1. Introduction
In the arboreal environment, visual signals are often difficult to perceive through the dense vegetation. Sound, however, travels easily and widely through foliage. This has led to the evolution of intraspecific vocal communication systems in many arboreal animals [
1]. Among nocturnal mammals, prosimian primates [
2,
3,
4] and tree hyraxes [
5] rely extensively on acoustic communication, and have complex vocal repertoires. Paterson’s ‘recognition species concept’ predicts the importance of shared communication systems in conspecific mate recognition [
4,
6,
7]. Loud calls that are used in long-range advertising are qualitatively different between species [
8]. For example, many recently described galago species have been originally identified by their advertisement calls [
9,
10,
11,
12,
13]. In
Paragalago cocos and
P. zanzibaricus taxonomic separation based on acoustic properties of an advertisement call was subsequently confirmed by genetic analysis [
14]. In contrast to advertisement calls, alarm calls are conserved, phylogenetically homologous, and predicted to be less variable than those influenced by sexual selection, making them more useful as grouping criteria [
15,
16].
In the field, different species of nocturnal primates are often very difficult to tell apart from each other by visually observing them from a distance, and to capture these animals for the purpose of closer examination is often not feasible. Thus, vocal analysis has become an important, non-invasive tool in their study.
As vocalizations convey information about the caller’s identity, physical condition, motivation and/or external events, there must be variation in frequency modulations and temporal patterns within the calls [
17,
18]. Animal vocalizations typically vary over time and space, and calls may have multiple sequences that interact with each other [
19]. Signal repertoires of mammals have graded variation within and between call types [
20]. Signal repertoires may also be mixed when they combine forms of signal types within and between call types [
10,
21]. Mixed calls can be identified from differences in the context in which the calls occur, and the responses of other animals.
This study focuses on the vocalizations of three poorly known, nocturnal, arboreal mammals in the moist montane forests of the Taita Hills, Kenya. Our aim is to, for the first time, describe the most frequently used loud, long-range calls of each species on the basis of acoustic recording data, and then, when possible, make inferences about the taxonomic affinities of these species. The species studied are the local tree hyrax
Dendrohyrax sp. (
Figure 1A,B), the small-eared greater galago
Otolemur garnettii (
Figure 1C,D), and the local dwarf galago
Paragalago sp. (
Figure 1E,F). Both the tree hyrax and the dwarf galago are of uncertain taxonomic identity (see below). The nomenclature of hyraxes and galagos is confusing, and not all species have well-established, vernacular English names [
22]. Thus, unless stated otherwise, we will refer to our study animals by their ‘Latinized vernacular’ names, i.e., as ‘Dendrohyrax’, Otolemur’, and ‘Paragalago’, respectively.
1.1. Tree Hyrax Taxonomy
Tree hyraxes (genus
Dendrohyrax) are medium-sized (3–5 kg), afrotherian mammals [
23,
24,
25]. Tree hyraxes are sometimes active by day [
26,
27,
28,
29], but, for the main part, they are nocturnal, especially in areas where they are hunted [
28]. They are believed to be solitary, and to rely on acoustic communication for advertising their position relative to one another during the night [
5,
24,
25,
29,
30,
31,
32,
33].
Since the publication of Hahn in 1934, there has been general agreement among zoologists that there are three species of tree hyrax: the southern tree hyrax
Dendrohyrax arboreus, the western tree hyrax
D. dorsalis, and the eastern tree hyrax
D. validus [
34]. Due to their arboreal and often nocturnal habits, tree hyraxes are difficult to study in the wild, and much of what is known, or inferred, about their distributions is based on museum records. The known range of
Dendrohyrax dorsalis versus the other two species is almost allopatric; there is only a small area of geographical overlap between
D. dorsalis and
D. arboreus in east-central Africa [
35,
36].
