Antecedentes de la UAQ Campus Cadereyta

On the basis of current evidence, it is likely that all or most australopiths were partly arboreal (Senut et al. 2017 and references therein). Falls from a height are a major cause of morbidity in primates (Kraft et al. 2014 and

known cause of infant mortality in, for example, chimpanzees (van Lawick-Goodall 1967). Arboreality should, therefore, select for infants who can cling well, and/or for carriers who can support-carry very effectively.

Carrying clinging infants is also an energetically

expensive behaviour (Altmann and Samuels 1992, Williams et al. 2002, Pontzer and Wrangham 2004, Snowden and Cronin 2007) and carrying non-clingers without devices is likely to be even more expensive. Wall-Scheffler et al. (2007) found that their human subjects used 16% more energy to carry a 6 mo in their arms than in a sling. High levels of adipose tissue in the human infant (see chapter 5), and high human IMMRs (De Silva 2011) may make the costs of human support-carrying particularly high, but Wall-

Scheffler et al.’s results are nonetheless suggestive of high support-carrying costs in all primates.

Carrying non-clingers can also compromise arboreal activities such as effective foraging. Van Lawick-Goodall (1967) described chimpanzee mothers of small, non-clinging infants as “severely handicapped” when moving through trees (1967:336). Nonhuman anthropoids typically support- carry infants only for short periods (see chapter 4),

suggesting that there are multiple reasons why the

behaviour is difficult to maintain, probably including higher energetic costs as infants increase in size and weight.

It is valuable to consider the basic infant transport strategies which would have been available to partly arboreal australopiths. Use of a nonhuman anthropoid analogue suggests these would have been three-fold: effective early clinging on the part of infants; effective, long-term support-carrying of non-clingers; or the arboreal parking of non-clingers (see chapter 4). Use of a

strategies of terrestrial parking and the use of baby-carrying devices (see chapter 5).

Few specific data are available for arboreal support- carrying in nonhuman primates. Some support modes have, however, been noted, such as thigh support of the very young by brachiating gibbons (Palombit 1995, and see Appendix B for captive siamangs), and thigh support or one- handed support of the very young by vertically-climbing chimpanzees (van Lawick-Goodall 1967). Stanley (1992) proposed that climbing australopiths could have transported non-clingers by draping them over one shoulder, or across the nape of the neck. The second behaviour is reported as occasional in capuchins (Nakamichi and Yamada 2009). It is, however, highly likely that australopith infants carried habitually in this way would be at increased risk of falling, and also that the energetic costs of carrying older and heavier infants in this fashion would be high.

Another potential strategy, arboreal parking, is reported for pig-tailed langurs (Simias concolor) (Fuentes and Tenaza 1995) and some muriquis (Strier 1992), but the behaviour is rare among primates (Rosenberg et al. 2004). Infant parking in the above species also occurs only after clinging onset (Fuentes and Tenaza 1995), and only for short periods. In the case of muriquis, it also usually occurs only when baby-sitters are nearby (Strier 1992). The

parking of older, non-clinging australopith infants is hypothetically possible, but it would not mitigate the carrier’s difficulties with ascent and descent.

A H. sapiens analogue suggests the possibility of baby-carrying devices. However, as chapter 4 noted, australopiths may have lacked the technological ability to fashion them (see e.g. Watson et al. 2008). More

range of highly effective devices (reviewed by Ayres 1973, Jelliffe 1975, Small 1998, Schön & Silven 2007) and also climb trees to procure resources in a number of cultures (Kraft et al. 2014), there are no reports of them habitually combining these behaviours. Regular tree-climbing is a predominantly male activity among contemporary humans (Kraft et al. 2014), and one which females tend to avoid (Venkataraman et al. 2013 and references therein), but women do climb to procure resources in some cultures (Kraft et al. 2014 and references therein). Neither sex, however, is reported to climb habitually while carrying infants in devices. This suggests either that humans are unable to climb well in this fashion, that they fear for the safety of carrier or infant, or that both factors are involved.

Contemporary humans in some cultures cache infants terrestrially (Lozoff and Brittenham 1979), which might suggest that arboreal australopiths could have cached non- clingers on the ground. Falk (2004) argued that

(exclusively terrestrial) members of early Homo would have needed to lay their infants down while they foraged,

because their young would not have been able to cling. A major objection to her hypothesis (Sokol and Thompson 2004) is, however, equally relevant for arboreal

australopiths: the behaviour would put the infant at risk. Material culture also plays a crucial role in human caching: infants are, for example, cached in cots within rooms (Lozoff and Brittenham 1979), or on cradle-boards in dwellings, minimizing risks from factors such as weather. Hunter-

gatherer mothers rarely lay their babies down, and typically do so only briefly and while maintaining close (< 1 metre) proximity (Locke 2004 and references therein). It is

possible that australopiths could have handed non-clingers to allomothers before climbing, but this raises the question

of how the allomothers could then have engaged in arboreal activities.

None of the possibilities discussed above can be dismissed. Nonetheless, the most parsimonious way to model infant transport in arboreal australopiths is to assume that infants clung to their carriers, like all other nonhuman anthropoids, and gained the capacity to do so at similarly young ages.

A consensus has not yet been reached about australopith climbing. Debate about this issue centred initially on the morphology of A.L. 288-1 (Au. afarensis), at a time when this hominin’s remains were the most extensive known. The course of that debate, prior to 2000, has been reviewed, from the perspective of an advocate of retained arboreality, by Stern (2000), and, from the perspective of an advocate of little or no arboreality, by Ward (2002). The debate about climbing in Au. afarensis is ongoing (see Ward 2013 and references therein for continued advocacy of minimal arboreality). However, the first opportunity to observe the species’ scapula, which occurred after discovery of the Dikika Child (Alemseged et al. 2006), lent

considerable support to the arboreality hypothesis (Wong 2006), since it indicated strong climbing ability (Green and Alemseged 2012), and the majority view, in recent years, is probably that Au. afarensis was partly arboreal (Stanford 2012). The view has also been strengthened by further analysis of A.L.288-1 which has indicated that this female’s cause of death, based on the pattern of forelimb injuries, was probably falling out of a tree (Kappelman et al. 2016). Climbing has also been inferred for Au. africanus (Berger 2000), for Au. sediba (Williams et al. 2013), and, on the basis of a partial skeleton, for P. boisei (Dominguez-Rodrigo et al. 2013).

Recent studies of H. sapiens’ climbing ability are also extremely relevant, because those who have argued that australopiths were very poor climbers have used the

supposedly minimal climbing abilities of modern humans as an analogue. Two studies have challenged this view.

Venkataraman et al. (2013) tested the hypothesis that habitual climbing by hunter-gatherers should leave skeletal signatures in the ankle. They documented substantial ankle dorsiflexion during some human climbing modes, when these were practised from childhood, but found no

detectable hard tissue signatures, and they discussed the difficulty of using ankle morphology to infer a lack of arboreality in earlier hominins. Kraft et al. (2014) also recently challenged the entrenched view of humans as incompetent climbers. They documented three major, highly effective, unassisted climbing modes used by hunter- gatherers, and discussed the implications of their findings for reconstructing early hominin locomotor behaviours. Both studies have confirmed that human climbing abilities are inferior to those of great apes, but they have also re-cast humans as an analogue for positive, not negative,

australopith climbing abilities.

The above discussion indicates that australopiths are likely to have climbed, and that their infants are likely to have developed clinging abilities at a young age. Further support for this view also comes from the phenotypically plastic australopith trait of phalangeal curvature, which is discussed in 6.5, below, in the context of locomotor

ontogeny.