Vocal repertoire of Amur Tiger cubs
In this study, nine call types produced by captive Amur tiger cubs were identified. The number of calls of each of the nine call types was not equal. The numbers of Ar-1 and Ar-2 were higher than those of other call types, comprising 32.3 and 36.6% of the whole sample dataset, respectively. This is the result of random sampling and the behavioral rhythm of Amur tiger cubs. Because of the experimental environment (breeding beds) and the motor and sensory capabilities of tiger cubs under two months old, calls mostly occurred in an isolation context. According to Yu’s study , except for sleep and rest, play is the main routine behavior of Amur tigers after two months of age (play accounted for 12.12%, exercise for 8.11%, feeding for 6.33%, and others for 7.09%). Therefore, Ar-1 and Ar-2, respectively, as the signal-specific calls of these two contexts (Isolation and Play), had a large number of calls.
Three of nine Amur tiger call types are also uttered by captive adult Sumatran Tigers, including Chuff, Hiss, and Roar . Although Growl was also identified in adult Sumatran Tigers, the Growl in our study was different from that in Rose et al.’s study  which had no rhythmic pulsation. The definition herein used for this call type was based on Volodina  and Stanton : Growl was a low-pitched, rumbling sound consisting of numerous short pulses with a long duration. Comparing the spectrograms and audios of Growl from Rose et al.’s study  and the calls of our study, we found that the calls of Haer were similar to those of Growl. A short harsh and repeated call named ‘coughing snarl’, which was used when attacking , was similar to Haer in our study, which was similar to coughing and was also used to show offensive behavior toward a conspecific.
In the hierarchical cluster analysis and DFA, the samples of eee and Growl were always classified together, consistent with aural and visual sense. Both are calls with rhythmic pulsation. However, the calls of Growl were relatively rare with lower fundamental frequency and lower three quartiles frequency, and it was mostly used in the offensive context (87% in Offensive-to-Human, 12% in Offensive-to-Conspecific), while the calls of eee were mostly used in the Play context (88%), which occurred when a cub felt uncomfortable in a playing situation. The samples of Ar-2, Er, Haer, and Roar were also assessed together in the quantitative analysis; however, unlike eee and Growl, they are not consistent with aural and visual sense. Spectrograms showed that their power was mostly concentrated at low frequencies, but the spectrograms of Ar-2 and Er had clear harmonics (Fig. 2B–C). For aural sense, the calls of Haer and Roar exhibited higher power and shorter duration than the calls of Ar-2 and Er. Ar-2 vocalizations were a tonal sound similar to the “a” vowel, while Er vocalizations were also a tonal sound, but sounded like the “e” vowel. Roar and Haer sounded alike, but Haer usually occurred repeatedly in quick succession, while Roar was usually given alone or with a long interval. Regarding the behavioral context, the use of Haer was different from that of Ar-2, Er, and Roar (Table 4). Although the Ar-2, Er, and Roar vocalizations were often given in the Play context, Roar vocalizations were used to show stronger unwillingness to join in playing, sometimes even occurring with some threatening actions. Roar vocalizations of tiger cubs are powerful, harsh, and alarming, like the roar of an adult tiger . Haer vocalizations were short and harsh, but less powerful than Roar and usually uttered continuously. Regarding the behavioral context, Haer was usually used to express offensive to conspecifics. It is possible that the acoustic parameters could not reflect the characteristics of the calls very well so that they could not be classified precisely in the quantitative analyses.
Meow (Mew/Meow) is the most well-known felid call [32, 47]. In our study, Meow was not recorded in Amur tigers. However, according to several other studies [24, 25], Meow has been recorded in adult tigers. In Peters , Mew calls were found in tiger cubs, but the author did not indicate from which tiger subspecies the calls were. During this study period, we found that Meow does exist in adult Amur tigers. Therefore, there may be vocal repertoire changes in the ontogeny of Amur tigers. More studies on the vocalization of adult Amur tigers and the comparison of vocalizations between adults and cubs need to be carried out to confirm this assumption.
Contextual use of call types
Based on previous observations, we found that eyesight can influence the behavior of Amur tiger cubs. When a newborn tiger cub without good eyesight is separated from its mother, it behaves anxiously, constantly crawling and uttering a loud sound, even with recognizes its littermates. However, when the eyesight of a tiger cub improves during the ontogeny, it realizes its littermates seemingly, and it behaves anxiously only when it is separated from both its mother and littermates. Therefore, the definition of the isolation context considered the eyesight of tiger cubs. The cubs uttered six call types (Ar-1, Ar-2, Chuff, eee, Er, Hiss), with Ar-1 (1658/2556) as the major call in this situation. Ar-1 is the context-specific call and signal-specific call of the isolation context. We, therefore, deemed Ar-1 given in the isolation context as the isolation call of Amur tiger cubs. Comparing the spectrograms of isolation calls between domestic cat kittens [8, 33] and Amur tiger cubs, we found that they were similar and both very intense and tonal in quality with a number of clear resonant frequency components. However, according to Moelk  and Brown et al. , the isolated call of domestic kittens was a complex vowel call (“mi-a:ou”), that is similar to Meow, while the isolated call of Amur tiger cubs (Ar-1) sounds like a monosyllabic “a”, different from kittens.
Haskins  found that kitten vocalization (from the isolation context) could prompt maternal behavior. However, we have no explicit evidence to show that the isolation call of Amur tiger cubs can influence maternal behavior. During the study, we noted that the tiger cubs would be able to emit isolation calls at a few hours after birth. From the observation of a female tiger and her babies, we found that the maternal-reared tiger cubs would not utter isolation calls until their mother left for a period of time. When keepers hold the tiger cub and make a daily check, they would be more likely to utter intense isolation calls. This situation is similar to that of kittens that were confronted with a high-arousal condition (experimenters grasped the kitten and lifted it off the ground) . It was found that the kittens’ isolation calls varied their acoustic parameters between low and high-arousal conditions . In our study, we did not consider the effect of arousal conditions in data collection because of the difficulty in identifying the different arousal conditions.
Three call types, Growl, Haer, and Hiss, were usually used in the offensive context by Amur tiger cubs. However, Haer was commonly used in the Offensive-to-Conspecific context, whereas Growl and Hiss were commonly used in the Offensive-to-Human context. In our study, the Offensive-to-Conspecific context always occurred during feeding. Thus, we are not sure if Haer is a specific call type for Offensive-to-Conspecific or for food conflicts. During the study, clash during non-feeding time was extremely rare. We suggest that this is caused by the captivity circumstance and the lack of territory consciousness in tiger cubs. Chuff was the signal-specific call of both Conspecific-Contact and Human-Contact contexts. It seems that Amur tiger cubs used the same vocal signal to show their amity to the other tigers and humans.
In the Play and Suckled context, the Amur tiger cubs who bit or sucked others would rarely utter calls. The calls (Ar-2, eee, and Roar) from these two contexts were all given by the cubs who were sucked or bitten to express uncomfortable feelings. It has been hypothesized that the playing of kittens is a way to practice predation . According to the feeders, a cub was killed by another cub or cubs during play. Therefore, we suggest that the calls may be a vital indicator to ask for help from the mother.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.