Children,but not chimpanzees,have facial correlates of determination

Facial expressions have long been proposed to be important agents in forming and maintaining cooperative interactions in social groups. Human beings are inordinately cooperative when compared with their closest-living relatives, the great apes, and hence one might expect species differences in facial expressivity in contexts in which cooperation could be advantageous. Here, human children and chimpanzees were given an identical task designed to induce an element of frustration (it was impossible to solve). In children, but not chimpanzees, facial expressions associated with effort and determination positively correlated with persistence at the task. By contrast, bodily indicators of stress (self-directed behaviour) negatively correlated with task persistence in chimpanzees. Thus, children exhibited more behaviour as they persisted, and chimpanzees exhibited less. The facial expressions produced by children, could, therefore, function to solicit prosocial assistance from others.     

Male chimpanzees prefer mating with old females

Cross-cultural studies indicate that women's sexual attractiveness generally peaks before motherhood and declines with age. Cues of female youth are thought to be attractive because humans maintain long-term pair bonds, making reproductive value (i.e. future reproductive potential) particularly important to males. Menopause is believed to exaggerate this preference for youth by limiting women's future fertility. This theory predicts that in species lacking long-term pair bonds and menopause, males should not exhibit a preference for young mates. We tested this prediction by studying male preferences in our closest living relative, the chimpanzee (Pan troglodytes). We show that despite their promiscuous mating system, chimpanzee males, like humans, prefer some females over others. However, in contrast to humans, chimpanzee males prefer older, not younger, females. These data robustly discriminate patterns of male mate choice between humans and chimpanzees. Given that the human lineage evolved from a chimpanzee-like ancestor, they indicate that male preference for youth is a derived human feature, likely adapted from a tendency to form unusually long term mating bonds.     

Majority-biased transmission in chimpanzees and human children, but not orangutans

Cultural transmission is a key component of human evolution. Two of humans' closest living relatives, chimpanzees and orangutans, have also been argued to transmit behavioral traditions across generations culturally [1-3], but how much the process might resemble the human process is still in large part unknown. One key phenomenon of human cultural transmission is majority-biased transmission: the increased likelihood for learners to end up not with the most frequent behavior but rather with the behavior demonstrated by most individuals. Here we show that chimpanzees and human children as young as 2 years of age, but not orangutans, are more likely to copy an action performed by three individuals, once each, than an action performed by one individual three times. The tendency to acquire the behaviors of the majority has been posited as key to the transmission of relatively safe, reliable, and productive behavioral strategies [4-7] but has not previously been demonstrated in primates.     

Eastern chimpanzees, but not bonobos, represent a simian immunodeficiency virus reservoir

Chimpanzees in west central Africa (Pan troglodytes troglodytes) are endemically infected with simian immunodeficiency viruses (SIVcpzPtt) that have crossed the species barrier to humans and gorillas on at least five occasions, generating pandemic and nonpandemic forms of human immunodeficiency virus type 1 (HIV-1) as well as gorilla SIV (SIVgor). Chimpanzees in east Africa (Pan troglodytes schweinfurthii) are also infected with SIVcpz; however, their viruses (SIVcpzPts) have never been found in humans. To examine whether this is due to a paucity of natural infections, we used noninvasive methods to screen wild-living eastern chimpanzees in the Democratic Republic of the Congo (DRC), Uganda, and Rwanda. We also screened bonobos (Pan paniscus) in the DRC, a species not previously tested for SIV in the wild. Fecal samples (n = 3,108) were collected at 50 field sites, tested for species and subspecies origin, and screened for SIVcpz antibodies and nucleic acids. Of 2,565 samples from eastern chimpanzees, 323 were antibody positive and 92 contained viral RNA. The antibody-positive samples represented 76 individuals from 19 field sites, all sampled north of the Congo River in an area spanning 250,000 km(2). In this region, SIVcpzPts was common and widespread, with seven field sites exhibiting infection rates of 30% or greater. The overall prevalence of SIVcpzPts infection was 13.4% (95% confidence interval, 10.7% to 16.5%). In contrast, none of the 543 bonobo samples from six sites was antibody positive. All newly identified SIVcpzPts strains clustered in strict accordance to their subspecies origin; however, they exhibited considerable genetic diversity, especially in protein domains known to be under strong host selection pressure. Thus, the absence of SIVcpzPts zoonoses cannot be explained by an insufficient primate reservoir. Instead, greater adaptive hurdles may have prevented the successful colonization of humans by P. t. schweinfurthii viruses.     