D. arboreus and
D. validus both occur in parts of Kenya and Tanzania, but the exact limits of their respective ranges are not well known [
37]. Kingdon [
38,
39] considered
D. validus to be the least specialised of the tree hyraxes, and suggested that it has been displaced by
D. arboreus in many localities in eastern Africa. Where both species occur,
D. validus usually lives at greater altitudes in montane forests [
23]. However,
D. validus may occur at sea level in areas where it is the only tree hyrax species present [
40]. In recent years, there have been suggestions that the species-level diversity of tree hyraxes is underestimated [
37,
41] Bearder et al. (2015) [
42] recorded unusual
Dendrohyrax calls in the field in southern Nigeria; the authors noted that these calls do not resemble those of
D. dorsalis, the only tree hyrax species supposedly found in this area. This suggests that there might be at least one other, scientifically undescribed tree hyrax species living in western Africa.
Dendrohyrax validus has been divided into a number of subspecies, of which four are widely accepted. The type locality of the nominate subspecies
D. v. validus is “Mt. Kilima-njaro” [
43,
44]. The subspecies
D. v. neumanni occurs on the island of Zanzibar [
45], whereas the type locality of the subspecies
D. v. schusteri is the Uluguru Mountains [
46]. Finally, the type locality of the subspecies
D. v. terricola is eastern Usambara Mountains, a few hours’ distance away from the Amani Research Station, and a few minutes’ distance away from the village of “Monga”, according to the original description [
47]. Notably, the type localities of all these four subspecies are in present-day Tanzania; none is located in present-day Kenya. Until relatively recently,
Dendrohyrax validus was not believed to occur in Kenya, and the country’s only native tree hyrax species was thought to be
Dendrohyrax arboreus. The presence of
D. validus in Kenya was finally confirmed in the 1970’s, when a small population was discovered circa 30 km north of Mombasa [
40]. Subsequent sightings and sound recordings have shown that
D. validus also occurs in the Taita Hills [
5]. The Taita Hills tree hyraxes are thought to represent the subspecies
D. v. terricola [
33]. However, this assumption is based on the relative geographical proximity to the Usambara Mountains across the Tanzanian border rather than on any detailed morphological or genetic studies. In fact, to our knowledge, as of yet no tree hyrax specimens have been collected from the Taita Hills [
48,
49]. The only published specimen records of
D. validus from Kenya that we are aware of are those of Seibt et al.; these authors discovered one complete and one partial skull, and a ‘mummified’ carcass of an adult male in the vicinity of the village of Vipingo, circa 30 km north of Mombasa along the coast of Kenya [
40].
Roberts analysed recordings from 16 forest populations of
Dendrohyrax validus and found at least three distinct subpopulations with extreme acoustic qualitative differences: (1) a subpopulation in the Taita Hills in Kenya and the East Usambara Mountains in Tanzania, (2) a subpopulation in the Uluguru and Udzungwa Mountains in southern Tanzania, and (3) a subpopulation in the islands of Zanzibar and Pemba at the Tanzanian coast [
5]. Roberts suggested that these populations should be subject to further behavioural and genetic research [
5].
1.2. Galago Taxonomy
Galagos are strictly nocturnal [
50,
51]. Consequently, they are often difficult to observe in their natural habitat. Like tree hyraxes, galagos are, however, very vocal animals, and their nocturnal calls are characteristic sounds of the African night.
The greater galagos, or thick-tailed galagos, of the genus
Otolemur are the largest of the galagos, weighing about 1–1.5 kg [
52]. The number of species in this genus has been a matter of debate [
4,
10,
53,
54,
55,
56,
57,
58,
59], but most authors agree that there are three species. According to Groves, these are the brown greater galago
Otolemur crassicaudatus, the silvery greater galago
O. monteiri, and the northern, or small-eared, greater galago
O. garnettii [
60]. All three species are found in various parts of eastern Africa, and
G. crassicaudatus and
O. garnettii locally occur in sympatry [
61]. Kingdon suggested that a fourth, undescribed
Otolemur species may live in south-eastern Tanzania [
59]. The only
Otolemur species recorded from the Taita Hills is
O. garnettii [
60].
