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PubMed2006

Arboreal bipedalism in wild chimpanzees: implications for the evolution of hominid posture and locomotion.

Field observations of bipedal posture and locomotion in wild chimpanzees (Pan troglodytes) can serve as key evidence for reconstructing the likely origins of bipedalism in the last prehominid human ancestor. This paper reports on a sample of bipedal bouts, recorded ad libitum, in wild chimpanzees in Bwindi Impenetrable National Park in southwestern Uganda. The Ruhija community of chimpanzees in Bwindi displays a high rate of bipedal posture. In 246.7 hr of observation from 2001-2003, 179 instances of bipedal posture lasting 5 sec or longer were recorded, for a rate of 0.73 bouts per observation hour. Bipedalism was observed only on arboreal substrates, and was almost all postural, and not locomotor. Bipedalism was part of a complex series of positional behaviors related to feeding, which included two-legged standing, one-legged standing with arm support, and other intermediate postures. Ninety-six percent of bipedal bouts occurred in a foraging context, always as a chimpanzee reached to pluck fruit from tree limbs. Bipedalism was seen in both male and female adults, less frequently among juveniles, and rarely in infants. Both the frequency and duration of bipedal bouts showed a significant positive correlation with estimated substrate diameter. Neither fruit size nor nearest-neighbor association patterns were significantly correlated with the occurrence of bipedalism. Bipedalism is seen frequently in the Bwindi chimpanzee community, in part because of the unusual observer conditions at Bwindi. Most observations of bipedalism were made when the animals were in treetops and the observer at eye-level across narrow ravines. This suggests that wild chimpanzees may engage in bipedal behavior more often than is generally appreciated. Models of the likely evolutionary origins of bipedalism are considered in the light of Bwindi bipedalism data. Bipedalism among Bwindi chimpanzees suggests the origin of bipedal posture in hominids to be related to foraging advantages in fruit trees. It suggests important arboreal advantages in upright posture. The origin of postural bipedalism may have preceded and been causally disconnected from locomotor bipedalism.

Age Factors↗
PubMed2006

Twinning and heteropaternity in chimpanzees (Pan troglodytes).

Unlike monozygotic (MZ) twins, dizygotic (DZ) twins develop from separate ova. The resulting twins can have different sires if the fertilizing sperm comes from different males. Routine paternity testing of a pair of same-sexed chimpanzee twins born to a female housed with two males indicated that the twins were sired by two different males. DNA typing of 22 short-tandem repeat (STR) loci demonstrated that these twins were not MZ twins but heteropaternal DZ twins. Reproductive data from 1926-2002 at five domestic chimpanzee colonies, including 52 twins and two triplets in 1,865 maternities, were used to estimate total twinning rates and the MZ and DZ components. The average chimpanzee MZ twinning rate (0.43%) equaled the average human MZ rate (0.48%). However, the chimpanzee DZ twinning rate (2.36%) was over twice the human average, and higher than all but the fertility-enhanced human populations of Nigeria. Similarly high twinning rates among African chimpanzees indicated that these estimates were not artifacts of captivity. Log-linear analyses of maternal and paternal effects on recurrent twinning indicated that females who twinned previously had recurrence risks five times greater than average, while evidence for a paternal twinning effect was weak. Chimpanzee twinning rates appear to be elevated relative to corresponding estimated human rates, making twinning and possibly heteropaternity more important features of chimpanzee reproductive biology than previously recognized.

Animals↗
PubMed1989

Cytoplasmic antigen in hepatocytes of chimpanzees infected with non-A, non-B hepatitis virus or hepatitis delta virus: relationship to interferon.

