Introduction

Tigers (Panthera tigris) are the largest of all living felids, Asia’s largest terrestrial predator, and one of the most recognizable and charismatic species on earth (Sunquist 1981; Seidensticker and McDougal 1993). They were historically classified into nine sub-species (GTI 2011), with a recent genetic evolutionary analysis by Liu et al. (2018) identifying six extant sub-species (Bengal tiger, Amur tiger, South China tiger, Sumatran tiger, Indochinese tiger, and Malayan tiger) and three extinct sub-species (Javan tiger, Bali tiger, and Caspian tiger). The International Union for Conservation of Nature (IUCN) red list identifies tigers as an ‘endangered’ species, and the Convention on International Trade of Endangered Species of Flora & Fauna (CITES) has retained tigers in Appendix I (CITES 2022), thereby prohibiting or regulating their international trade. The historical distribution of tigers once ranged from Eastern Turkey to the sea of Japan and throughout southeast Asia including many large Asian islands (Seidensticker et al. 1999), but their density and range has now contracted to a suite of relatively small patches throughout this former range. Tiger populations are thought to remain in just 13 tiger range countries (TRCs) including Bangladesh, Bhutan, Cambodia, China, India, Indonesia, Lao PDR, Malaysia, Myanmar, Nepal, Russia, Thailand and Vietnam (GTI 2011), although Goodrich et al. (2022) showed that tigers have vanished completely from Cambodia, Lao PDR and Vietnam. The global population of tigers is currently estimated to be between 3,726 and 5,578 individuals, located in only 10 tiger range countries (Goodrich et al. 2022). Tigers remain a species of high conservation value.

Tigers are also an umbrella species, which means that their presence is associated with relatively healthy ecosystems, and tiger-focused conservation can therefore benefit multiple species at lower trophic levels (Karanth 2003; GTI 2011). Tigers are now facing substantial environmental challenges, climate change, population growth and infrastructure development (Seidensticker 2010). South China tigers exist only in captivity while Amur, Malayan and Indochinese tigers exist only between 150 and 250 in number and Sumatran tiger constitute ∼ 600 individuals (Jhala et al. 2021). Most sub-species retain very small population that are not expected to persist. However, the Bengal tiger is by far the largest population and can be found in Bangladesh, Bhutan, India and Nepal, where ∼ 3,800 animals exist (GTI 2011). Understanding the trends and status of research on Bengal tigers is critical to ensuring that conservation effort and resources are allocated wisely, for the benefit of tigers and the many other species that coexist with them in tiger-suitable habitats.

Here, we review Bengal tiger studies published globally between 2010 and 2022, focusing on research themes, as well as spatial and temporal publication trends. Previous reviews have focused on limited areas and themes (Sarkar et al. 2021a; Yadav et al. 2022). For example, country-specific reviews from India (Rastogi et al. 2012) and Nepal (Ghimire 2022) have also focused on just one theme tiger conservation. Other reviews focusing on tiger pheromones (Brahmachary and Poddar-Sarkar 2015), conservation status (Kumar 2021; Sarkar et al. 2021a) and recovery (Seidensticker 2010) have also been carried out in the past, but none of these sought to synthesize the breadth of work similar with what we have undertaken here. Similar reviews to our have recently been conducted for greater one-horned rhinoceros Rhinoceros unicornis (Pant et al. 2020), red panda Ailurus fulgens (Karki et al. 2021), and various large felids and other mammals (Ghosal et al. 2013; Akash and Zakir 2020; Bist et al. 2021; Srivathsa et al. 2022). Our work therefore expands the breadth and depth of these reviews, necessary for identifying global trends and status of Bengal tiger research.

Consequently, our aims are to (1) examine the number and geographic distribution of studies, including the types and sources of those studies, as well as institutional and research collaboration; (2) examine the primary themes and sub-themes discussed; and (3) identify important gaps in the current knowledge. We hope that this review will give direction towards future research and conservation efforts for Bengal tigers, and related sub-species.

