Status, distribution and research gaps of rodents (Mammalia: Rodentia) in North-Eastern States of India

In North Eastern states of India (NE India) there are almost 65% of mammal species of the country but baseline information on small mammals, particularly rodents, for the region is scanty. Present study recorded a total of 59 species of rodents from the NE India out of 100 species reported from Indian Subcontinent. The list contains all the valid taxonomic names and their distribution in the states of NE India. Additionally, five species has been added to the checklist of rodents in India. The list provided 59 species belonging to 30 genera under 5 families of 7 subfamilies. Among them Muridae was recorded to be with highest number of species (31 species), followed by Sciuridae with 22 species, Cricetidae with three species, while Spalacidae and Hystricidae have recorded only two species in each group. Among the eight states of NE India the highest number of species was recorded from Arunachal Pradesh (76 %) and the lowest, from Tripura (22%). Two Threatened, three Near Threatened, two Not Listed and six Data Deficient species have been listed from the present work with four endemic species from this region. The findings of this study indicated the requirements for intensive and extensive surveys in the north-eastern States of India and taxonomic revisions of many species.


Introduction
Past few decades have shown promising results in the discoveries of rodent species from the south-eastern Asia. Several new species of genus Petaurista Link, 1795 have been discovered or recorded from India (Choudhury 2007;2009;2013) while genus Biswamoyopterus Saha (1981) stands no longer endemic to India (Sanamxay et al. 2013;Li et al. 2019). Sommeromys macrorhinos Musser and Durden (2002) has been discovered as a new genus and a species from central Sulawesi. Auffray et al (2003) described a new species of subgenus Mus as M. fragilicauda from Thailand. Jenkins et al (2005) described a new family Laonastidae, genus Laonastes and a species L. aenigmamus from Lao People's Democratic Republic. Heany et al (2011) described seven new species from Philippines with a new description of a subgenus Apomys (Megapomys). Ge et al. (2020) reviewed the Niviventer species complex from the specimens studied at China through morphological and molecular approach and described Niviventer fengi as a novel species. Despite of these recent studies, rodents have been always neglected and less progressive than the other taxa (Lee and Liao 1998). Among all mammals, in spite of being the most diverse order, rodents are also less cared animals for conservation (Amori and Gippoliti 2003). Lesser advancement in the studies of rodent taxa may be due to its larger consideration as a pest species although evidence suggests that rodents are tool for promoting biodiversity (Reichman and Seabloom 2002). Majority of the rodent species has been marked as exploiters of human agricultural resources and several species are known to carry potential harmful diseases for humans (Singleton et al. 1999). Despite of being a harmful animal to humans, rodents are known to contribute several important aspects in balancing the natural world. Studies have shown that rodents are not only an important seed dispersal animal and serves as a prey base for most of the mammalian and avian carnivores in forests but also act as a supressing agent for alien invasive species (Eviner and Chapin 2003), contribute significantly in biogeochemical cycles, an indicator species for industrial pollution, forest health and fragmentations, and are also known to act as sensitive barometers for changing climate (Singleton et al. 1999).
Scientific studies of rodents in India can be dated back to pre-independent British colonial years (Agrawal 2000). One of the major taxonomic reviews and systematics on rodents of Indian subcontinent were conducted by Ellerman (1961). Compilation of such work fuelled years of scientific progress in the upcoming years. Following major revision of the rodent taxa focusing only on the family Muridae and Hystricidae was accomplished by Agrawal (2000). Since the works of Agrawal (2000) several changes have occurred in global taxonomy of rodents as subsequently the present paper will discuss later in details. New information on distributions and systematics based on molecular evidences has also provided several insights in the rodent taxonomy of world (Yu et al. 2006;Sanamxay et al. 2013;Ge et al. 2020). Last validation of rodent species was provided by Pradhan and Talmale (2011) listing a total of 103 species from India.
Covering about 8% of the total geographic area of the country, North Eastern states of India (NE India) is considered to be the highest in terms of mammalian diversity including 65% of the country's total mammal species (Choudhury 2013). However, only a handful and scattered reports on Rodents are so far available from the NE India. This region was selected as study site owing to its importance in terms of biodiversity because of its unique flora and fauna. North Eastern States of India harbours two biodiversity hotspots viz. Eastern Himalaya and Indo-Burma Hotspot (Chandra et al. 2018). Studies for assessing the presence of faunal species in the NE India have been conducted in the past and are compiled as State Fauna Series by the Zoological Survey of India (Das et al. 1995;Bhattacharya and Ghosh 2002;Chattopadhyay et al. 2006;De et al. 2006). Most of these studies are survey based or based on secondary information. Few articles are although based on some aspects of the rodents viz. rodents and the bamboo flowering (Bhattacharjee et al. 1979;Pathak and Kumar 2000;Chauhan 2003;Biswas et al. 2016), threatened rodent species of Arunachal Pradesh (Kumawat et al. 2013), diversity of gliding squirrels (Krishna et al. 2016). Ecology, population, demography and effect of human induced habitat fragmentation and encroachments are not known. Moreover, presence of several cryptic species assemblage viz. Leopoldamys, Niviventer, Rattus or Mus species complex and poor taxonomic studies on rodents with both morphological and molecular tools and inaccessibility at several locations may have slowed down the rodent research exploration in NE India. Current and regional conservation status for the rodents in these regions are in need of assessment. Considering the lack of substantial and basic information, a checklist of rodent species has been worked out to act as a base line information for the future studies in NE India.