O. garnettii is divided into several subspecies; the one that occurs in the Taita Hills is presumed to be
O. g. lasiotis, but the known distribution limits of another subspecies,
O. g. panganiensis, are not located far off [
13]. The subspecific identity of the Taita Hills greater galago thus requires confirmation.
Historically, most of the small-bodied galagos, as well as the
Otolemur species, were placed in only one genus,
Galago [
15,
60,
62]. Subsequently it has become evident that the extant galago taxa represent deeply divergent evolutionary lineages, despite their superficial similarity [
8,
63,
64]. Currently, several different genera of galagos are recognised. Apart from the lesser galago genus
Galago (
sensu stricto) and the greater galago genus
Otolemur, most authors recognise the dwarf galago genus
Galagoides, the needle-clawed galago genus
Euoticus, and the squirrel galago genus
Sciurocheirus. Recently, Masters et al. reviewed the systematics of the
Galagoides species group of dwarf galagos and found that the western and eastern African species of this genus do not form a monophyletic clade [
15]. Thus, Masters et al. separated the latter into a separate genus, which they named
Paragalago. The type species of this new genus is the Zanzibar galago, originally described by Matschie in 1893 as
Galago zanzibaricus [
45].
In 2002, Perkin et al. presented preliminary evidence for the existence of a scientifically undescribed dwarf galago taxon in the Taita Hills [
65]. Based on general similarities in the vocal repertoire between the Taita Hills dwarf galago and other eastern dwarf galagos, they suggested that this possibly new species, too, belongs to the East African clade [
65]. However, in the absence of a physical type specimen, this taxon is at present still unnamed [
66].
2. Materials and Methods
2.1. Study Area
The study was conducted in the Taita Hills, Kenya (03°22′ S, 38°20′ E) (
Figure 2). The Taita Hills represent the northernmost extension of the Eastern Arc Mountains in south-eastern Kenya. The Taita Hills rise abruptly from the surrounding dry plains at ca. 600–1000 m altitude above sea level (a.s.l.) to a series of mountain ridges, reaching 2208 m a.s.l. at the highest peak, Vuria [
67]. Average yearly temperature in Wundanyi, is 19.7 °C, the coldest month is July with the average temperature of 17.3 °C, and the warmest month is March with the average temperature of 21.8 °C [
68]. Average yearly rainfall is 1140 mm. Extended rains occur in March, April, and May, and shorter rains in November and December. The upper slopes of the mountains receive moisture brought by the trade winds, and the moisture is captured in sufficient extent to sustain evergreen montane forests. The moist and relatively cool climate sustains lush montane forests with a considerable epiphyte biomass [
69,
70,
71]. The University of Helsinki has a research station in Taita Hills, which served as a base for the fieldwork described in this paper.
The crystalline block-faulted Eastern Arc Mountains were formed 290–180 Myr BP [
72], and represent a well-known biodiversity hot spot [
73,
74,
75]. While the natural vegetation on the upper slopes of the Taita Hills consists of closed montane forest, long-lasting and intensive human influence has reduced the indigenous forest into remnant patches [
68,
76,
77]. The largest remaining patches of montane forests are Mbololo (220 ha) and Ngangao (120 ha). In addition, there are numerous small fragments of indigenous forest vegetation, many of them at sites regarded by the local people as sacred, and which therefore are afforded protection [
78]. Despite the efforts to conserve remaining forest patches and to reforest degraded areas with indigenous trees, the cover of montane forest further decreased between 2003 and 2018 [
79].