Materials and methods

Literature search strategies

We followed the systematic literature review approach demonstrated by Pant et al. (2020) and Karki et al. (2021), which is based on the recommendations of Pullin and Stewart (2006) and the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA ) framework described by Moher et al. (2009). Peer reviewed articles published in English between 2010 and 2022 in three web-based databases (i.e., − Scopus, Web of Science and Science Direct) were searched for the following terms: “Tiger” OR “Bengal tiger” OR “Panthera tigris tigris”.

Article selection criteria

Our literature search returned 35,744 articles. We then examined the article title, abstract and keywords of each article and only retained peer-reviewed empirical and review articles on wild and captive Bengal tigers. Irrelevant articles associated with fish, plants, insects, entertainment and sports were excluded. This resulted in sourcing 2,118 articles in Scopus, 1,744 in Web of Science, and 767 in Science Direct database after applying our first level of exclusion criteria (see Supplementary Material, Table 1). All the duplicates were then removed by combining all articles in “Zotero” software, followed by re-screening each article by title and abstract to further exclude irrelevant articles related to other species. Tiger related articles were then segregated at the sub-species level for further analysis. Articles related to the other five sub- species of tiger were excluded, and only articles related to Bengal tigers were used for our analysis. Thus, we chose to review a total of 491 articles about Bengal tigers (see Supplementary Material, Fig. 1). These articles encompass non-specified sub-species as well as review papers.

Data compilation and analysis

We categorized each article according to publication year, authors, country of research (i.e. where the work originated from), journal of publication, data types, and thematic areas for our analysis (see Supplementary Material, Table 2). We defined ‘primary data’ to reflect empirical studies based on original data generated by researchers through surveys, questionnaires and/or experiments, and ‘secondary data’ to reflect empirical studies based on data originally generated by others. The thematic areas adopted by Pant et al. (2020) and Karki et al. (2021) were considered for this review as well, and were further reclassified into sub-themes for critical appraisal. The thematic areas we assessed were broadly categorized into eight themes−: habitat, biology, population, conflict, genetics, poaching, socio-cultural dimension, and general (Table 1). The data frame with the specified categories are presented in detail in the Supplementary Material, Table 2. The analysis of qualitative data was performed using VOSviewer software, and spatial information was displayed using ArcGIS 10.2, while descriptive quantitative data were analyzed using Microsoft Excel.

Table 1 Thematic areas with key contents and number of articles
Full size table

Why 2010 to 2022?

A global tiger workshop was held in October 2009 in Kathmandu, Nepal with a theme “Saving wild tiger is our test; if we pass, we get to keep the planet” which was attended by more than 250 tiger experts from 13 tiger Range Countries (TRCs). The workshop recommended celebrating 2010 as the year of tiger throughout the world. The recommendation was carried forward to the First Asian Ministerial Conference on Tiger Conservation (1st AMC) which was held in January 2010 in Hua Hun, Thailand, which set the ambitious goal of total protection of critical tiger habitats and doubling the global number of wild tiger (TX2) by 2022 in the 13 TRCs. Similarly, the follow-up meeting amongst TRCs and experts held in July 2010 at Bali, Indonesia solidified the foundation for doubling tiger numbers and creating a Global Tiger Recovery Program. Finally, in November 2010, the first “Tiger Summit” or International Tiger Conservation Forum was held at St Petersburg, Russia. The significant declarations and outputs of this summit were (1) to double the number of wild tigers across their range by 2022, (2) endorse the Global Tiger Recovery Program (2010–2022), and (3) celebrate 29th July as global tiger day across the world. Hence, the year 2010 is a pivotal year for the history of tiger conservation throughout the world, and 2022 represents the global benchmark denoting the date that many of the proposed goals should have been achieved by. The global tiger number is presented in Supplementary Material, Table 3.

