G. Gergely, H. Bekkering, and I. Király (2002) showed that 14-month-old infants imitate rationally, copying an adult's unusual action more often when it was freely chosen than when it was forced by some constraint. This suggests that infants understand others' intentions as rational choices of action plans. It is important to test whether apes also understand others' intentions in this way. In ...

Here, we show how the mechanical properties of a thick-shelled tropical seed are adapted to permit them to germinate while preventing their predation. The seed has evolved a complex heterogeneous microstructure resulting in hardness, stiffness and fracture toughness values that place the structure at the intersection of these competing selective constraints. Analyses of different damage mechani...

A literature search revealed that for over 100 years there has been a consensus that the ligament of the head of the femur (LHF) is absent in the orangutan and elephant. A dissection of the hip joints of an adult orangutan and an adult Indian elephant exposed, in each joint, a robust LHF that is functionally important. These LHFs are easily overlooked during a cursory examination of the hip joi...

The nightly construction of a 'nest' or sleeping platform is a behavior that has been observed in every wild great ape population studied, yet in captivity, few analyses have been performed on sleep related behavior. Here, we report on such behavior in three female and two male captive orangutans (Pongo spp.), in a natural light setting, at the Indianapolis Zoo. Behavioral samples were generate...

Over the past 20 years several studies have attempted to clarify orangutan systematics based on DNA sequences and karyological and morphological data; however, the systematic and phylogenetic relationships among orangutan taxa remain controversial. Surprisingly, few systematic studies have used data from wild-living orangutans of exactly known provenance. Furthermore, most of these studies pool...

Deferred imitation of object-related actions (e.g., picking up a cloth with a set of tongs) was assessed in 3 enculturated juvenile orangutans (Pongo pygmaeus) and 3 enculturated juvenile chimpanzees (Pan troglodytes). For each task, animals were given 4 min to explore the objects (baseline), followed by a demonstration of the target behavior, and 10 min later, were re-presented the objects (de...

Objects from the Middle Paleolithic period colored with ochre and marked with incisions represent the beginning of non-utilitarian object manipulation in different species of the Homo genus. To investigate the visual effects caused by these markings, we compared humans who have different cultural backgrounds (Namibian hunter-gatherers and German city dwellers) to one species of non-human great ...

Nest-building orangutans must daily build safe and comfortable nest structures in the forest canopy and do this quickly and effectively using the branches that surround them. This study aimed to investigate the mechanical design and architecture of orangutan nests and determine the degree of technical sophistication used in their construction. We measured the whole nest compliance and the thick...

Inductive learning and reasoning, as we use it both in everyday life and in science, is characterized by flexible inferences based on statistical information: inferences from populations to samples and vice versa. Many forms of such statistical reasoning have been found to develop late in human ontogeny, depending on formal education and language, and to be fragile even in adults. New revolutio...

Primate facial expressions are widely accepted as underpinned by reflexive emotional processes and not under voluntary control. In contrast, other modes of primate communication, especially gestures, are widely accepted as underpinned by intentional, goal-driven cognitive processes. One reason for this distinction is that production of primate gestures is often sensitive to the attentional stat...