Our studies on nocturnal mammals were conducted in the two largest remaining fragments of indigenous montane forest in Taita Hills, Mbololo (3°19′37″ S, 38°27′4″ E, 1550–1700 m a.s.l.) and Ngangao (3°22′9″ S, 38°20′33″ E, 1700–1870 m a.s.l.) (
Figure 2). Both fragments are classified as moist montane forests [
70], and are situated on the steep upper slopes of north-south-oriented mountain ridges. While parts of Ngangao have been subjected to relatively intensive selective logging in the past, the largest upper canopy trees are over 50 m tall [
80,
81]. The multi-layered canopy of Ngangao is formed by many different tree species including
Tabernaemontana stapfiana,
Macaranga capensis,
Strombozia scheffleri,
Pouteria adolfi-friedericii, and
Newtonia buchananii. While no long-term climatic data are available, some data on climatic conditions of Ngangao have been published [
82,
83]. Parts of Mbololo receive abundant moisture from low-lying clouds and fog and appear wetter than most sites in Ngangao. Human–wildlife conflict is ongoing in and near the forest patches in the area; notably, blue monkeys
Cercopithecus mitis frequently raid farms close to the indigenous forests [
84].
2.2. Data Collection
Nocturnal observations and recordings were made from sunset at 6 p.m. until 1 a.m., a total of seven hours, or 2–7.30 a.m., a total of five and a half hours. We had one observation period for each night, usually shifting from evening to morning. During rainy nights, no observations were made, as animals were then generally quiet. Forests were studied in detail also during the day, to gain information of the forest characteristics, tree species, and tree hollows used by Paragalagos. Recordings were made by placing a recorder close to the location known to have high animal density with GPS location. Notes were taken from about a 5-m distance away, as this reduced sound disturbance caused by small movements of the observer. Forests were searched with transects during 20 nights between 2 January to 25 January to gain knowledge of which sites were preferred by the animals. Later we confined our movement to forest paths as this helped to reduce excessive noise. Our speed along the paths was circa 500 m/h. Whenever Paragalagos were encountered recordings were made opportunistically at standstill. Recordings were also made in proximity of the dwarf galago daytime sleeping sites at dawn and dusk, when family groups typically vocalize communally.
Fenix TK 25 red beam handheld flashlights (Fenix Lighting, Littleton, CO, USA) were used for visual observations. Only red light was used, as it does not disturb the animals. Flashlights were held at the level of the eye to be able to spot the animals’ eyeshine. Animals were photographed with an Eos D50 camera (Canon, Tokyo, Japan) equipped with a Canon EF 70–200 zoom and a Canon Speedlight 550EX flashlight. Data on vocalizations (total 256 h) were collected with an LS-12 recorder (Olympus, Tokyo, Japan) and an AT8015© microphone (Audio-Technica, Tokyo, Japan).
Animals were often first identified by the colour of their eyeshine. Dendrohyrax has yellowish eyeshine, but sometimes eyeshine was not visible. Dendrohyraxes were seen typically unmoving, resting on the large branches. Otolemur usually has reddish, and occasionally yellowish, eyeshine. Paragalago has the most reddish and distinct eyeshine, and can be separated from Otolemur by the fact that its eyes are set more closely to each other. Otolemur and Paragalago can be also be told apart from each other by their movement patterns and speed, and their pronounced difference in body size.
The material collected in January–March 2019 included 90 h of recordings during 55 observation nights in the Ngangao and Mbololo Forests. These data were used for preliminary analysis, and they were compared with recordings stored by the Nocturnal Primates Research group and by Wildsolutions,
http://www.wildsolutions.nl. Wildsolutions is the website of the Eastern Africa Primate Diversity and Conservation Program. The site also has vocal profiles for galagos and tree hyraxes. Names of the calls used here are the same as used by Wildsolutions. The loud calls of each species were chosen for analysis from preliminary material. The analysed vocalizations included Dendrohyrax strangled thwack (
n = 174) and hac (
n = 79) calls, Otolemur trailing (
n = 78) and cluster squawk (
n = 47) calls, and Paragalago incremental (
n = 31), chatter (
n = 43), chirrup (
n = 54) and yap (
n = 45) calls. Dendrohyrax song syllables are described briefly and here named for the first time.