Results

Spatial and temporal trends of research

The number of Bengal tiger studies published annually has increased over time (Fig. 1a). The fewest number of studies were published in 2010 (n = 22), while the highest number of studies were published in 2022 (n = 61).

Fig. 1
figure 1

(a) The number of peer reviewed articles on Bengal tigers, (b) duration of empirical studies on Bengal tigers and (c) the top 11 journals that most frequently published research on Bengal tigers,2010 to 2022. (Note: NA denotes not mentioned/applicable)

Full size image

Most studies were geographically focused on India (n = 256), followed by Nepal (n = 56) and Bangladesh (n = 22) (see Supplementary Material, Fig. 2a) and 53 studies were not specific to any country or area. A total of 12 studies were conducted at a transboundary level covering two or more TRCs, and only 13 studies were conducted across all TRCs. Tiger research from the USA, Australia, UK, Italy, France, Germany, Pakistan and other non-TRCs were mainly associated with captive tigers and illegal trade.

Research types and duration

Of the 491 articles we reviewed, ∼75% of them were empirical studies based on primary data and ∼22% of them were based on secondary data; ∼5% of the studies were based on use of both primary and secondary data. Of the empirical studies that mentioned duration (n = 252), most of them were short-term studies with the study periods of < 6 months (Fig. 1b). Approximately 87% of the studies focused on wild tigers, ∼11% focused on captive tigers, and ∼3% focused on both. Most of the studies on captive tigers were carried out in the USA (n = 16) and India (n = 10), followed by Australia (n = 7) and China (n = 6).

Thematic fields

We have categorized the research into wide range of themes and sub theme. Thematic diversity increased over time (see Yadav et al. 2022). Between 2010 and 2022, Biology (∼22%) was the most common research theme, followed by Genetics (∼16%), Conflict (∼14%) and Habitat (∼12%); Poaching (∼7%) was the least studied theme (Table 1). The General category included studies on conservation and collaboration, review papers, technologies, history, veterinary and economic analysis.

Biology

Bengal tiger biology was the most studied theme (∼22% of studies). Within this theme, dietary (n = 42) aspect of their biology was the most studied (Aziz et al. 2020; Lahkar et al. 2020; Letro et al. 2022; Pun et al. 2022), followed by behavior (n = 28) (Sarkar et al. 2021b; Singh et al. 2020) and physiology (n = 22) (Singh et al. 2014; Williams et al. 2015; Long et al. 2017; Pereira et al. 2022). Tiger interactions (n = 13) with other wild animals was the least studied aspects in this theme (Ullas Karanth et al. 2017; Bhandari et al. 2021; Thapa et al. 2021).

Genetics

Within the genetics theme, most research centered on species and individual identification (n = 37) (Nittu et al. 2021; Jiang et al. 2020; Dalton et al. 2020; Karmacharya et al. 2018), followed by diversity (n = 28) (Aziz et al. 2022; Mondol et al. 2013; Naidenko et al. 2019; Thapa et al. 2018). Population estimation (n = 3) (Aziz et al. 2017; Aylward et al. 2022) and faecal and blood analysis (n = 6) (Shrivatav et al. 2012; Parnell et al. 2015; Kumari Patel et al. 2021) studies were also undertaken. Many genetic-related studies originated from non-TRCs such as Australia, Korea, USA, UK and South Africa.

Conflict

Conflict was the third most studied theme. Within this category, over 50% of the research (n = 38) were related with human and tiger conflict (Karanth et al. 2012; Sharma et al. 2020; Shahi et al. 2022) including some other species like leopard (Panthera pardus) and dhole (Cuon alpinus). Conflict associated with interactions (n = 12) (Penjor et al. 2022; Puri et al. 2022b; Carter et al. 2022) such with infrastructures were also undertaken. Livestock depredation by tigers (Borah et al. 2018; Letro and Fischer 2020; Li et al. 2020) were studied from Bhutan, Nepal, India and China. Coexistence (Inskip et al. 2016; Rakshya 2016; Badola et al. 2021) was the least studied aspect with the ‘conflict’ theme .

