A study of scientific publications on the greater cane rat (Thryonomys swinderianus, Temminck 1827)

Abstract Background The greater cane rat (GCR), reputed to be African's second largest rodent, is a precocial hystricomorph with an uncommon phenotype and life history. Scientific and socio‐economic interests in the GCR have led to heightened research efforts targeted towards a better understanding of its biology and exploration of its economic and translational usefulness. Methods Records of all online scientific publications on the GCR from Google, Google Scholar, PubMed, science.gov, Ebscohost and Worldwide science, with the exception of research theses, proceedings, unpublished projects and abstracts, were collated and analyzed using descriptive statistics. Results A total of 146 published scholarly articles spanning about six decades were retrieved, with 98% of the GCR publications originating from African countries. Nigeria boasts the highest number of publications (58.22%) followed by Ghana (21.23%) and South Africa (5.48%) while Senegal contributed the least (0.69%). Publications were sorted into ten field categories. The field with the highest number of articles (41.78%) was animal breeding and management recording, closely followed by anatomy (37.67%). Lesser contributions were made by parasitology (5.48%), biochemistry/hematology (4.8%), pharmacology/toxicology (4.11%), pathology (2.06%), and surgery/anesthesia and physiology (1.37% apiece). The fields with fewest contributions were microbiology and developmental biology (0.69% each). Conclusion This study chronicles the spectrum of knowledge available on the GCR, highlighting the knowledge gap that still exists in various fields in order to provide advocacy for new frontiers in research efforts on this rodent. We suggest the need for a clearly defined and well integrated national/regional policy aimed at establishing Africa's foremost micro‐livestock rodent, the greater cane rat, on the world's scientific radar.


| INTRODUC TI ON
African cane rats belong to the family Thryonomyidae, which is represented by a single genus, Thryonomys. Nearly all the earliest fossil thryonomyids are African, suggesting that the family originated there in the early Miocene. The group diversified to a maximum of six genera during the middle Miocene, 1

and by the
Pliocene only the genus Thryonomys remained. 2,3 Misonne 4 and Woods 5 have provided short reviews of extant species of thryonomyids, concluding that only two species exist at present. These are the greater cane rat (GCR) (Thryonomys swinderianus, Temminck 1827) and the lesser cane rat, (Thryonomys gregorianus, Thomas 1894). 6 Although, other species such as T logani 7 and T arkelli 8 were initially described, they are no longer recognized as taxons distinct from T swinderianus. 9 The GCR is commonly referred to in Nigeria as "oya" in the Yoruba parlance, "nchi" by the Igbos, "Gegbi" by the Hausas, "agouti" by French-speaking Africans, "grasscutter" in West Anglophone Africa, and "hedgehog" in Central Africa. 10 They are predominantly found in the humid and sub-humid regions of Africa, from the far west (Senegal) through the grasslands of East Africa to Southern Africa, extending to the eastern Cape. 11 The presence and predominance of dense thick cane-like grasses such as elephant grass (Pennisectum purpureum) and guinea grass (Panicum maximum) influence the geographical distribution of these rodents. As obligate herbivores, they feed primarily on reeds, roots, shoots and stems of grasses using their broad and sharp incisors to cut easily through tough plant materials. However, they can be major pests in sugar cane, wheat and maize fields. 12,13 The skin of GCR has very coarse hairs speckled with yellow or grey, giving it a bristly appearance. 14,15 With a broad head, round muzzle, small round ears and a short tail, these hystricomorphic rodents are heavily built, reaching up to about 9 kg. 16,17 Unsurprisingly therefore, they are reputed to be the fourth largest extant rodent 18,19 and the second largest African rodent after the African porcupine, Hystrix africaeaustralis. [20][21][22][23] Their large carcass yield and the high nutritional value of their meat make them a premium alternative source of protein, especially in sub-Saharan Africa, where they command high prices leading to huge economic returns. [24][25][26][27] Furthermore, their meat is widely accepted, with few to no cultural and religious prohibitions, thus making them Africa's foremost micro-livestock. 6,28,29 This precocial African rodent has an uncommon phenotype and life history, with an unusually long gestational period of 150 days. 30 Reproduction occurs all year round and litter sizes vary from 2 to 4. 6,31 Pups are born with eyes wide open with thick fur on their skin and become accomplished runners shortly after birth. 6 Recently, scientific and socio-economic interest in the GCR has led to increased research efforts to better understand of its biology and explore of its economic and translational benefits. The aim of this paper therefore is to chronicle the spectrum of knowledge available on the GCR over a period of six decades, highlighting the knowledge gap that still exists in order to provide advocacy for new frontiers in research efforts on this rodent.

| ME THODS
This review of published literature on the greater cane rat (GCR) was conducted at the Federal University of Agriculture Abeokuta (FUNAAB). It was predicated on a database search using the search engines Google, Google Scholar, PubMed, science.gov, Ebscohost and Worldwide science, in no particular order, to access online journals. The keywords used were "greater cane rat", "GCR", "grasscutter", "African greater cane rat". All publications up until July 2019 were collated and analyzed. Theses, proceedings, unpublished projects and abstracts were excluded from this search. All pooled publications were broadly classified based on the following field categories: anatomy, animal breeding and management, biochemistry/hematology, surgery/anesthesia, microbiology, parasitology, pathology, pharmacology/toxicology, physiology, and developmental biology. The field of anatomy was further subdivided into various body organ systems, namely: circulatory, digestive, endocrine, immune and lymphatics, integumentary, muscular, nervous, respiratory, skeletal, and urinary systems. In addition, publications were grouped into originating countries based on the institutional affiliations of lead authors. Publication rate per decade was defined as the total number of publications within a decade divided by the time period under consideration (10 years), while publication differentials was calculated as the differences in publication rates between successive decades. All data generated were presented in pictorial (bar and pie) charts and linear graphs using Microsoft Excel (version 2016).

