Species identity and cave-dwelling tree hyraxes of the Kenyan coast

The eastern tree hyrax is thought to be a solitarily living arboreal species of the forests of East Africa. However, on the coast of Kenya, indigenous forests have been almost entirely cleared, and some of the last tree hyrax populations live in limestone rocky formations and caves. Interestingly, they seem to be living in social groups. Here, we describe and document photographically these unique tree hyrax populations. We also describe their acoustical communication and their calling activity in three different habitats. Based on these animals’ physical appearance and acoustic analyses of their calls, they represent the species eastern tree hyrax, Dendrohyrax validus. Due to immence pressure from humans, the future of these small and isolated, cave-living tree hyrax populations does not seem bright.

Methods
Study sites

We visited three tree hyrax habitats on the Kenya coast in Jan 26–Feb 1, 2022. The first study site, the Shimba Hills Natural Reserve, 4.214° S, 39.451° E, has a total area of 192.5 km. The reserve supports forests (Figure 2a) and grassland areas that are kept open by periodic burning. The local tree hyraxes live in trees. The second site, Vipingo, is on private land (coordinates not published). It represents a 25-ha patch of old indigenous forest with large baobab trees (Figure 2b), surrounded by agricultural lands and villages. The site also has three large sacred limestone caves with bat colonies. The local tree hyraxes live in caves and trees. The third site, Chasimba, 3.739° S, 39.693° E, represents a small (2 ha) rock outcrop with sacred caves and small indigenous trees (Figure 2c), surrounded by agricultural fields and banana groves. The local tree hyraxes live in rocky formations higher up. Unfortunately, the site has already lost most of its tree cover and is in imminent threat of being turned into a limestone quarry, which would undoubtedly exterminate the tree hyrax population, as there are no other suitable habitats nearby.

We used a thermal imaging camera, Pulsar Helion 2 XP50 (Yukon Advanced Optics Worldwide, Vilnius, Lithuania) and Fenix TK25 RED (Fenix Lighting, Littleton, CO, USA) flashlights. For recordings, we used AudioMoth automatic recorders (v1.1.0 Open Acoustics Devices, Southampton, UK) and Song Meter4 (Wildlife Acoustics, Maynard USA).

Spectrograms and call analyses were done with Raven Pro 1.6. Different call types were identified, and clear recordings with no background noise were isolated. Tree hyrax calling activity for each hour was calculated by using band-limited energy detection function. A band-limited interactive detector was used with the following settings: min frequency 1000 Hz, max frequency 4500 Hz, min duration 0,01 s, max duration 2 s, min separation 0.23 s. Signal-to-noise ratio 70 %, SNR threshold above 10. False positive calls, (mostly short-eared greater galago Otolemur garnettii calls), were removed from the calculated calls by visually inspecting the calls.

We compared our photographic and video material of living tree hyraxes with photographs of tree hyrax skins that we have studied in the collections of the Natural History Museums in Nairobi, Kenya, and in the Swedish Museum of Natural History in Stockholm, Sweden. This allowed us to compare the physical appearance of the Kenya coast tree hyraxes with representatives of other tree hyrax populations and species.

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东非树蹄兔（eastern tree hyrax）曾被认为是一种独居的树栖物种，栖息于东非的森林中。然而在肯尼亚沿海地区，原生森林已几乎被完全砍伐，现存的部分树蹄兔种群栖息于石灰岩岩层与洞穴中。令人关注的是，它们似乎以社群群体形式生活。本研究通过摄影手段对这些独特的树蹄兔种群进行描述与记录，同时阐述它们在三种不同生境中的声学通讯与鸣唱活动。结合该动物的外形特征与鸣叫声的声学分析，确认其为东非树蹄兔（Dendrohyrax validus）。由于人类活动带来的巨大生存压力，这些体型小巧、栖息于洞穴且种群孤立的树蹄兔，其未来前景不容乐观。

## 研究方法
### 研究样地
本研究于2022年1月26日至2月1日期间，对肯尼亚沿海地区的三处树蹄兔栖息生境开展调查。
第一处样地为希姆巴丘陵自然保护区（Shimba Hills Natural Reserve），坐标为南纬4.214°、东经39.451°，总面积192.5平方千米。该保护区内分布有森林（图2a）与经定期焚烧维持开阔状态的草原生境，当地树蹄兔栖息于树木之上。
第二处样地为Vipingo（坐标未公开），位于私人土地之上。该区域为一片面积25公顷的古老原生森林，生长有大型猴面包树（图2b），周边被农田与村落环绕；同时存在三处带有蝙蝠群落的大型神圣石灰岩洞穴，当地树蹄兔同时栖息于洞穴与树木中。
第三处样地为Chasimba，坐标为南纬3.739°、东经39.693°，为一处面积2公顷的小型岩石露头，带有神圣洞穴与小型原生树木（图2c），周边被农田与香蕉园环绕；当地树蹄兔栖息于高处的岩层中。遗憾的是，该样地已丧失大部分林木覆盖，且正面临被改造为石灰岩采石场的紧迫威胁——由于周边无其他适宜生境，此举无疑将导致该树蹄兔种群彻底灭绝。

本研究使用的设备包括：Pulsar Helion 2 XP50热成像相机（立陶宛维尔纽斯Yukon Advanced Optics Worldwide公司生产）与Fenix TK25 RED手电筒（美国科罗拉多州利特尔顿Fenix Lighting公司生产）；声学录音设备则为AudioMoth自动录音机（v1.1.0版本，英国南安普顿Open Acoustics Devices公司出品）与Song Meter4声学记录仪（美国马萨诸塞州梅纳德Wildlife Acoustics公司出品）。

声谱图绘制与鸣叫声分析采用Raven Pro 1.6软件完成：首先识别不同类型的鸣叫声，筛选出无背景噪音的清晰录音；随后通过带限能量检测函数计算每小时的树蹄兔鸣唱活动量，使用带限交互式检测器，参数设置为：最低频率1000Hz、最高频率4500Hz、最短时长0.01s、最长时长2s、最小间隔0.23s，信噪比阈值为70%，信噪比门槛值高于10。通过人工目视检查鸣叫声，剔除误判的鸣唱信号（多数为灰颊大婴猴（Otolemur garnettii）的叫声）。

本研究将拍摄的活体树蹄兔影像资料，与我们在肯尼亚内罗毕自然历史博物馆及瑞典斯德哥尔摩自然历史博物馆馆藏的树蹄兔标本照片进行比对，以此将肯尼亚沿海树蹄兔的外形特征与其他树蹄兔种群及物种的标本进行对比分析。

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