Habitat

A total of 58 studies focused on the habitat theme, including studies on species distribution (n = 17) (Hossain et al. 2018; Mohan et al. 2021; Shrestha et al. 2022; Vernes et al. 2022), connectivity (n = 17) (Mondal and Nagendra 2011; Kanagaraj et al. 2013; Schoen et al. 2022), habitat suitability (n = 14) (Kanagaraj et al. 2011; Carter et al. 2013; Thinley et al. 2021), threats (n = 6) (Aziz et al. 2013; Saxena and Habib 2022) and climate change impacts (n = 3) (Deb et al. 2019; Mukul et al. 2019; Rather et al. 2020). The few studies related to climate change effects on habitat were originated from India, Nepal, and Bangladesh.

Socio cultural

Socio-Cultural themed studies (n = 40) covered a wide variety of subjects, including people’s perceptions of tigers (n = 21) (Carter and Allendorf 2016; Doubleday and Rubino 2022; Dhungana et al. 2022), and research related to tourism (n = 12) (Ghosh and Uddhammar 2013; Thapa et al. 2017a; Macdonald et al. 2017), investigations into tradition practices (n = 5) (Liu et al. 2016; Saif et al. 2016), and ecosystem services (n = 2) (Zabel and Engel 2010; Bhattarai et al. 2021).

Population

A total of 34 studies focused on the population theme, and within this, studies on tiger density (n = 27) (Harihar et al. 2017, 2020; Thapa and Kelly 2017; Tempa et al. 2019) emerged as the most extensively researched subject. A limited number of studies also focused on tiger diversity (n = 4) (Debata and Swain 2018; Ahmed et al. 2021) and dispersal (n = 3) (Singh et al. 2013; Thapa et al. 2017b; Lamichhane et al. 2018).

Poaching

Poaching (n = 32) was the least studied theme, even though we included poaching related articles wherever tiger sub-species was unknown. Poaching studies concentrated on tiger trade (n = 18) (Li and Hu 2021; Khanwilkar et al. 2022; Dang Vu et al. 2022), illegal killing (n = 11) (Saif et al. 2018; Davis et al. 2020) and illegal wildlife trade network (n = 3) (Patel et al. 2015; Paudel et al. 2020; Domínguez et al. 2022) connection of criminal groups in and out of the country.

General

Articles that could not be classified under the previous themes were categorized under the General theme (n = 81). These could be further classified into groups that focused on conservation-related studies (n = 14) (Wikramanayake et al. 2011; Harihar et al. 2018; Puri et al. 2022a), reviews (n = 14) (Seidensticker 2010; Akash and Zakir 2020; Srivathsa et al. 2022), tiger recovery (n = 5) (Wilting et al. 2015; Gubbi et al. 2016; Karanth et al. 2020), technology (n = 5) (Miller et al. 2010; Maheswari et al. 2022), veterinary associated topics (Guthrie et al. 2021; McCauley et al. 2021; Kadam et al. 2022) and other subjects (n = 23) (Yamaguchi et al. 2013; Sadath et al. 2013; Kafey et al. 2014; Whitfort 2019; Sanderson et al. 2019; Nayak et al. 2020) included articles related to economic analysis, farming and history.

Journals, authors and affiliations

Journals

We identified 171 journals which published articles pertaining to Bengal tigers. Among these, the highest number of publications (∼9%, n = 37) were found in PLoS ONE, followed by Biological Conservation with around (6%, n = 27) and Oryx (∼5%, n = 23; Fig. 1c). The top eleven publishing journals, primarily from Europe and USA, are categorized as Q1 and Q2 publications.