We previously described a cytoplasmic antigen, detected by monoclonal antibodies, in hepatocytes of chimpanzees experimentally infected with the parenterally transmitted form of non-A, non-B hepatitis virus or with the hepatitis delta virus. The expression of this antigen appears to be a host-specified response to infection with these two hepatitis viruses but not with hepatitis A virus, hepatitis B virus or enterically transmitted non-A, non-B hepatitis virus. To determine whether this antigen, found in parallel with the hepatocyte cytoplasmic structures described previously, is associated with interferon, as suggested by others, we studied by immunofluorescence liver biopsies from chimpanzees treated with an interferon inducer or exogenous interferon for the presence of the antigen. In two hepatitis B virus carrier chimpanzees and one normal chimpanzee treated with the interferon inducer polyinosinic-polyribocytidylic acid-poly-l-lysine carboxymethylcellulose, the antigen became detectable in hepatocytes within 2 weeks of initiation of the treatment, remained detectable throughout the treatment and disappeared within 4 weeks after treatment was terminated. Electron microscopy revealed that the biopsies positive for the antigen exhibited the hepatocyte cytoplasmic changes; convoluted membranes and microtubular aggregates, identical to those described originally for chimpanzees infected with non-A, non-B hepatitis virus. The antigen was not detected in any of the biopsies from a control chimpanzee that received only the carboxymethylcellulose used to stabilize the interferon inducer. In addition, liver biopsies obtained from a hepatitis B virus carrier chimpanzee during treatment with exogenous human leukocyte interferon were found to be positive for the antigen as well.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1978

Effect of ethanol during hepatitis B virus infection in chimpanzees.

To determine whether the use of ethyl alcohol (ethanol, C2H5OH) may increase the liver damage caused by hepatitis B virus infection, ethanol was infused into four chimpanzees on one or two occasions during the course of natural or experimentally induced hepatitis B virus infections. A fifth chimpanzee, without active hepatitis B virus infection, served as a control. Moderate elevations of serum aspartate or alanine aminotransferases occurred in four of the five chimpanzees, including the control chimpanzee, in direct association with ethanol infusion; pre-existing enzyme elevations persisted in a fifth chimpanzee. No alteration occurred in the titers of hepatitis B surface antigen or of antibody to hepatitis B core antigen in three of the four infected chimpanzees. There was no significant alteration in the course of hepatitis B virus infection by ethanol infusion in these chimpanzees.

Acute Disease↗
PubMed1985

Neutralization of hepatitis B virus infectivity by a murine monoclonal antibody: an experimental study in the chimpanzee.

Two study chimpanzees were inoculated intravenously with approximately 1,000 chimpanzee infectious doses of hepatitis B virus (HBV), one with subtype adr and one with subtype ayw, each previously incubated with 0.1 ml of a murine monoclonal antibody (IgG 1(K) class) directed against a single epitope on hepatitis B surface antigen common to most or all HBV. Two control chimpanzees received identical doses of HBV not incubated with the murine anti-HBs. Neither study chimpanzee developed HBV infection during 12 months of follow-up as judged by normal serum aminotransferase activity, normal liver biopsies, and negative serological tests for HBV-associated antigens and antibodies. In contrast, both control chimpanzees became infected by HBV as evidenced by elevated serum aminotransferase activity, liver biopsy changes characteristic of viral hepatitis, and the appearance of hepatitis B surface antigen (HBsAg) in their sera. Both study chimpanzees were shown to be fully susceptible to infection with these same HBV inocula when challenged 15 months after the initial inoculations at a time when passively administered anti-HBs was no longer detectable. Prior to challenge with HBV, one of the two study chimpanzees received a second injection of the same volume of the murine monoclonal anti-HBs. The survival of this anti-HBs in serum was reduced from six weeks (after the initial injection) to approximately two weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1997

A primatized MAb to human CD4 causes receptor modulation, without marked reduction in CD4+ T cells in chimpanzees: in vitro and in vivo characterization of a MAb (IDEC-CE9.1) to human CD4.