An analysis of 176 h of recordings taken in June–August 2019 is presented here (see
Supplemental Data S1). This material was collected in the Ngangao Forest. Only distinct, close-range recorded calls with no interfering katydid songs [
85,
86] or other animal calls were included in the analysis. In all species, graded and mixed calls were included in the analysis, and categorized as the call type that it resembled the most. As calling animals were impossible to recognize individually, more than one call by the same animal may have been included in the analyses. For Dendrohyrax, the probability of repeatedly recording the vocalizations of the same individual was reduced by analysing only one call for each sequence and locality. The recordings included thousands of Dendrohyrax calls, which reduced the probability of analysing the same animal repeatedly. For Otolemur, all high-quality trailing calls were analysed, and a number of cluster squawks were randomly selected for analysis. The trailing calls were recorded in many different locations; however, it is possible that calls of some individual animals were analysed more than once. For Paragalago, all high-quality incremental calls (
n 31) were analysed. As we could only obtain calls from the single known population in Ngangao, consisting of only ca. 10 individuals, we could not avoid recording individual animals repeatedly. In Paragalago chatter, chirrup, and yap samples, calls were sampled randomly as our recordings included hundreds of individual chatters, chirrups, and yaps. Thus, we were able to provide a detailed picture of variation in the calls of this single population. Unit distances of calls were calculated from recordings when only one individual was vocalizing.
2.3. Data Analysis
The audio data were analysed with sound software RAVEN PRO 1.5 (Cornell University, Ithaca, NY, USA) using the following spectrogram parameters: DFT size 512, 50% overlap, hann 86.1 Hz, sample rate 44,100 Hz, 16-bit signed. Ten parameters were measured for each call. These were: (a) fundamental frequency; the lower frequency bound of the selection (Hz), (b) high frequency; the upper frequency bound for the selection (Hz), (c) call duration – delta time; difference between start time and end time for the selection (s), (d) maximum frequency; frequency with maximum power (Hz), (e) Q1 frequency; divides the selection into two frequency intervals containing 25% and 75% of the energy in the selection (Hz), (f) centre frequency; divides the selection into two frequency intervals of equal energy (Hz), g) max entropy; describes entropy distribution within the call. Higher values correspond to greater disorder (bits), (h) delta frequency; difference between upper and lower frequency limits on the selection (Hz), (i) energy; total energy within selection bounds (dB), (j) peak power (dB); maximum decibels within the call measured at the maximum point.
When analysing calling strategies from whole sequences, we investigated inter-unit interval (time between two calls, measured in s), redundancy, combination of calls, and answers coming from conspecifics. Inter-unit intervals of tree hyrax calls were calculated from sequences when the animal was calling alone. Otolemur trailing calls and Paragalago incremental calls were compared with calls described in previous studies [
12,
13,
52].
Data analysis was carried out using R Studio version 1.2.5033 (The R Foundation for Statistical Computing, Vienna, Austria) and SPSS, version 25 (SPSS Inc., Cary, NC, USA, following protocols described by Zuur et al. [
87] (
Supplementary Figure S2). We used multinomial regression analysis to test whether we had classified the calls correctly.
Covariates entered into analysis were delta frequency, energy, maximum entropy, peak power, delta time, maximum frequency, first quartile frequency, and central frequency. We examined homogeneity of variances from scatterplots by using residuals of multiple regression by plotting residuals vs. fitted values and making boxplots of the residuals. We examined normality for each group and covariate by using histograms. As the data did not meet the assumptions of homogeneity required for discriminant function analysis, we used multinomial linear regression (MLR) instead. Outliers were identified using Cleveland dotplots and Cook’s distance. Outliers left in the data are natural outliers, showing true variation in the calls. All assumptions required for multinomial regression analysis were met. Data exploration graphs are available in the
Supplemental information (Figure S2).
Variables entered in the multinomial regression analysis were delta time (s), maximum frequency (Hz), and max entropy (bits). All variables were significant in MLR analysis. MLR analysis classified 82.8% of the calls correctly (
Table 1).
If we had used energy or peak power in the analysis there would have been 2.8% better results from MRL analysis (85.6%). However, peak power and energy are related to the distance of the calling animal from the microphone. As distance was unknown, we chose to leave peak power and/or energy out of the MLR.