Country of first author, affiliation and co authorship

We identified first authors from 34 different counties, with ∼38% (n = 192) from India and ∼25% (n = 111) from the USA and ∼8% (n = 38) from the UK ( see Supplementary Material, Fig. 2b). About 74% of these studies were conducted by first authors based within the same country where the research was undertaken. Although ∼18% of the first authors were not based in the country of research, a substantial portion of such studies involved co-authors who were situated in the country where the research was conducted.

We further found that authors from India have collaborated with the highest number of co-authors, closely followed by those from Nepal. We also identified the presence of eight author groups engaged in collaborative efforts (Fig. 2). Among these, we found two distinguishable groups: one with five interconnected clusters highly interlinked with prominent researchers from India, and another group with three clusters highly interlinked with the prominent researchers from Nepal, USA and India.

Fig. 2
figure 2

Network of co-authorship groups publishing studies on Bengal tigers, 2010 to 2022

Full size image

Key word co-occurrence analysis

Keyword analysis revealed that obvious words such as ‘tiger’ and ‘Panthera tigris’ were most frequently used words (> 100 times) in titles and abstracts of published articles (Fig. 3), but ‘protected area’, ‘endangered species’, ‘conservation’, ‘human’ and ‘biodiversity’ also featured heavily.

Fig. 3
figure 3

Density visualization of terms co-occurrence in the studies

Full size image

Discussion

Spatial and temporal trend of research

The average annual publications count (n = 40) is higher than the greater one-horned rhinoceros and red panda (Pant et al. 2020; Karki et al. 2021), which are also of great conservation concern in the same region (Fig. 1a).

Although most research is conducted in India, Nepal and Bangladesh (see Supplementary Material, Fig. 2a), researchers from Nepal and Bangladesh are less prominent as lead authors compared with those from India, the USA and UK (see Supplementary Material, Fig. 2b). Causes of this are unknown, but may be associated with funding and/or publication biases. Bhutan is underrepresented on both counts, necessitating greater attention on tigers there from both perspectives.

Although many contiguous protected areas exist among India-Nepal, India-Bangladesh and India-Bhutan, where tigers are moving freely across them, yet there is a surprising lack of cross-country or transboundary studies (Harihar et al. 2017; Naidenko et al. 2019; Sharma et al. 2020). This limited number of transboundary research may be due to the complexity of tiger management regimes and lack of transboundary coordination. This barrier must be overcome if tiger conservation efforts are to succeed in the future.

Research types and duration

Three-quarters of the research conducted relied on primary data sources indicating the trustworthy research trend of Bengal tigers. The intricate studies such as behavior ecology, effects of climate change, population dynamics, threats, and human-tiger conflict demand long term efforts. Our findings reveal that only 23% of the studies carried out for more than three years (Harihar et al. 2020; Krishnakumar et al. 2022) indicating the scarcity of long term research on those intricate studies. The long term studies on those subjects need to be highlighted in the future. Although, there has been notable research on captive tigers across many countries in non-TRCs (Lefebvre et al. 2020; Pandey et al. 2021), which is positive step. However, it is equally important to conduct captive tiger studies in TRCs as well.

Thematic fields

Biology

The study conducted by Ghosal et al. (2013) indicated that Biology was the most extensively explored theme and still it persists. Under this theme, the dietary aspects were highly investigated across the range countries and the studies showed that wild boar Sus scrofa and sambar Rusa unicolor are the preferred prey (Hayward et al. 2012) of Bengal tigers, while the largest ungulate, Gaur Bos gaurus also played a significant role in the tiger’s biomass consumption (Krishnakumar et al. 2022).

The spatial ecology of tigers, including dispersal (Bhardwaj et al. 2021; Singh et al. 2021) and home range size (Barlow et al. 2011; Majumder et al. 2012) were commonly studied, while the physiological aspect was less studied in the TRCs. The majority of tigers behavior studies were originated from India (Singh et al. 2020; Bhardwaj et al. 2021) with only one from Bangladesh (Barlow et al. 2011). Such type of research is important if the expansion on tiger habitats beyond the protected areas remains as international goal. We did not find any studies directly related to tiger behavior from Nepal and Bhutan, the nations that play vital role in doubling the tiger number.

