Published Papers - Matsudaira Kazunari

The mitochondrial D-loop is a robust maternal-species identifier in gibbons (Hylobatidae)

Lauren Lansdowne, Kazunari Matsudaira, Takafumi Ishida, Sîan McHugh, Richard M. Badge

Conservation Genetics Resources ( Springer Science and Business Media LLC )    2025.07 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Abstract Some gibbon species are difficult to distinguish phenotypically. This issue is compounded by recent changes in the gibbon taxonomy, which have sometimes occurred long after individuals were taken from the wild for captive breeding programmes. Furthermore, gibbon species within the Hylobates and Nomascus genera have been documented to hybridize to produce viable, fertile offspring, both in captivity and in the wild. This raises the possibility of cryptic hybrids within captive populations, threatening the genetic integrity of these genera. Phylogenetic methods using the mitochondrial D-loop and cox1 gene were compared to genetically identify gibbon species. Both loci identified lineages with cryptic hybrid ancestry. However, the D-loop outperformed the cox1 gene, providing higher resolution, particularly for Nomascus spp., which for some species were not monophyletic when using the cox1 gene. The D-loop also revealed a significantly higher number of unique sequences, making it more suitable for investigations into relatedness, such as female dispersal patterns.

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Viral load of Torquetenovirus correlates with Sano's score and levels of total bilirubin and aspartate aminotransferase in Kawasaki disease

Pietro Giorgio Spezia, Kazunari Matsudaira, Fabio Filippini, Takako Miyamura, Keiko Okada, Yoshiro Nagao, Takafumi Ishida, Tetsuya Sano, Mauro Pistello, Fabrizio Maggi, Junichi Hara

Scientific Reports ( Springer Science and Business Media LLC )  13 ( 1 )   2023.10 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Abstract Cause of Kawasaki disease (KD) is unknown. KD is often resistant to treatment with intravenous immunoglobulin (IVIG). Sano's score, which is derived from total bilirubin (TBIL), aspartate aminotransferase (AST) and C-reactive protein (CRP), is predictive of IVIG resistance in Japan. A recent study reported that Torquetenovirus (TTV), especially TTV7, was present at a high viral load in the patients with KD. We used PCR to quantify TTV load and amplicon next generation sequencing to detect individual TTV species. We used serum samples that were collected between 2002 and 2005 from 57 Japanese KD patients before IVIG treatment. Correlations between TTV load and Sano's score, the biomarkers that constitute this score, and IVIG resistance were examined. TTV load was positively correlated with Sano's score (P = 0.0248), TBIL (P = 0.0004), and AST (P = 0.0385), but not with CRP (P = 0.6178). TTV load was marginally correlated with IVIG resistance (P = 0.1544). Presence of TTV7 was correlated with total TTV load significantly (P = 0.0231). The correlations between biomarkers for KD and TTV load suggested that TTV may play a role in the pathophysiology of KD. We hypothesize that TTV7 may be associated with a higher total viral load in KD.

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Time-series analysis of satellite imagery for detecting vegetation cover changes in Indonesia

Takuro Furusawa, Takuya Koera, Rikson Siburian, Agung Wicaksono, Kazunari Matsudaira, Yoshinori Ishioka

Scientific Reports ( Springer Science and Business Media LLC )  13 ( 1 ) 8437   2023.05 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Abstract Indonesia has one of the world's largest tropical forests; thus, its deforestation and environmental degradation are a global concern. This study is the first to perform comprehensive big data analyses with coherent vegetation criteria to measure the vegetation change at a high temporal resolution (every 16-day period) for 20 years and the high administrative resolution (regency or city) all over Indonesia. The normalized difference vegetation index (NDVI) of the Moderate Resolution Imaging Spectroradiometer is analyzed through state space modeling. The findings reveal that the NDVI increases in almost all regencies, except in urban areas. A high correlation between the NDVI change and the time is observed in Sumatra, Papua, and Kalimantan. The gain of the NDVI values is obvious in the Central and Eastern Java Island. Human activities, such as the expansion of agriculture and forestry and forest conservation policies, are the key factors for the observed pattern.