Study Area
NE India comprises of eight states namely, Assam, Arunachal Pradesh, Mizoram, Manipur, Meghalaya, Nagaland, Tripura and Sikkim. These states cover three biogeographic zones viz. the Trans Himalaya, Himalaya and North East India representing 5 provinces namely Trans Himalaya Sikkim (1C), Central Himalaya (2C), East Himalaya (2D), Brahmaputra Valley (9A) and North East Hills (9B) ( Rodgers and Panwar 1988) and holds distinguished 51 types of forest types under 13 broad categories viz. tropical wet evergreen forests, tropical semi-evergreen forests, tropical moist deciduous forests, littoral and swamp forests, tropical dry deciduous forests, subtropical broad-leaved hill forests, subtropical pine forests, montane wet temperate forests, Himalayan moist temperate forests, Himalayan dry temperate forests, sub-alpine forests, moist alpine scrub and dry alpine scrub (Champion and Seth 1968). NE India shares international borders with Bangladesh, Bhutan, China, Myanmar and Nepal which is almost about 2000 km 2 . The area acts as a geographic gateway for its distinctive flora and fauna due to its sub-tropical belt extending from the foothills of Himalaya in the west to east up to the south eastern China ).

Methodology
The present checklist was compiled through examining the specimens present in the National Zoological Collection, Kolkata. Also, literatures from the archives of Zoological Survey of India, State fauna series, and published peer reviewed research articles and reports available online/offline was used to document the basic species checklist for the present study. Taxonomic positions and the validity of individual species as well as their present status was confirmed using Wilson and Reeder 2005;Pimsai et al. 2014;Ellerman 1961;Musser, 1973Musser, , 1981Agrawal 2000. This compilation has taken into account all the species with current valid/updated names based on the present available online literatures. Map of the study area was prepared using ArcGIS 10.4 (ESRI 2016). Graphs were created using online data visualization website (Anonymous 2020).