A HERV-K provirus in chimpanzees, bonobos and gorillas, but not humans

Evidence from DNA sequencing studies strongly indicated that humans and chimpanzees are more closely related to each other than either is to gorillas [1-4]. However, precise details of the nature of the evolutionary separation of the lineage leading to humans from those leading to the African great apes have remained uncertain. The unique insertion sites of endogenous retroviruses, like those of other transposable genetic elements, should be useful for resolving phylogenetic relationships among closely related species. We identified a human endogenous retrovirus K (HERV-K) provirus that is present at the orthologous position in the gorilla and chimpanzee genomes, but not in the human genome. Humans contain an intact preintegration site at this locus. These observations provide very strong evidence that, for some fraction of the genome, chimpanzees, bonobos, and gorillas are more closely related to each other than they are to humans. They also show that HERV-K replicated as a virus and reinfected the germline of the common ancestor of the four modern species during the period of time when the lineages were separating and demonstrate the utility of using HERV-K to trace human evolution.     

Chimpanzees empathize with group mates and humans,but not with baboons or unfamiliar chimpanzees

Human empathy can extend to strangers and even other species, but it is unknown whether non-humans are similarly broad in their empathic responses. We explored the breadth and flexibility of empathy in chimpanzees, a close relative of humans. We used contagious yawning to measure involuntary empathy and showed chimpanzees videos of familiar humans, unfamiliar humans and gelada baboons (an unfamiliar species). We tested whether each class of stimuli elicited contagion by comparing the effect of yawn and control videos. After including previous data on the response to ingroup and outgroup chimpanzees, we found that familiar and unfamiliar humans elicited contagion equal to that of ingroup chimpanzees. Gelada baboons did not elicit contagion, and the response to them was equal to that of outgroup chimpanzees. However, the chimpanzees watched the outgroup chimpanzee videos more than any other. The combination of high interest and low contagion may stem from hostility towards unfamiliar chimpanzees, which may interfere with an empathic response. Overall, chimpanzees showed flexibility in that they formed an empathic connection with a different species, including unknown members of that species. These results imply that human empathic flexibility is shared with related species.     

Male orangethroat darters,Etheostoma spectabile, do not prefer larger females

Synopsis Male orangethroat darters,Etheostoma spectabile, did not show a preference for larger females as measured by the number of matings or time until mating in a field enclosure experiment. The lack of male preference for female size may be due to the male-biased sex ratio at the site (seven males per female) or to low variation in the number of eggs deposited per spawning.     

HIV-infected humans, but not chimpanzees, have circulating cytotoxic T lymphocytes that lyse uninfected CD4+ cells

It has been suggested that autoimmune phenomena contribute to the depletion of CD4+ T cells and the development of AIDS in HIV-1 infected humans based, in part, on observations that some HIV-1-infected humans have autoantibodies reactive with Ag expressed on uninfected CD4+ cells. In this study, 11 of 14 asymptomatic HIV-1-infected homosexuals and hemophiliacs, but none of 17 uninfected homosexuals or heterosexuals, were found to have cytotoxic lymphocytes in blood that can lyse uninfected CD4+ T cells from humans and chimpanzees but not human B lymphoblastoid cells or mouse T cells. The cytotoxic PBL were concluded to be CTL rather than NK cells, with the phenotype being CD3+, TCR-1 alpha beta+, CD8+, CD4-, CD16- based on findings that PBL-mediated lysis of uninfected CD4+ cells was 1) blocked by a mAb to CD3, which inhibits CTL but not NK activity; 2) diminished by treatment of PBL with a mAb to CD8 and C, but not by treatment with mAb to CD4 or CD16 and C; and 3) blocked by mAb WT31 directed against the TCR-1 alpha beta. In contrast, PBL from HIV-1-infected chimpanzees, which to date have not developed AIDS, lacked detectable CTL lytic for uninfected CD4+ cells.     

Dogs (Canis familiaris), but not chimpanzees (Pan troglodytes), understand imperative pointing

Chimpanzees routinely follow the gaze of humans to outside targets. However, in most studies using object choice they fail to use communicative gestures (e.g. pointing) to find hidden food. Chimpanzees' failure to do this may be due to several difficulties with this paradigm. They may, for example, misinterpret the gesture as referring to the opaque cup instead of the hidden food. Or perhaps they do not understand informative communicative intentions. In contrast, dogs seem to be skilful in using human communicative cues in the context of finding food, but as of yet there is not much data showing whether they also use pointing in the context of finding non-food objects. Here we directly compare chimpanzees' (N = 20) and dogs' (N = 32) skills in using a communicative gesture directed at a visible object out of reach of the human but within reach of the subject. Pairs of objects were placed in view of and behind the subjects. The task was to retrieve the object the experimenter wanted. To indicate which one she desired, the experimenter pointed imperatively to it and directly rewarded the subject for handing over the correct one. While dogs performed well on this task, chimpanzees failed to identify the referent. Implications for great apes' and dogs' understanding of human communicative intentions are discussed.