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Macaque progressions: passing order during single-file movements reflects the social structure of a wild stump-tailed macaque group

Aru Toyoda, Tamaki Maruhashi, Suchinda Malaivijitnond, Kazunari Matsudaira, Zin Arai, Ikki Matsuda, Hiroki Koda

Primates ( Springer Science and Business Media LLC )    2023.02 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Genetic Diversity of White-Handed Gibbon Hylobates lar (Linnaeus, 1771) at Ywar Kaing Kaung Village in Dawna Mountain Range Corridor

No No Wai, Kazunari Matsudaira, Takafumi Ishida, Aye Mi San

Journal of the Myanmar Academy of Arts and Science   21 ( 3 ) 1 - 10   2023 [ Peer Review Accepted ]

Type of publication: Research paper (other science council materials etc.)

Phylogeography of Long-tailed Macaque, Macaca fascicularis aurea I. Geoffroy [1831] from Mon and Kayin States, Myanmar

Phyu Pyar Tin, Kazunari Matsudaira, Aye Mi San

Journal of the Myanmar Academy of Arts and Science   21 ( 3 ) 11 - 20   2023 [ Peer Review Accepted ]

Type of publication: Research paper (other science council materials etc.)

Introgression and mating patterns between white-handed gibbons (Hylobates lar) and pileated gibbons (Hylobates pileatus) in a natural hybrid zone

Kazunari Matsudaira, Ulrich H. Reichard, Takafumi Ishida, Suchinda Malaivijitnond

PLOS ONE ( Public Library of Science (PLoS) )  17 ( 3 ) e0264519 - e0264519   2022.03 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Gibbons (Family Hylobatidae) are a suitable model for exploring hybridization in pair-living primates as several species form hybrid zones. In Khao Yai National Park, Thailand, white-handed gibbons (Hylobates lar) and pileated gibbons (Hylobates pileatus) are distributed parapatrically and hybridize in a narrow zone. Their phenotypic characteristics suggest limited inter-species gene flow, although this has never been assessed. To uncover the history and degree of gene flow between the two species, we studied the genetic structure of gibbons in the hybrid zone by analyzing fecal DNA samples, phenotypic characteristics, vocalizations and individuals' social status. We determined eight autosomal single nucleotide variant (SNV) loci, and mitochondrial DNA (mtDNA) and Y-chromosomal haplotypes of 72 gibbons. We compared these markers with reference types of wild pureblood white-handed gibbons (n = 12) in Kaeng Krachan National Park and pureblood pileated gibbons (n = 4) in Khao Soi Dao Wildlife Sanctuary. Autosomal genotypic analyses confirmed the various levels of mixed ancestry for several adult gibbons with or without atypical phenotypic traits in Khao Yai National Park. In some other adult gibbons, the mixed ancestry was not detected in either autosomal SNVs or their phenotypic traits but the mtDNA. Both male and female adult hybrids formed reproductive units mainly with a phenotypic pureblood partner and many of them produced offspring. Taken together, our results suggest that once hybridization occurs, white-handed-pileated-gibbon hybrids can reproduce with either parental species and that the backcrossing and thus introgression may occur in successive generations, with no drastic changes in phenotypic appearance.

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Mating and Reproductive Success in Free-Ranging Stump-Tailed Macaques: Effectiveness of Male–Male Coalition Formation as a Reproductive Strategy

Aru Toyoda, Tamaki Maruhashi, Yoshi Kawamoto, Kazunari Matsudaira, Ikki Matsuda, Suchinda Malaivijitnond