A Primatized anti-CD4 monoclonal antibody (MAb), CE9.1, with V-domain from cynomolgus macaque (showing 92% homology with human consensus sequence V-domains), and a human IgG1 constant region, was characterized in vitro and in vivo in chimpanzees. This MAb binds human CD4 with Kd of 1.0 nM and was also able to bind to human IgG Fc receptors (Fc gamma R). However, despite being of the IgG1 subclass, CE9.1 did not bind to complement component C1q, nor did it mediate complement-dependent cytotoxicity. Examination of T cells from a number of species showed restricted reactivity for CE9.1, recognizing only human and chimpanzee CD4. In both human and chimpanzee MLRs, it had an IC50 of about 10.0 ng/mL. Therefore, a chimpanzee in vivo model was used to characterize CE9.1, CE9.1 caused transient decrease in the number of lymphocytes bearing the CD4 receptor starting at doses of 0.3 mg/kg in an in vivo dose ranging study in one chimpanzee. This effect was reversed within approximately 7 days. In a multiple high-dose study in which 10.0 mg/kg of CE9.1 was administered at intervals of 1-3 months, there was a dramatic loss of CD4 marker with a reciprocal increase in the number of CD3+ CD8- CD4- cells. The CD4 receptor was totally undetectable on these lymphocytes for 1-2 weeks, with a gradual, but complete, reversal within 4 weeks. We interpret these observations as receptor modulation because, although there was apparent loss of CD4+ lymphocytes, an equivalent number of CD3+CD8- T lymphocytes were present in circulation in all four chimpanzees treated with 10.0 mg/kg CE9.1. Even at this high dose, only limited reduction of CD4+ T lymphocytes was observed in these animals. These observations are in sharp contrast to what has been reported in rodents or in human clinical studies using other IgG1 mAbs to human CD4. CD8 counts, although variable, remained unaffected by CE9.1 treatment. No adverse events were observed following administration of CE9.1 to chimpanzees, and there was no detectable host immune responses to the Primatized MAb.

Animals↗
PubMed1995

Onchocerca volvulus: parasitologic and serologic responses in experimentally infected chimpanzees and mangabey monkeys.

Six chimpanzees (Pan troglodytes) and six mangabey monkeys (Cercocebus atys) were inoculated with Onchocerca volvulus third-stage larvae (L3) of West African origin. Two chimpanzees each received 200, 300, or 400 L3, while three mangabeys each received either 50 or 250 L3. All six chimpanzees became microfilaria positive between 11 and 25 months postinoculation (PI), while two of the six mangabeys were skin-snip positive at 24 and 37 months PI, respectively. All chimpanzees developed antibodies to two native antigens of 14 and 22 kDa and to the recombinant antigens OV16, OC3.6, and OC9.3. Marked antibody responses were observed in the mangabey monkeys, and in general, the responses were similar to those observed in the chimpanzees. However, in the mangabeys, these responses did not generally manifest themselves until later in the infection. The results of this study suggest that in chimpanzees, the smallest inoculum used, 200 L3, was sufficient to initiate consistent infections that had parasitologic and immunologic parameters equivalent to animals inoculated with larger numbers of larvae. Similarly, inoculation of mangabey monkeys with small numbers of larvae appeared to be as likely to establish infection and induce immunologic responses as did inoculation of larger numbers of larvae. Microfilaria-positive chimpanzees and mangabey monkeys were examined by three conventional imaging techniques (X ray, ultrasound, and magnetic resonance imaging (MRI)), but no adult worms or nodules could be identified in any animal.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1997

Ontogeny of locomotion in mountain gorillas and chimpanzees.

The African apes are a group of closely related taxa that differ considerably in body size. In spite of the large body size difference, the African apes are similar in many aspects of their morphology; it has been suggested that most of their differences result from selection for these body size differences (Shea, 1988). The influence of body size on locomotion has been well-documented, but what is less clear, is whether these behavioral differences occur throughout ontogeny because few studies have directly addressed the influence of ontogeny (and changing body size) on locomotion. This study documents the ontogeny of mountain gorilla locomotion and compares it with that of chimpanzees in order to consider how changing body size during ontogeny influences locomotion in the two species. Results indicate that gorilla locomotor development is greatly accelerated compared with chimpanzees, and that much of the interspecific variation in age can be explained by body size. When chimpanzees and gorillas are at similar sizes (although widely disparate ages), they perform very similar locomotor activities. However, it is incorrect to view a gorilla as a faster growing and ultimately larger chimpanzee. Throughout ontogeny, gorillas have broader scapulae and relatively shorter phalanges and metacarpals than chimpanzees (Susman, 1979; Shea, 1981; Jungers & Susman, 1984; Inouye, 1992) which are associated differences in mountain gorilla and chimpanzee suspensory behavior; gorillas never show as high an incidence of suspensory behavior as chimpanzees during ontogeny.

Aging↗
PubMed2001

Mortality rates among wild chimpanzees.