Concerning species interactions, tigers were mostly studied in association with leopards and dholes, though greater one-horned rhinoceros (Rhinoceros unicornis), Asian elephants (Elephas maximus) and even lion (Panthera leo) interactions have been evaluated (Thuppil and Coss 2013; Williams et al. 2015; Medhi and Saikia 2019; Saikia et al. 2020) However, there remains a dearth of studies regarding tiger interactions with sloth bears (Melursus ursinus) Indian jackals (Canis aureus indicus), and hyenas (Hyaena hyaena), which all share the same habitat.

Genetics

Despite the growing volume of genetic research, a notable gap exists in Bhutan, where such research remains absent. Countries like Bangladesh (Aziz et al. 2022) and Nepal (Karmacharya et al. 2019) yield fewer studies as compared to India. Genetic analysis is widely accepted tool safeguarding species like tigers, both within their natural habitats and in controlled environments (Luo et al. 2010). Investigation into the genetic makeup of populations reveals the adverse effects of inbreeding depression on small population (Khan et al. 2021). Such genetic analysis and population estimation approach studies are still lacking in the counties like Bhutan, Bangladesh and Nepal. Furthermore, the captive tigers studies on genetics were mostly unrelated to TRCs, except few from India (Maity et al. 2022; Dehnhard et al. 2015; Mishra et al. 2014). This suggests that more resourceful and developed countries have taken the lead in such studies. To address this disparity, it is essential to find ways to overcome the research resources and expertise in these TRCs countries.

Some of the genetic studies showed that population isolation is caused by human disturbance, fragmentation and topographic complexity (Reddy et al. 2017, 2019; Thatte et al. 2020). Various studies in landscape genetics have tried to integrate genetic data with landscape features to pinpoint barriers to gene flow (Mondol et al. 2013; Sharma et al. 2013; Yumnam et al. 2014). Such barriers could be overcome by the establishment of habitat corridors and connectivity facilitating of the movement and gene flow of tigers (Kolipakam et al. 2019; Thapa et al. 2018). However, these studies were limited mostly in India and few from Nepal. Such studies focusing on landscape genetics, emphasizing the importance of corridors and connectivity between the adjacent transboundary protected areas, could be a critical areas of study for preserving genetic diversity, which is completely lacking at present.

Conflict

Goodrich (2010) suggested that as tiger populations rise, so does the potential for human-tiger conflicts, and TRCs must proactively address this issue. Various strategies have been proposed to mitigate these conflicts, including prompt compensation, insurance scheme (Dhungana et al. 2016) radio collaring of problematic tigers (Barlow et al. 2013; Dhungana et al. 2016) and payment of ecosystem services (PES) (Khadija et al. 2022). But, some additional mitigating factors such as efficacy of awareness programs and use improved corral for domestic animals are still unexplored.

We noticed fewer studies examining human-tiger interactions resulting to conflicts, particularly due to infrastructures such as roads (Carter et al. 2020; Quintana et al. 2022) at overall context. Following the rapid infrastructure development in the TRCs, such studies need to be further intensified at local level to gain deeper understanding of these issues.

Studies indicates that leopards and dholes tend to avoid tigers (Thinley et al. 2018; Ramesh et al. 2020; Penjor et al. 2022), residing in peripheral forest near human settlements. Consequently, livestock depredation occurred by these animals at the peripheral forest. Thus, these aggravate the potential human –wildlife conflict scenario.

Habitat

Our findings reveal that habitat is the third most studied theme, while Yadav et al. (2022) emphasized habitat and ecology as the most studied theme. We have identified studies that focus on habitats situated at higher altitudes in regions beyond those typically considered suitable for Bengal tigers i.e. India (Mohan et al. 2021), Nepal (Bista et al. 2021; Shrestha et al. 2022) and Bhutan (Dorji et al. 2019). Despite these investigations, the precise cause behind the existence and significance of these high-altitude habitats for tiger populations remains as unexplored aspect of research.