4. Discussion
Our analyses of loud calls of Dendrohyrax, Otolemur, and Paragalago in the Taita Hills provide new insights into their taxonomic status. In the Taita Hills, Dendrohyraxes make a distinctive loud call, the strangled twack, the like of which has not been recorded elsewhere. Here, we characterized this call in detail, as well as another distinct call type, the hac (
Figure 4). The recorded loud calls are distinctly different from those recorded from
Dendrohyrax arboreus (
Figure 12A
Supplemental Sound S19) or
Dendrohyrax validus (
Figure 12B
Supplemental Sound S20). [
5,
88]. However, similar calls have been previously recorded in East Usambara and West Usambara and Pare Mountains, and possibly also in Mt Kilimanjaro and Mt Meru [
5,
33]. Strangled thwacks are most likely advertisements of the social status of male Dendrohyrax, comparable to the snorts of rock hyrax males [
89]. This type of call has only been recorded in the Taita Hills and the Usambara Mountains [
5], which indicates that these animals may represent an as of yet undescribed taxon.
The strangled thwack of Dendrohyrax is a very loud call with a large frequency range of over 16,000 Hz. This call was never repeated for more than 20 times during one vocalization session, but the sequence often continued as a series of hac calls that have a smaller frequency range of about 9000 Hz. Calling in sequences that last for minutes is energetically costly. This is especially true for a herbivorous mammal such as Dendrohyrax, which lives on an energy-poor diet of fibrous leaves and which presumably, like other hyraxes, has a low metabolic level [
23,
24]. Vocal turn-taking and counter calling is essential part of Dendrohyrax acoustic communication. Our recordings also suggest that Dendrohyraxes are adding variation and nonlinear noises to their calls. This may facilitate individual recognition and elicit responses from other individuals [
90]. The active counter-calling of Dendrohyraxes in the Taita Hills suggests that their social life is more complex than previously thought [
24].
In the Taita Hills, Dendrohyraxes occasionally sing, and while singing they use long tonal whistles and repeated syllables in tight, short, broadband pulses. One type of syllable is repeated 10–70 times, after which the animal switches to another type of syllable (
Figure 6). To our knowledge, our recordings and descriptions of Dendrohyrax songs are the first from anywhere in Africa. Song structures have been previously described in the rock hyrax
Procavia capensis [
91,
92,
93,
94]. Rock hyrax males use these songs to advertise their fitness to females. In
Procavia capensis, only one in three of the adult males sing, these songs being individually different [
92]. Songs of
Procavia capensis [
89,
91,
92] and Dendrohyrax in the Taita Hills have similarities in their syllables. Strictly harmonic songs are rare, and deterministic chaos is very common. More research is needed on these songs and their behavioural meaning.
Otolemur garnettii has an extensive vocal repertoire with 12 different call types [
13,
50]. Otolemurs in Taita Hills also have several different calls, but the only loud calls they use are the trailing call and the cluster squawk. The trailing call is a loud and long call that is used for advertising and for maintaining social distance. It begins loudly with several notes and then trails away [
95]. The squawk call is shorter, and bark-like. It may be used as a counter-call with trailing calls, or as a mild alarm. The Taita Hills are situated near the transition zone between the ranges of two small-eared greater galago subspecies,
Otolemur garnettii lasiotis and
O. g. panganiensis [
96]. We thus compared the trailing calls of
O. g. panganiensis [
13],
O. g. lasiotis [
11], and the Otolemurs from the Taita Hills (
Table 5). Acoustic comparison confirms that the subspecies in the Taita Hills is
O. g. lasiotis. The main difference between the calls of different subspecies is the peak frequency of the call, which in the Taita Hills is 1593 Hz, whereas peak frequency is 819 Hz in
O. g. panganiensis and 1320 Hz in
O. g. lasiotis. The peak frequency in
O. g. lasiotis is much closer to the peak frequency found in the Taita Hills, and for all calls in the Taita Hills, the peak frequency was above 1200 Hz.