In order to compare evolved human and chimpanzees' life histories we present a synthetic life table for free-living chimpanzees, derived from data collected in five study populations (Gombe, Taï, Kibale, Mahale, Bossou). The combined data from all populations represent 3711 chimpanzee years at risk and 278 deaths. Males show higher mortality than females and data suggest some inter-site variation in mortality. Despite this variation, however, wild chimpanzees generally have a life expectancy at birth of less than 15 years and mean adult lifespan (after sexual maturity) is only about 15 years. This is considerably lower survival than that reported for chimpanzees in zoos or captive breeding colonies, or that measured among modern human hunter-gatherers. The low mortality rate of human foragers relative to chimpanzees in the early adult years may partially explain why humans have evolved to senesce later than chimpanzees, and have a longer juvenile period.

Animals↗
PubMed2000

Full-genome sequence analyses of hepatitis B virus (HBV) strains recovered from chimpanzees infected in the wild: implications for an origin of HBV.

Hepatitis B virus (HBV) belongs to the genus Orthohepadnavirus of the family Hepadnaviridae. Having been found in various animals (duck, heron, woodchuck, ground squirrel, and primates), hepadnaviruses must have undergone a long history of evolution and may comprise more members than currently recognized. Chimpanzees may also have their own hepadnavirus, even if it might be very close to HBV. We analyzed HBV-like sequences from three chimpanzees (Pan troglodytes) that were most likely infected during their life in Africa in the wild. Two chimpanzees (Ch256 and Ch258) possessed a viral genome of 3182 nt in length with a 33-nt deletion in the preS1 region, which could not be classified into any of the six genotypes (A-F) of human HBV but was very homologous to a previously reported isolate from a London Zoo chimpanzee. Phylogenetically distinct from the HBV-like sequences from gibbons, orangutans, and a gorilla so far reported, the Ch256 and Ch258 isolates would represent an indigenous chimpanzee HBV (tentatively ChHBV). A third chimpanzee (Ch195) had a 3212-nt genome, classifiable into the genotype E of HBV. Because HBV-E has been found mostly in Africans, Ch195 may have been infected from a human source in Africa. However, an inverse scenario is also possible: a spread of HBV-E might have occurred from chimpanzees to humans a long time ago in Africa. Analysis of the arginine-rich C-terminal region of the core protein, which is well conserved among mammalian hepadnaviruses, indicated that HBV-E/F and nonhuman primate hepadnaviruses are much closer than HBV-A/B/C/D to the hepadnaviruses of woodchuck and ground squirrel. Our results support an "ex-nonhuman primate" hypothesis for the origin of HBV.

Africa↗
PubMed1995

Sequence diversification and exon inactivation in the glycophorin A gene family from chimpanzee to human.

In humans, the allelic diversity of MNSs glycophorins (GP) occurs mainly through the recombinational modulation of silent exons (pseudoexons) in duplicated genes. To address the origin of such a mechanism, structures of GPA, GPB, and GPE were determined in chimpanzee, the only higher primate known to have achieved a three-gene framework as in humans. Pairwise comparison of the chimpanzee and human genes revealed a high degree of sequence identity and similar exon-intron organization. However, the chimpanzee GPA gene lacks a completely formed M- or N-defining sequence as well as a consensus sequence for the Asn-linked glycosylation. In the case of the GPB gene, exon III is expressed in the chimpanzee but silenced, as a pseudoexon, in the human. Therefore, the protein product in the chimpanzee bears a larger extracellular domain than in the human. For the GPE genes, exon III and exon IV have been inactivated by identical donor splice-site mutations in the two species. Nevertheless, the chimpanzee GPE-like mRNA appeared to be transcribed from a GPB/E composite gene containing no 24-bp insertion sequence in exon V for the transmembrane domain. These results suggest a divergent processing of exonic units from chimpanzee to human in which the inactivation of GPB exon III preserved a limited sequence repertoire for diversification of human glycophorins.

Amino Acid Sequence↗
PubMed1984

Ultrastructural alterations in serial liver biopsy specimens from chimpanzees experimentally infected with a human non-A, non-B hepatitis agent.