A range of habitat studies were conducted, covering habitat preference by reintroduced tigers (Sarkar et al. 2017), the utilization of agriculture lands as seasonal habitats (Warrier et al. 2020), and assessment of habitat change at different land management regimes in human dominated areas (Carter et al. 2013). Nevertheless, a noteworthy gap persists in study of behavior change of tigers at these distinct habitats. Various studies concluded that infrastructures such as roads and railways (Carter et al. 2020, 2022; Quintana et al. 2022; Saxena and Habib 2022) as well as dam and hydropower projects (Kenney et al. 2014; Palmeirim and Gibson 2021) pose substantial threats to tiger populations. However, studies on the effects of irrigation canals, high tension lines and industries on tigers remains unexplored.

Regarding habitat connectivity, numerous studies have highlighted the importance of corridors and connectivity (Anwar and Borah 2019) and its mapping (Dutta et al. 2016). These studies were limited and only from India and Nepal, thus need to be further intensified across the TRCs. Research prioritizing the identification of threats (Aziz et al. 2013) has identified climate change as another significant threat (Naha et al. 2016). Mukul et al. (2019) found out that the suitable habitat for tiger in Sundarbans will disappear by 2070. Furthermore, climate change studies concerning tigers have often studied with other species like leopards (Rather et al. 2020) and other mammals (Deb et al. 2019). All of these climate change studies use habitat suitability modeling and the consistent outcome of these studies is the projected reduction of suitable habitats of tigers in the future. We found none of studies from Nepal and Bhutan directly related to climate change.

Socio cultural

Few studies have addressed socio-economic issues in the past (Ghosal et al. 2013), and our results also confirm the same result. In the domain of ecosystem services, aspect related with willingness to pay by the visitors of protected areas in Nepal (Bhattarai et al. 2021) and performance payment to tiger-livestock conflict in India (Zabel and Engel 2010) has been conducted. These innovative approaches hold the potential to foster sustainable conservation practices, hence need to be further intensified across the TRCs. The studies exploring traditional knowledge, art and literature, ethics and values, and religion and rituals associated with tigers within diverse communities is currently very insufficient.

Several tourism-related studies (Ghosh and Ghosh 2019; Rao and Saksena 2021) have revealed that it was unable to benefit local communities. But, tourism is often considered as a significant component of both local and national economies due to its substantial multiplier effects. Hence, it should be extensively studied across the TRCs from the prospective of employment generation and promotion of local products, which is still lacking. It has also got some adverse consequences such as threats to local culture and traditions, pollution, and habitat loss. These aspects also need to be explored for better understanding and address the challenges associated with sustainable tourism.

Population

Harihar et al. (2017) suggested that tiger population estimates should be at annual or biennial intervals. To achieve this, Gopalaswamy et al. (2012) emphasized the robustness of combining camera trap data with fecal DNA analysis to obtain more accurate density estimates. However, Harihar et al. (2020) suggested that population increases alone may not reflect tiger conservation success given that prey depletion and human disturbance play the major roles in tiger loss at local levels (Karanth et al. 2011). All of these studies exclusively focused on India. Therefore, it is crucial to investigate the impact of prey depletion and human disturbance on tiger populations at each distinct locality across the range countries. Our investigation revealed that there is a total lack of population-related research from Bangladesh.

In an unprecedented discovery, a study done in China’s Tibet region recorded the presence of Bengal tigers for the first time (Li et al. 2021). We found only limited studies on tiger dispersal in Nepal (Thapa et al. 2017b; Lamichhane et al. 2018) and India (Singh et al. 2013), which revealed that tigers are dispersing from seemingly suitable areas towards seemingly unsuitable or peripheral areas. Causes for this are unknown, but will be important to identify if we are to improve reintroduction success and effectively protect key tiger habitats.