Frontiers in Ecology and Evolution ( Frontiers Media SA )  10   802012   2022.03 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Male coalition-like formation, recently found in stump-tailed macaques (Macaca arctoides), occurs when several top-ranking males collaboratively guard females to prevent mating with other rival males and actively share secured mating opportunities with their allies. We lack a comprehensive understanding of the proximate mechanisms underlying such male coalitions, e.g., the effect of genetic relatedness. Such cooperative partner choice among males is particularly interesting in animals, such as M. arctoides, that live in matrilineal (female philopatry/male dispersal) society. Theoretically, in such a social system, females, and not males, are often related to each other by kinship. Thus, the occurrence of cooperative behavior between related females are generally common, but it would be rare between unrelated/non-kin males in such matrilineal society. Herein, for the first time, we report detailed copulatory behaviors, including mating and reproductive success, in relation to male coalition-like formation in free-ranging M. arctoides following direct behavioral observation and genetic analysis. We found that coalition-forming male–male alliances often occur among both individuals that are highly related and those that are less related. We identified two groups with different mating strategies, i.e., single-male monopoly and coalition-male monopoly groups. In both groups, nearly 80% of copulations were monopolized by a single male or by coalition males. However, the single-male monopoly strategy allows opportunistic/sneaky copulations by other males with a relatively high probability. Thus, the degree of reproductive success did not reflect mating success. In contrast, the males employing a coalition strategy successfully shared their mating and reproductive success, particularly in the largest group. Compared with single-male monopolized groups, the coalition-male monopoly groups copulated with a considerably more number of females, suggesting that coalition males can effectively guard against opportunistic/sneaky copulation by rival males. We also found that coalition-forming male–male alliances often occur regardless of the degree of kinship/relatedness, indicating the complexity and flexibility inherent in the male social bond of M. arctoides.

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Highly versatile, non-invasive method for collecting buccal DNA from free-ranging non-human primates.

Aru Toyoda, Kazunari Matsudaira, Tamaki Maruhashi, Suchinda Malaivijitnond, Yoshi Kawamoto

Journal of Tropical Biology and Conservation   18   251 - 267   2021.10 [ Peer Review Accepted ]

Type of publication: Research paper (other science council materials etc.)

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Divergence and introgression in small apes, the genus Hylobates, revealed by reduced representation sequencing

Kazunari Matsudaira, Takafumi Ishida

Heredity     2021.06 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Gibbons of the genus Hylobates, which inhabit Southeast Asia, show great diversity and comprise seven to nine species. Natural hybridisation has been observed in several species contact zones, but the history and extent of hybridisation and introgression in possibly historical and the current contact zones remain unclear. To uncover Hylobates species phylogeny and the extent of introgression in their evolution, genotyping by random amplicon sequencing-direct (GRAS-Di) was applied to 47 gibbons, representing seven Hylobates species/subspecies and two outgroup gibbon species. Over 200,000 autosomal single-nucleotide variant sites were identified. The autosomal phylogeny supported that divergence from the mainland species began ~3.5 million years ago, and subsequently occurred among the Sundaic island species. Significant introgression signals were detected between H. lar and H. pileatus, H. lar and H. agilis and H. albibarbis and H. muelleri, which all are parapatric and form ongoing hybrid zones. Furthermore, the introgression signals were detected in every analysed individual of these species, indicating a relatively long history of hybridisation, which might have affected the entire gene pool. By contrast, signals of introgression were either not detected or doubtful in other species pairs living on different islands, indicating the rarity of hybridisation and introgression, even though the Sundaic islands were connected during the Pliocene and Pleistocene glacial events.

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Species Identification of Captive Gibbons in Japan by Using Mitochondrial DNA (cytochrome b) Sequences

Kazunari MATSUDAIRA, Yoichi MAEDA, Naoko SHIDEHARA, Takafumi ISHIDA

Japanese Journal of Zoo and Wildlife Medicine ( Japanese Society of Zoo and Wildlife Medicine )  26 ( 2 ) 35 - 42   2021.06 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Testing sex-biased admixture origin of macaque species using autosomal and X-chromosomal genomic sequences