Four chimpanzees experimentally infected with an agent of human non-A, non-B hepatitis were studied to determine the sequence of ultrastructural alterations in hepatocytes during infection. Three of the four types of cytoplasmic alterations previously described in association with non-A, non-B hepatitis were observed in the hepatocytes. Sponge-like cytoplasmic inclusions (designated C-I) were detected at or near the time of peak serum aminotransferase elevations in two of the four chimpanzees. Undulating membranes (designated C-II) were observed in all four chimpanzees, at the time of the first elevation of serum aminotransferase levels. Cytoplasmic tubules (designated C-III) were first observed four, eight, and twelve weeks, respectively, after inoculation in three of the chimpanzees. Four weeks after the peak of serum aminotransferase elevations, cytoplasmic alterations could no longer be detected in hepatocytes of the four chimpanzees. Intranuclear inclusions consisting of 20-27 nm granules and vermicular particles were observed in hepatocytes from preinoculation liver biopsy specimens, as well as biopsies obtained during non-A, non-B hepatitis. The number of these particles was greatest near the time of peak elevation of serum aminotransferase levels, however. Tubulo-crystalline inclusions were noted as well in the endothelial cells from both preinoculated and infected chimpanzees. Cytoplasmic alterations in hepatocytes of chimpanzees experimentally infected with an agent of non-A, non-B hepatitis appear characteristic of infection with this agent. In contrast, intranuclear particles were not specifically related to the non-A, non-B hepatitis infection.

Animals↗
PubMed2004

Causal knowledge and imitation/emulation switching in chimpanzees (Pan troglodytes) and children (Homo sapiens).

This study explored whether the tendency of chimpanzees and children to use emulation or imitation to solve a tool-using task was a response to the availability of causal information. Young wild-born chimpanzees from an African sanctuary and 3- to 4-year-old children observed a human demonstrator use a tool to retrieve a reward from a puzzle-box. The demonstration involved both causally relevant and irrelevant actions, and the box was presented in each of two conditions: opaque and clear. In the opaque condition, causal information about the effect of the tool inside the box was not available, and hence it was impossible to differentiate between the relevant and irrelevant parts of the demonstration. However, in the clear condition causal information was available, and subjects could potentially determine which actions were necessary. When chimpanzees were presented with the opaque box, they reproduced both the relevant and irrelevant actions, thus imitating the overall structure of the task. When the box was presented in the clear condition they instead ignored the irrelevant actions in favour of a more efficient, emulative technique. These results suggest that emulation is the favoured strategy of chimpanzees when sufficient causal information is available. However, if such information is not available, chimpanzees are prone to employ a more comprehensive copy of an observed action. In contrast to the chimpanzees, children employed imitation to solve the task in both conditions, at the expense of efficiency. We suggest that the difference in performance of chimpanzees and children may be due to a greater susceptibility of children to cultural conventions, perhaps combined with a differential focus on the results, actions and goals of the demonstrator.

Animals↗
PubMed2004

Fruit availability and chimpanzee party size at Kahuzi montane forest, Democratic Republic of Congo.

The effect of fruit availability on chimpanzee party size was investigated in the montane forest of Kahuzi. Seasonal variation in both fruit availability and party size was examined. Fruit abundance per se does not affect chimpanzee party size. However, seasonality and distribution patterns of fruits are both determinant ecological factors that control the size of chimpanzee parties at Kahuzi. There was no correlation between fruit abundance and the spatial distribution of fruits. When fruits were clumped and available in large amounts for a long period, chimpanzee party size increased, or otherwise decreased when fruits were highly available for only a limited period. Tree species that produced only a small amount of ripe fruit throughout the year did not affect the foraging party size of chimpanzees. Temporal and spatial variability in fruit abundance seems to constrain grouping patterns of chimpanzees at Kahuzi more so than in other chimpanzee habitats previously described.

Animals↗
PubMed2004

When does play panting occur during social play in wild chimpanzees?

To clarify the social functions of play panting in chimpanzees, I investigated when they emitted play panting in social play and how the interactions were affected by the occurrence of play panting. The subjects were the M-group chimpanzees living in Mahale, Tanzania. The following observations were made: (1) chimpanzees emitted play panting when they were tickled or chased but rarely did so when they tickled or chased others. Chimpanzee play panting does not have the function of a play signal communicating that these "aggressive" actions are performed not as aggression but as play. (2) Chimpanzees emitted play panting more often when they received "aggressive" actions that supposedly elicited higher arousal. (3) A chimpanzee tended to continue to perform "aggressive" actions when the target emitted play panting. Play panting activates the interaction of social play by encouraging the performer to continue tickling or chasing. These results can be summarized as showing that chimpanzee play panting serves as positive feedback to the play partner for continuing somewhat fragile interactions, which may contain the risk of excessive arousal and the risk of confusing "defensive" actions by the target of the "aggressive" actions with real efforts to escape the situation.