| RE SULTS
A total of one hundred and forty-six (146) published scholarly articles were found using a combination of selected search engines. The field of physiology was the first to be explored in this rodent, with a report by Ewer 12 on the form and function of the GCR describing its structural and behavioural primary feeding adaptations.
No other significant contribution was made in this field until 43 years later when Yapi et al 32 (Figures 6 and   7). This field was further classified based on body organ-systems.
The sub-field with the greatest number of publications was reproductive anatomy (36.4%), followed by neuroanatomy (23.7%), and the digestive (16.4%), skeletal (10.9%) and the integumentary and urinary systems (3.6% apiece). Fewest publications were allocated to the fields of muscular, immune and lymphatics and endocrine systems, each comprising 1.8% of the total (Figure 8). No article on the cardiovascular and respiratory systems of the GCR was found in this search.
Eight publications, representing 5.48% of the total number of articles found on the GCR, were allocated to the field of parasitology (Figures 6 and 7). In a survey of ecto-and endo-parasites carried out in Ghana by Yeboah and Simpson, 41  trematodes and 2 cestodes were identified in domesticated rats. 43 In Gabon, Jori et al 44

identified Paralibyostrongylus hebreniticus
and Taenia species at post mortem in captive cane rats. Natural occurrence of hemoparasites such as Trypansoma spp., Plasmodium spp. and Babesia spp. in wild and captive-reared GCRs have also been reported. 42 To date, only 7 species of ticks have been found on the body of wild GCRs and none has been identified on their captive-bred counterparts. These ticks include: Ixodes aulacodi,

Ixodes sp Rhipicephalus simpsoni, Rhipicephalus (Boophilus) microplus, Amblyomma compressum, Haemaphysalis parmata, and
Haemaphysalis leachi. 41,45 Only 2.06% of the total publications on the GCR were allocated to the field of pathology (Figures 6 and 7). Stressful conditions as a result of transportation, poor handling, traumatic injuries, respiratory diseases, especially pneumonia, and septicaemia have been identified as the leading causes of mortality in the GCR. [46][47][48] In most instances, Klebsiella pneumonia, Staphylococcus aureus, Streptococcus D haemolyticum and Candida albicans were isolated from postmortem lung samples of this rodent. 44 Neoplasia, although rare, has been reported by Jori and Cooper. 49 The least number of publications were allocated to the fields of microbiology and developmental biology, each comprising 0.69% of the total (Figures 6 and 7).
Recently, developmental milestones across the entire period of gestation in GCR using external morphogenetic features were reported by Mustapha et al. 31 The authors noted that the GCR had a relatively longer period of embryogenesis and a consequently shorter period of fetogenesis compared to other precocial mammals like the guinea pig, sheep and pig. They also reported evidence-based findings that prenatal development in the GCR might be associated with a reproductive delay. 31

| D ISCUSS I ON
One hundred and forty-six (146) scientific publications on the GCR were found from an online search covering a period of six decades.
Considering the exclusion criteria deployed in this study, this publication pool, while extensive, is not exhaustive. The increasing rate of publications on the GCR, particularly the spurt in the last ten years (2009-2019), attests to a recent rise in research activity on this rodent. This is traceable to the current drive by African scientists to identify indigenous animals suitable for domestication and use as spontaneous research models, especially within the African context. 19,31,50,51 The publication spurt also highlights an increase in awareness and exploitation of this rodent for economic purposesthe GCR is regarded as providing premium, choice meat, particularly in West Africa, where they command high prices leading to huge economic returns. 28,29 It is therefore unsurprising that the fields of animal breeding and management, and anatomy were most highly represented among all publications on the GCR. The lower percentages recorded for other fields such as parasitology, physiology and developmental biology, amongst others, reflect a huge dearth of information on the biology of this rodent in health and disease. For instance, while pneumonia has been reported as one of the major causes of mortalities in the GCR, 44,48 it remains to be ascertained whether there are morpho-physiological configurations and/or predispositions that make the GCR susceptible to this condition.
All publications excluding three from France originated from African countries. This comes as no surprise as the GCR is predominantly found in Africa. 11 Specifically, the GCR is found in the humid and sub-humid areas of Africa, south of the Sahara, inhabiting virtually all countries of west, east and southern Africa. 52  Evidenced-based findings suggesting a reproductive delay in the GCR were recently reported by Mustapha et al 31 However, the exact type(s) of delay exhibited by this rodent remain to be fully ascertained. More studies are therefore needed in order to investigate, characterize and confirm this. It will also be interesting to know whether this delay is specific to the GCR or rather a phenomenon of precocial hystricomorph rodents.
This study has highlighted the spectrum of research opportunities yet to be explored in various fields of study that will contribute to a better understanding and further domestication of this rodent.
It also underscores the need for a clearly defined and well integrated national push to establish Africa's foremost mini-livestock rodent on the world's scientific radar. This can be achieved by concerted and purposeful scientific investigation targeted towards exploring its suitability as a spontaneous indigenous research animal model while also leveraging its huge economic potential, especially in sub-Saharan Africa.