investigations into the role of selection
in the origin of human bipedalism using ape
models have relied heavily on behavioral
frequency data. However, analysis of video
of wild apes has the advantage of capturing
the details of the entirety of each rare,
brief bipedal bout witnessed, not just the
moment detected in observational studies.
We used video to explore the behavioral
context and effects of several variables
on bipedalism across all ages in wild
forest-dwelling chimpanzees from Ngogo,
Uganda. We found, as in earlier studies,
that adult chimpanzees used bipedalism in
the context of foraging; however, unlike
earlier studies, we found that while
foraging was the predominant behavioral
context during arboreal bipedalism,
terrestrial bipedalism was more varied in
contextual composition. We also found that
these different behavioral contexts of
bipedalism were associated with different
variables. Specifically, foraging was
associated with arboreality, hand
assistance, and adulthood; antagonism was
associated with adulthood, locomotion,
and males; play was associated with
terrestriality and subadulthood; and
travel was associated with locomotion and
females. Given that several variables
influence bipedalism across multiple
behavioral contexts in chimpanzees, it is
likely that the early evolution of human
bipedalism occurred under the influence
of numerous factors. This exploratory
study thus suggests that more
comprehensive models should be used when
reconstructing the transition to
bipedalism from the Last Common Ancestor
of humans and chimpanzees.


"When all bipedal bouts were examined,
feeding/foraging was the most prevalent
behavioral context, occurring in 32.7% of
the 425 bipedal bouts (Table 1). However,
as illustrated in Figure 2, when data are
assessed by age and by substrate (arboreal
vs. terrestrial), behavioral context
varied (Figure 2). Feeding/foraging was
still the most prevalent behavioral
context for both adults and subadults when
arboreal. However, the effect of behavioral
context on terrestrial bipedality was more
varied, with visibility, play and
antagonistic interactions all having
increased occurrence. In the case of
subadults, play was the predominant context
of terrestrial bipedality."

"The video-based approach used in this
study facilitated the collection of the
largest sample of bipedal instances yet
documented for any ape. It is apparent from
this data set that bipedality in chimpanzees
occurs under different behavioral contexts
and is associated with multiple variables.
Arboreal vs. terrestrial substrate use
emerged as a particularly relevant variable
(Figures 2, 3), and it was found that
terrestrial bipedalism occurred under more
contextually diverse circumstances than
did arboreal bipedalism (Figure 2)."

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