Humans and Scavengers: The Evolution of Interactions and Ecosystem Services
Excerpt: Diet of early humans: Food provisioning and the onset of cultural services
Around the time of the Pliocene–Pleistocene transition, increasing seasonality in precipitation occurred in African savannas (Cerling et al. 2011a). This forced the australopithecine ancestors of humans to diversify their diet in order to cope with the developing seasonal bottleneck in fruits and other soft plant foods. While hominins of the genus Paranthropus became adapted to exploit durable seeds, roots, and sedges (Cerling et al. 2011b, Klein 2013, Sponheimer et al. 2013), the lineage leading to Homo turned to the meat provided by large vertebrate carcasses to overcome the effects of the increasingly seasonal production of fruits and new plant growth (Foley and Lee 1989, Bunn and Ezzo 1993, Ungar et al. 2006, Klein 2013). Although the relative role of hunting and scavenging by early humans remains controversial (Domínguez-Rodrigo 2002, Ungar et al. 2006, Pickering 2013), many anthropologists contend that the earliest humans obtained animal food largely through confrontational scavenging (also called power scavenging and aggressive scavenging) by driving large carnivores from their kills (figure 1; O’Connell et al. 1988, Bunn and Ezzo 1993, Brantingham 1998, Ragir 2000, Domínguez-Rodrigo and Pickering 2003, Klein 2009, Bickerton and Szathmáry 2011). Indeed, it has been proposed that the emergence of endurance running could have helped early humans to secure sufficient access to the scattered and ephemeral resource that is carrion, although this might have been a later feature facilitating the hunting of live ungulate prey (Bramble and Lieberman 2011).
Interference and resource competition probably accounted for most of the interactions among the earliest humans, vultures, bone-cracking hyenids, and other vertebrate scavengers (Bunn and Ezzo 1993, Owen-Smith 1999, Bickerton and Szathmáry 2011, Bramble and Lieberman 2011). In addition, confrontational scavenging would have exposed early humans to increased risks of injury or death while they were driving away the large carnivores that had killed the carcasses or driving away other fearsome scavengers present at them (Bunn and Ezzo 1993, Bickerton and Szathmáry 2011). But facilitatory interactions could also have been a feature, as it happens in current vertebrate scavenger guilds (Cortés-Avizanda et al. 2012, Pereira et al. 2014). For instance, observations of contemporary hunter–gatherers who actively exploit scavenging opportunities suggest that watching the behavior of vultures and large mammalian carnivores could have helped early humans locate carcasses (O’Connell et al. 1988). Such food provisioning probably represents the first ecosystem service that humans gained from scavenging vertebrates.
Moreover, a major function of the earliest stone tools crafted by early hominins was the processing of large carcasses to yield meat and marrow, a pattern of butchery that extended well into the Pleistocene (de Heinzelin et al. 1999). Competition with other scavengers probably contributed to the refinement of these tools and their use and, therefore, to cultural diversity. In addition, selective pressures associated with confrontational scavenging—specifically, the spatiotemporal unpredictability of carcasses and exposure to predation—probably contributed to the most distinctive features of humans: collaborative cooperation and language development (both of which were used to express where the resource was imagined to be awaiting; Bickerton and Szathmáry 2011). In turn, the improved diet quality due to increasing meat consumption has been related, along with other factors, to the extraordinary brain enlargement within the human lineage (Bramble and Lieberman 2011, Navarrete et al. 2011). Therefore, (confrontational) scavenging helped shape our modern cognitive identity.
Amensalism: any interaction between two individuals or groups of the same or different species in which one organism or group is harmed but the other is unaffected.
Carrion: any type of dead animal tissue.
Coevolution: reciprocal selective pressure that makes the evolution of one taxon partially dependent on the evolution of another (Brantingham 1998).
Commensalism: any interaction between two individuals or groups of the same or different species in which one organism or group benefits without affecting the other.
Competition: any interaction between two individuals or groups of the same or different species that reduces access to a shared resource or set of resources. Competition is direct (interference) if one organism or group affects the ability of another to consume a given limiting resource or indirect (exploitation) if the consumption of a given limiting resource by one organism or group makes the resource unavailable for another.
Ecosystem services: benefits people obtain from ecosystems (MA 2005) or the set of ecosystem functions that are useful to humans (Kremen 2005). These include provisioning (products obtained from ecosystems), regulating (related to the regulation of ecosystem processes), and cultural (nonmaterial benefits) services that directly affect people, as well as the supporting services needed to maintain other services. Provisioning, regulating, and cultural services typically have relatively direct and short-term impacts on people, whereas the impact of supporting services is often indirect or occurs over a very long time period (MA 2005).
Facilitative processes: those processes whose effects on a given organism are beneficial and increase its development or fitness.
Facultative scavenger: an animal that scavenges at variable rates but that can subsist on other food resources in the absence of carrion. All mammalian predators (e.g., jackals, hyenas, and lions in Africa and southern Asia; foxes, raccoons, wolves, and bears in temperate ecosystems), numerous birds of prey (e.g., kites, most large eagles), and corvids (e.g., ravens, crows), as well as other vertebrates (e.g., crocodiles), can be considered, to a greater or lesser extent, facultative scavengers (DeVault et al. 2003, Pereira et al. 2014).
Mutualism: any beneficial and reciprocal interaction between two individuals or groups of different species. This relationship of mutual dependence can be obligate (when a given organism or group cannot survive or reproduce without its mutualistic partner).
Obligate scavenger: a scavenger that relies entirely or near entirely on carrion as food resource. Among Quaternary terrestrial vertebrates, only vultures (both Old and New World species—families Accipitridae and Cathartidae, respectively) are considered obligate scavengers.
Predation: an interaction in which one animal kills and eats all or part of another. Predation can affect prey through the two fundamental mechanisms of direct consumption and capture risk.
Scavenging: an interaction in which one animal eats all or part of a dead animal. Scavenging is active (also called confrontational, aggressive, or power scavenging) when the predator that was responsible for the kill is chased away and most of the meat on the carcass is procured, or it is passive when the bones, which may contain fragments of meat, marrow, and skull contents, are collected.
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