Naoki Osada, Kazunari Matsudaira, Yuzuru Hamada, Suchinda Malaivijitnond

Genome Biology and Evolution ( Oxford University Press (OUP) )  13 ( 1 ) evaa209   2021.01 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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<title>Abstract</title> The role of sex-specific demography in hybridization and admixture of genetically diverged species and populations is essential to understand the origins of the genomic diversity of sexually reproducing organisms. In order to infer how sex-linked loci have been differentiated undergoing frequent hybridization and admixture, we examined 17 whole-genome sequences of seven species of the genus Macaca, which shows frequent inter-specific hybridization and predominantly female philopatry. We found that hybridization and admixture were prevalent within these species. For three cases of suggested hybrid origin of species/subspecies, M. arctoides, M. fascicularis ssp. aurea, and Chinese M. mulatta, we examined the level of admixture of X chromosomes, which is less affected by male-biased migration than that of autosomes. In one case, we were able to determine that M. cyclopis and M. fuscata was genetically closer to Chinese M. mulatta than to the Indian M. mulatta, and the admixture level of Chinese M. mulatta and M. fuscata/cyclopis was more pronounced on the X chromosome than on autosomes. Since the mitochondrial genomes of Chinese M. mulatta, M. cyclopis, and M. fuscata were found to cluster together, and the mitochondrial genome of Indian M. mulatta is more distantly related, the observed pattern of genetic differentiation on X-chromosomal loci is consistent with the nuclear swamping hypothesis, in which strong, continuous male-biased introgression from the ancestral Chinese M. mulatta population to a population related to M. fuscata and M. cyclopis generated incongruencies between the genealogies of the mitochondrial and autosomal genomes.

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Complete mitochondrial genome of Macaca mulatta from Bangladesh

Kazunari Matsudaira, Takafumi Ishida

Archivio per l’Antropologia e la Etnologia   150   59 - 65   2020 [ Peer Review Accepted ]

Type of publication: Research paper (bulletin of university, research institution)

Short dispersal distance of males in a wild white-handed gibbon (Hylobates lar) population

Kazunari Matsudaira, Takafumi Ishida, Suchinda Malaivijitnond, Ulrich H. Reichard

American Journal of Physical Anthropology ( Wiley )  167 ( 1 ) 61 - 71   2018.09 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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OBJECTIVES: It has long been recognized that in gibbons both sexes disperse from the natal group. However, the fate of dispersed individuals was rarely documented. Here we provide the first detailed information on sex differences in dispersal patterns by analyzing the spatial genetic structure of a well-known white-handed gibbon (Hylobates lar) population. MATERIALS AND METHODS: Mitochondrial DNA (mtDNA) and Y-chromosomal haplotypes, and autosomal microsatellite genotypes were determined for individuals of the Mo Singto study site, Khao Yai National Park, Thailand. Mantel tests for the three genetic marker types were performed for 17 gibbon groups comprising 23 adult males and 18 adult females. RESULTS: Significant positive Mantel correlations were observed for spatial distance and both autosomal microsatellite-based as well as Y-chromosomal haplotype-based genetic distance among adult males. Neighboring adult males tended to be genetically related and share Y-chromosomal haplotypes. Conversely, no significant Mantel correlations were observed either in autosomal microsatellites or mtDNA among adult females. DISCUSSION: Our results confirm, at a genetic level, hypotheses from long-term demographic observations that white-handed gibbon males of the Mo Singto population primarily disperse into adjacent groups. Instead, females disperse more opportunistically either to adjacent or more distant groups. This sex-specific difference reflects an apparent greater tolerance between males than between females. The higher tolerance of adult males allows the formation of stable multimale groups and facilitates male dispersal into an adjacent group. Stable multifemale groups have never been documented for white-handed gibbons probably due to feeding competition between females.

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Whole Mitochondrial Genomic and Y-Chromosomal Phylogenies of Burmese Long-Tailed Macaque (Macaca fascicularis aurea) Suggest Ancient Hybridization between fascicularis and sinica Species Groups

Kazunari Matsudaira, Yuzuru Hamada, Srichan Bunlungsup, Takafumi Ishida, Aye Mi San, Suchinda Malaivijitnond

Journal of Heredity ( Oxford University Press )  109 ( 4 ) 360 - 371   2018.05 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Macaca fascicularis aurea(Burmese long-tailed macaque) is 1 of the 10 subspecies ofMacaca fascicularis. Despite having few morphological differences from other subspecies, a recent phylogeographic study showed that M. f. aurea is clearly distinct genetically from Macaca fascicularis fascicularis (common long-tailed macaque) and suggests that M. f. aurea experienced a disparate evolutionary pathway versus other subspecies. To construct a detailed evolutionary history of M. f. aurea and its relationships with other macaque species, we performed phylogenetic analyses and divergence time estimation of whole mitochondrial genomes (2 M. f. aurea, 8 M. f. fascicularis, and 16 animals of 12 macaque species) and 2871 bp of the Y chromosome (1 M. f. aurea, 2 M. f. fascicularis, and 5 animals of 5 macaque species) and haplotype network analysis of 758 bp of the Y chromosome (1 M. f. aurea, 2 M. f. fascicularis, and 21 animals of 19 macaque species). Whereas the Y chromosome of M. f. aurea clustered with those of the fascicularis species group in the phylogenetic and haplotype network analyses, its mtDNA clustered within the clade of the sinica species group. Based on this phylogenetic incongruence and the estimated divergence times, we propose that proto-M. f. aurea underwent hybridization with a population of the sinica species group between 2.5 and 0.95 MYA after divergence from the common ancestor of M. fascicularis. Hybridization and introgression might have been central in the evolution of M. f. aurea, similar to what occurred in the evolution of other macaque species and subspecies.