Aggression↗
PubMed2004

Incisor-molar relationships in chimpanzees and other hominoids: implications for diet and phylogeny.

In chimpanzees, the cutting edge of the incisor battery is longer in relation to the length of the molar row than in any other hominoid, extant or fossil, the only other lineage approaching it being the orangutan. Apart from their increased mesio-distal dimensions, the upper and lower incisors of chimpanzees differ in additional ways from those of almost all other hominoids. The I2/ is enlarged, so that the difference in size between it and the central upper incisor is less than it is in the heteromorphic upper incisors of other hominoids. The lower incisors are expanded mesio-distally, so much so that isolated I/2 crowns can resemble upper central incisors. In chimpanzees the lingual surface of the lower incisors is generally more procumbent than it is in other hominoids, which have more vertically oriented incisor crowns and there is a greater difference in enamel thickness between labial and lingual sides. The re-orientation of the lower incisor crown is reflected in the root, which in lateral view is anteriorly concave in chimpanzees whereas it is more orthogonal or convex in other hominoids. The molars of chimpanzees, especially the lowers, have extensive and relatively deep occlusal basins, and the main cusps are peripheralised and labio-lingually compressed, making them more trenchant than those of other hominoids. This paper examines the incisor-lower molar proportions in extinct and living hominoids and develops a new hypothesis about the evolution of the dentition of chimpanzees and links it to their diet. It also examines the incisor-molar proportions of hominids and African apes in order to throw light on the phylogenetic relationships between them. It is shown that chimpanzees are highly derived in this respect and that several recent ideas concerning the chimp-like appearance of the last common ancestor of hominids and African apes are likely to be incorrect.

Animals↗
PubMed2004

How does stone-tool use emerge? Introduction of stones and nuts to naive chimpanzees in captivity.

Nut-cracking behavior has been reported in several communities in West Africa but not in East and Central Africa. Furthermore, even within nut-cracking communities, there are individuals who do not acquire the skill. The present study aimed to clarify the cognitive capability required for nut-cracking behavior and the process through which the the nut-cracking behavior emerges. To examine emergence, we provided three naive adult chimpanzees with a single opportunity to observe human models. A human tester demonstrated nut-cracking behavior using a pair of stones and then supplied stones and nuts to the chimpanzee subjects. Two out of three chimpanzees proceeded to hit a nut on an anvil stone using a hammer stone, one of whom succeeded in cracking open the nuts during the first test session. The third chimpanzee failed to crack open nuts. We used four variables (object, location, body part used, and action) to describe stone/nut manipulation in order to analyze further the patterns of object manipulation exhibited by the subjects. The analysis revealed that there were three main difficulties associated with nut-cracking behavior. (1) The chimpanzee who failed at the task never showed hitting action. (2) The chimpanzee who failed at the task manipulated nuts but rarely stones. (3) The combination of three objects was not commonly observed in the three chimpanzees. We also discuss our results with reference to the effect of enculturation in captivity and the social context of learning in the wild.

Animals↗
PubMed2006

Chimpanzees (Pan troglodytes) learn to act with other individuals in a cooperative task.

We presented two chimpanzees with a task in which they were required to pull each end of a rope simultaneously to drag blocks supporting food into reach. The chimpanzees did not succeed in initial tests. They did not immediately understand the necessity for cooperation, and they did not adjust their behavior to work with the partner. However, the frequency of success gradually increased as the number of sessions increased and the task was varied. They began to look at the partner frequently, wait if the partner was not holding the rope, and pull the rope in synchrony with the partner. However, they did not use interactive behaviors or eye contact to synchronize their behavior. One chimpanzee was then paired with a human partner in the same situation. After initial failures, the chimpanzee began to solicit the human partner for cooperation: looking up at his face, vocalizing, and taking the partner's hand. When this chimpanzee was again paired with the chimpanzee partner, no soliciting behavior was observed. Thus, the chimpanzees could learn to coordinate their behavior through trial and error. Communicative behavior emerged during the task, but the communication differed according to the identity of the partner.

Animals↗