Mother-infant vocal interactions play a crucial role in the development of human language. However, comparatively little is known about the maternal role during vocal development in nonhuman primates. Here, we report the first evidence of mother-daughter vocal interactions contributing to vocal development in gibbons, a singing and monogamous ape species. Gibbons are well known for their species-specific duets sung between mates, yet little is known about the role of intergenerational duets in gibbon song development. We observed singing interactions between free-ranging mothers and their sub-adult daughters prior to emigration. Daughters sang simultaneously with their mothers at different rates. First, we observed significant acoustic variation between daughters. Co-singing rates between mother and daughter were negatively correlated with the temporal precision of the song’s synchronization. In addition, songs of daughters who co-sang less with their mothers were acoustically more similar to the maternal song than any other adult female’s song. All variables have been reported to be influenced by social relationships of pairs. Therefore those correlations would be mediated by mother-daughter social relationship, which would be modifiable in daughter’s development. Here we hypothesized that daughters who co-sing less often, well-synchronize, and converge acoustically with the maternal acoustic pattern would be at a more advanced stage of social independence in sub-adult females prior to emigration. Second, we observed acoustic matching between mothers and daughters when co-singing, suggesting short-term vocal flexibility. Third, we found that mothers adjusted songs to a more stereotyped pattern when co-singing than when singing alone. This vocal adjustment was stronger for mothers with daughters who co-sang less. These results indicate the presence of socially mediated vocal flexibility in gibbon sub-adults and adults, and that mother-daughter co-singing interactions may enhance vocal development. More comparative work, notably longitudinal and experimental, is now needed to clarify maternal roles during song development.

The effect of hay bales and perches as environmental enrichment material was investigated on the behavior, heterophil/lymphocyte (H/L) ratio, footpad dermatitis (FPD), and productivity of Japanese broilers in a commercial farm. About 21,500 birds were allocated to 4 houses: male versus female×control versus treatment house. We observed maintenance behavior, the utilization of hay bales and perches, and the H/L ratio of birds at 3, 5, and 8 weeks of age. FPD was scored and productivity was calculated at about 60 days after hatching. Compared to birds in the control houses, birds in the treatment houses invested more time in stand-resting and moving behaviors (P＜0.01, for each behavior) and less time feeding, drinking, and sit-resting (P＜0.01, for each behavior). Females used the hay bales and perches more than the males (P＜0.1 and P＜0.01, respectively). The activity and utilization of hay bales and perches decreased with age (P＜0.01, for each behavior, respectively). The H/L ratio of birds in the treatment houses was lower than that in the control houses (P＜0.01). The prevalence of FPD was milder in the treatment houses compared to the control houses for females (P＜0.01), but not for males. In conclusion, hay bales and perches represent effective enrichment materials, allowing broilers to express normal behavior, alleviating stress in the younger and lighter birds of both sexes, and reducing FPD in female birds using the hay bales and perches more than males.

We examined acoustic individuality in wild agile gibbon Hylobates agilis agilis and determined the acoustic variables that contribute to individual discrimination using multivariate analyses. We recorded 125 female-specific songs (great calls) from six groups in west Sumatra and measured 58 acoustic variables for each great call. We performed principal component analysis to summarize the 58 variables into six acoustic principal components (PCs). Generally, each PC corresponded to a part of the great call. Significant individual differences were found across six individual gibbons in each of the six PCs. Moreover, strong acoustic individuality was found in the introductory and climax parts of the great call. In contrast, the terminal part contributed little to individual identification. Discriminant analysis showed that these PCs contributed to individual discrimination with high repeatability. Although we cannot conclude that agile gibbon use these acoustic components for individual discrimination, they are potential candidates for individual recognition.