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Mitochondrial DNA and two Y-chromosome genes of common long-tailed macaques (Macaca fascicularis fascicularis) throughout Thailand and vicinity

Srichan Bunlungsup, Hiroo Imai, Yuzuru Hamada, Kazunari Matsudaira, Suchinda Malaivijitnond

AMERICAN JOURNAL OF PRIMATOLOGY ( WILEY )  79 ( 2 ) e22596   2017.02 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Macaca fascicularis fascicularis is distributed over a wide area of Southeast Asia. Thailand is located at the center of their distribution range and is the bridge connecting the two biogeographic regions of Indochina and Sunda. However, only a few genetic studies have explored the macaques in this region. To shed some light on the evolutionary history of M. f. fascicularis, including hybridization with M. mulatta, M. f. fascicularis and M. mulatta samples of known origins throughout Thailand and the vicinity were analyzed by molecular phylogenetics using mitochondrial DNA (mtDNA), including the hypervariable region 1, and Y-chromosomal DNA, including SRY and TSPY genes. The mtDNA phylogenetic analysis divided M. f. fascicularis into five subclades (Insular Indonesia, Sundaic Thai Gulf, Vietnam, Sundaic Andaman sea coast, and Indochina) and revealed genetic differentiation between the two sides of the Thai peninsula, which had previously been reported as a single group of Malay peninsular macaques. From the estimated divergence time of the Sundaic Andaman sea coast subclade, it is proposed that after M. f. fascicularis dispersed throughout Southeast Asia, some populations on the south-easternmost Indochina (eastern Thailand, southern Cambodia and southern Vietnam at the present time) migrated south-westwards across the land bridge, which was exposed during the glacial period of the late Pleistocene epoch, to the southernmost Thailand/northern peninsular Malaysia. Then, some of them migrated north and south to colonize the Thai Andaman sea coast and northern Sumatra, respectively. The SRY-TSPY phylogenetic analysis suggested that male-mediated gene flow from M. mulatta southward to M. f. fascicularis was restricted south of, but close to, the Isthmus of Kra. There was a strong impact of the geographical factors in Thailand, such as the Isthmus of Kra, Nakhon Si Thammarat, and Phuket ranges and Sundaland, on M. f. fascicularis biogeography and their hybridization with M. mulatta.

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Development of Y-chromosomal Polymorphic Markers in Two Gibbon Species (Hylobates lar and Hylobates pileatus)

Kazunari Matsudaira, Takafumi Ishida

Japanese Journal of Zoo and Wildlife Medicine ( Japanese Society of Zoo and Wildlife Medicine )  20 ( 1 ) 15 - 19   2015.03 [ Peer Review Accepted ]

Type of publication: Research paper (other science council materials etc.)

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シロテテナガザル（<i>Hylobates lar</i>）およびボウシテナガザル（<i>H. pileatus</i>）を対象に，父系の遺伝的多様性の評価を可能とする 5つのマイクロサテライトおよび 3つの挿入欠失多型からなる 6座位のマーカーを Y染色体上に同定した。飼育テナガザル（各々12と 5個体）において，全ての個体が異なる Y染色体ハプロタイプを示し，今回同定した Y染色体マーカーの有用性が確認され，保全生物学への貢献が期待された。

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Extra-Pair Paternity Confirmed in Wild White-Handed Gibbons

Claudia Barelli, Kazunari Matsudaira, Tanja Wolf, Christian Roos, Michael Heistermann, Keith Hodges, Takafumi Ishida, Suchinda Malaivijitnond, Ulrich H. Reichard