O. garnettii is believed to be mainly solitary [
52]. However, in the Taita Hills, Otolemur appears to live mostly in pairs. In Kwa Kuchinja, Tanzania,
O. garnettii has also been observed to sleep in pairs or even in groups of four [
13]. It is possible that this species can alter its social behaviour according to environmental conditions.
There are clear behavioural differences between Paragalago individuals in the Ngangao and the Mbololo Forests. In Ngangao, the animals are either indifferent or inquisitive towards humans, sometimes even displaying mobbing behaviour towards observers. The animals in Mbololo are very shy and difficult to observe or photograph. In Mbololo, Paragalago advertisement call is a curtailed version that resembles the advertisement call of P. orinus. We never heard Paragalago use the incremental call in Mbololo. A possible explanation may be that the advertisement call of the geographically isolated Paragalago species from Mbololo has diverged over time from the more typical call of P. cocos. Alternatively, the Mbololo Forest Paragalago is indeed P. orinus.
Paragalago call recordings from Ngangao were listened to by several primatologists and identified unanimously as
P. cocos [
66]. Also, comparison of acoustic parameters demonstrates that
Paragalago cocos (recorded from Diani, Kenya) [
12] and the Paragalago in the Taita Hills use the same acoustic range in all parameters (
Table 6). However,
P. cocos lives in the coastal areas of Kenya, and in the lowlands along the Tana River. The closest
P. cocos populations live at a distance of 160 km from the Taita Hills. The intervening area between these areas and the Taita Hills consists of dry, mostly treeless savannah and shrub-land, which is presumably impossible for forest-living dwarf galagos to cross. Altitudinally,
P. cocos ranges up to at least 350 m a.s.l. on the northern slopes of the East Usambara Mountains in Tanzania [
12,
97].
P. cocos lives in dry, mixed, coastal forests with tree heights of 15–20 m [
52]. In the Taita Hills, the Paragalagos live at altitudes between 1550–1900 m a.s.l., in the most pristine parts of an indigenous moist montane forest where the upper canopy reaches 50 m. The dusk chorus of coastal
P. cocos is distinct whereas the morning chorus is modest [
12]. In contrast, in the Taita Hills, the morning chorus is loud and the evening chorus modest.
The closest relatives of
P. cocos are the Tanzania coast dwarf galago
Paragalago zanzibaricus and the Mosambique dwarf galago
Paragalago granti [
15]. Based on an analysis of vocalizations and DNA studies, Pozzi et al. [
14] concluded that
P. cocos and
P. zanzibaricus represent two cryptic species that probably underwent speciation in the Late Pliocene, when their populations became isolated from each other by the fragmentation of the East African forests. Molecular analyses and comparisons with other
Paragalago taxa are required to establishthe taxonomic identities of the Taita Hills Paragalagos.
The Paragalago population of Ngangao Forest is almost extinct with apparently no more than 10 individuals remaining. Several groups of Paragalago live scattered across the Mbololo Forest, but even there the species is not abundant. [
66]
From a biogeographical point of view, it is interesting that
Dendrohyrax,
Otolemur, and
Paragalago are all found also in the East Usambara Mountains in Tanzania, which are located only circa 160 km south of the Taita Hills.
Otolemur and
Paragalago are also known from Diani, Kenya, 170 km southeast of the Taita Hills. The remaining forests covering the Eastern Arc Mountains are home to a mammalian fauna with a high level of endemism [
38,
39,
72,
98]. Many of these Eastern Arc Mountain endemics or near-endemics are forest specialists that have poor abilities to disperse across extensive distances of non-forested habitat (e.g., [
99,
100]). Some taxa may be considered ‘relicts’, i.e., they represent old inhabitants of the area that have subsequently been replaced in the surrounding areas by later-arriving related taxa. Kingdon [
38,
39] suggested that
Dendrohyrax validus, in particular, represents such a case, and that its current restricted distribution in eastern Africa is a consequence of having been outcompeted by its ecologically similar relative
Dendrohyrax arboreus.