Results and Discussion
Current compilation resulted in 59 species of rodents under 30 genera from 5 families and 7 subfamilies from North-Eastern states of India as against to 100 species of rodents which belongs to 46 genera representing 7 families from India ( Kamalakannan and Venkatraman 2017).
Family Muridae (rats and mice) has contributed with maximum number of species (31), followed by Sciuridae (squirrels) with 22 species, Cricetidae (mountain voles) with three species, Spalacidae (bamboo rats) and Hystricidae (porcupines) with two species in each group (Fig. 3). Arunachal Pradesh (76%) recorded the highest number of rodent species and Tripura (22%) recorded the lowest number of rodent species among the NE states of India.
There is a wide gap between the number of rodents reported from Arunachal Pradesh and the rest of the states in NE region (Fig. 2). Being within a global biodiversity hotspot (Velho 2013) and also much more number of focused explorations could be a possible reasons for the higher record of rodents from Arunachal Pradesh. Similarly, the low record of rodents in Tripura may be attributed to poor field surveys undertaken in the state. Out of the recorded 59 species, Biswamoyopterus biswasi Saha, 1981, is considered to be as Critically Endangered as per IUCN Redlist, Hadromys humei (Thomas, 1886) as Endangered, Ratufa bicolor (Sparrman, 1778), Petaurista nobilis (Gray, 1842) and Petaurista mishmiensis Choudhury, 2009listed as Near Threatened, Belomys pearsonii (Gray, 1942, Petaurista mechukaensis Choudhury, 2007, Berylmys mackenziei (Thomas, 1916, Berylmys manipulus (Thomas, 1916), Dacnomys millardi Thomas, 1916 and Diomys crumpi Thomas, 1917 as Data Deficient and two species, Petaurista yunanensis (Anderson, 1875) and Petaurista siangensis Choudhury, 2013 has not been listed so far (Table 1). Also, the present checklist reports five species of Genus Petaurista which has not been mentioned in the checklist of Indian mammals or Indian rodents before (Kamalakannan and Venkatraman 2017;Kamalakannan et al. 2018) which may ensure the number of rodent species in India to be 105 species (Krishna et al. 2016). Rest of the recorded Rodent species are Least Concerned as per the IUCN Redlist (Table 1). Apart from their conservation status, four species viz. P. mishmiensis Choudhury, 2009; P. mechukaensis Choudhury, 2009; P. siangensis Choudhury, 2013 andH. humei (Thomas 1886) are known currently as endemic to India. Current status and distribution of the Rodents in north-eastern states of India is in need of focused survey and reassessment as plenty of misleading information and gap areas exist. For instance, there has been a long debate between the validation of Petaurista caniceps and P. elegans. While Corbet and Hill (1992) considered these two species to be sympatric, Wilson and Reeder (2005) kept P. caniceps as subspecies of P. elegans. Molecular studies (Yu et al. 2006) have shown promising results separating these two species and validating their distinct speciation as well as it is found in the morphological characteristics. P. caniceps has been found to maintain a distinct lineage from P. elegans with a difference of more than 16% in the molecular analysis. Such a large genetic distinction should be considered as separate species. P. caniceps lack any white speckling on its back and has a grey forehead. These characters alone separates the species from the rest of the flying squirrels of genus Petaurista (Li et al. 2013). Still IUCN and Checklist of mammals in India (Sharma et al. 2014) have not considered P. caniceps as a valid species. Although the species P. alborufus has been reported from East Khasi hill, Meghalaya, India since 1995 based on museum specimen collected way back in 1847 and recently  have stated how the identifications were validated in their article without examining a collected specimen. Neither does they explain how a species ( Funumbulus pennantii), which is well known to be restricted in its distribution up to the western side of Rajghat gap, has been located beyond this geographical barrier ( Thorington and Cifelli 1990 Kamalakannan et al. (2018) has also mentioned this species to be present in the eastern Himalayas. Another species Mus booduga has been enlisted in the species list of Kamalakannan et al. (2018) as distributed in eastern Himalayas, whereas the species has never been reported from any of the north eastern states so far (Agrawal 2000;Wilson and Reeder 2005;. The species Leopoldamys sabanus (Thomas 1887) is reported from the Changlang district of Arunachal Pradesh but is not confirmed whether it is L. sabanus or any other species. Its current status is therefore unconfirmed (Srinivasulu and Srinivasulu 2012). Under these circumstances it is highly recommended for intensive and extensive surveys for rodents in the North-eastern states of India and to put more focus on the taxonomic revisions of these species. Survey in the state of Tripura is also suggested, as it is the least recorded rodent species state in the north-eastern states. Apart from the taxonomic surveys, great deals of ecological knowledge of these rodent species calls for more studies required to be conducted on their population and habitat features. While examining the data it was observed that the distribution of Diomys crumpi was restricted only to the state of Manipur in the present study. The Northern most population of this species has been recorded from Nepal and the southernmost population from Myanmar. There is a possibility the species might also be therefore  distributed in other states of the study area. Golunda ellioti Gray 1837 is reported only from Assam in the present study, whereas it is reported to be present throughout the country and also in Nepal (Kamalakannan and Venkatraman 2017). Possibility of this species being distributed in the other states of North East India is also possible. A. stoliczkanus, N. sikkimensis and N. eha which are typically high altitude species, have been found to be restricted only to Sikkim. These species may also be distributed in the higher altitudes of Arunachal Pradesh. Distribution of B. biswasi, N. brahma, L. sabamus, A. draco, A. latronum and 5 species of Petaurista have been recorded only from Arunachal Pradesh (Table 1). M. himalayana, P. elegans, P.caniceps, P.nobilis and R. pyctoris are found only in Arunachal Pradesh and Sikkim within the North-eastern states of India (Table 1). Furthermore, status of the critically endangered species of B. biswasi Saha 1981, and also P. mechukaensis Choudhury 2007, P. siangensis Choudhury 2013, P. mishmiensis Choudhury 2009, andP. yunanensis (Anderson 1875) reported only from the state of Arunachal Pradesh, is recommended to be assessed as no report from the region after its discovery has been noted recently.