AMERICAN JOURNAL OF PRIMATOLOGY ( WILEY-BLACKWELL )  75 ( 12 ) 1185 - 1195   2013.12 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Knowledge of the genetic mating system of animal species is essential for our understanding of the evolution of social systems and individual reproductive strategies. In recent years, genetic methods have uncovered an unexpected diversity of paternal genetic contributions across diverse animal social mating systems, but particularly in pair-living species. In most pair-living birds, for example, genetic and behavioral observations have confirmed a previously unknown significance of extra-pair copulations (EPCs) and extra-pair paternity. Among mammals, white-handed gibbons (Hylobates lar) are also known to live in pairs and are traditionally believed to be single-male single-female breeders. However, at Khao Yai National Park, Thailand, behavioral observations have confirmed the occurrence of both EPCs and functional multi-male grouping, but knowledge about the genetic mating system is still unavailable. In this study, we genotyped 89 white-handed gibbons of the Khao Yai population based on fecal samplings and were able to determine paternity for 41 offspring through short tandem repeat analysis. We found that females' stable social partners sired the majority (90.5%) of offspring (N=38), while only a few (7.1%) offspring (n=2 confirmed cases; n=1 inferred case) were conceived with extra-pair partners. The paternity of one offspring remained inconclusive (2.4%), because the offspring's genotype did not mismatch with the genotypes of two potential sires. Like other predominantly pair-living species, gibbons appear to follow a mixed-reproductive strategy. The genetic mating system of wild white-handed gibbons is best described as flexible, primarily monogamous and opportunistically promiscuous. Am. J. Primatol. 75:1185-1195, 2013. (c) 2013 Wiley Periodicals, Inc.

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Molecular evidence for the introgression between Hylobates lar and H. pileatus in the wild

Kazunari Matsudaira, Ulrich H. Reichard, Suchinda Malaivijitnond, Takafumi Ishida

PRIMATES ( SPRINGER JAPAN KK )  54 ( 1 ) 33 - 37   2013.01 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Inter-specific hybrid zones for Hylobates gibbons are known in Southeast Asia. Among these, one hybrid zone between Hylobates lar and H. pileatus is located in Khao Yai National Park, Thailand. To find molecular evidence for the natural hybridization of the gibbons in this region, we studied mitochondrial DNA (mtDNA) of 68 gibbons of the H. lar phenotype living adjacent to the hybrid zone. Nucleotide sequencing of a fragment of mtDNA spanning hyper variable segment I showed that nine gibbons had an mtDNA haplotype of H. pileatus, and that seven of these nine gibbons belonged to a single maternal lineage over three generations. It is thus confirmed that introgression between H. lar and H. pileatus exists and the initial hybridization took place ages ago.

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Individual Variation in Behavioural Reactions to Unfamiliar Conspecific Vocalisation and Hormonal Underpinnings in Male Chimpanzees

Nobuyuki Kutsukake, Migaku Teramoto, Seijiro Homma, Yusuke Mori, Kazunari Matsudaira, Hisao Kobayashi, Takafumi Ishida, Kazuo Okanoya, Toshikazu Hasegawa

ETHOLOGY ( WILEY-BLACKWELL )  118 ( 3 ) 269 - 280   2012.03 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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It has been established that various species exhibit personality, defined as intra-individual consistency and inter-individual variation in behavioural phenotypes. For example, certain individuals may demonstrate consistently greater behavioural reactions and elevated stress responses. We conducted playback experiments and hormonal analyses on male chimpanzees (Pan troglodytes) in captivity to investigate the patterns and proximate mediators of individual variations in behavioural reactions. We found intra-individual consistency and inter-individual variation in behavioural reactions (intensive vigilance towards the direction of speakers) to vocalisations by unfamiliar chimpanzees. This behavioural reaction was positively correlated with changes in salivary cortisol concentration, suggesting that stress is a proximate factor mediating the variation in behavioural reactions. The males who were highly responsive to the conspecific vocalisation also exhibited high behavioural reactions towards the neutral broadcast stimulus (the jungle crows Corvus macrorhynchos ka vocalisation). This observation is consistent with the notion that male chimpanzees vary in intrinsic behavioural tendency to different stimuli.

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