Poaching

Our finding was also consistent with the findings of Yadav et al. (2022) and Bist et al. (2021) from Terai Arc Landscape in which poaching was identified as the least studied theme of tigers globally. Illegal wildlife trade is valued at more than USD$ 20 billion per year (Barber-Meyer 2010), and is multinational in scope (Paudel et al. 2020; Khanwilkar et al. 2022). Some of the studies showed that the efforts towards tiger farming will not stop the demand of wild-sourced products (Rizzolo 2021; Dang Vu et al. 2022). Such studies are limited only in few countries. Rampant snaring is one of the most common threats to large mammals in Southeast Asia (Figel et al. 2021). However, our understanding of various alternative techniques used by hunters at modern world need to be further investigated. Additionally, it is equally crucial to study the economic, social, and cultural factors that drive the behavior of hunters.

Tiger products experience varying degrees of demand. Tiger bone wine is the most demanded product of tiger (Li and Hu 2021) and tiger bone glue is mainly used for medicinal purposes (Davis et al. 2020). Additional items such as tiger skin, claws, bones and teeth are also demanded in markets (Nittu et al. 2022). Given the multinational nature of poaching, the studies originated from TRCs and even USA (Khanwilkar et al. 2022), revealing that China and Vietnam as major suppliers to the USA. Patel et al. (2015)explored China as the pivotal hub for the illegal wildlife trade network involving tigers, rhinos and elephants. Even though, it is complex to investigate organized crime and tough to research and publish, it should be emphasized in future.

Only one study focused on technology known as Management Information System (MIST) (Stokes 2010), which is the computerized management information system designed for ranger-based data collection for law enforcement, was recorded. Conducting additional research on the adopted anti-poaching technologies in countries like India and Nepal, such as Spatial Monitoring and Reporting Tool (SMART), Unmanned Aerial Vehicles(UAVs) and sniffer dogs, has the potential to make significant contributions to the conservation of the species.

General

Assessing the tigers protection using Conservation Assured/Tiger Standards (CA/TS) studies (Pasha et al. 2018; Dudley et al. 2020)provides a useful measuring scale for assessing the progress of conservation efforts. However,, there is not much research on expansion of protected area for tigers, with only one study identified from India (Gubbi et al. 2016, 2017).

The tiger related review papers that we found were primarily concentrates on policy, large felids, and prey species, with majority of these papers originating from India, with an exception from Bangladesh. The recent research conducted on novel technologies for animal detection through video analysis (Maheswari et al. 2022) and utilization of wireless sensor networks to monitor tigers (Badescu and Cotofana 2015), strive to contribute to species conservation and further intensified across the TRCs, particularly addressing the imperative of anti-poaching efforts for tigers in future.

Limited studies within the domain of veterinary science, demonstrated the transmission of canine distemper virus from dogs to tigers (Sidhu et al. 2019), tigers being infected by H5N1 avian influenza virus (He et al. 2015). Further, a broad spectrum of health perspectives could be explored. The recent Tiger summit in Russia also emphasize adoption of one health approach and safeguard against zoonotic disease transmission (VDTC 2022).

Furthermore, economic considerations related research such as the analysis of expenditures (Nayak et al. 2020) in India showed the allocated fund for tiger conservation is not proportionally allocated according to size of park or population of tigers. A financial analysis of tiger conservation in Nepal (Kafey et al. 2014) also found the current level of funding is inadequate. Similar financial gaps could be explored in other countries as well.

Publishing journals, authors, affiliation and key word co-occurrence

In the past 12 years, the research trend has shown a diverse array of journals interested on publishing tiger-related studies (Fig. 1c). All the top eleven journals are from natural science discipline, indicating consistent and valuable publishing trend that need to be continued. However, the dominance of journals originating from the USA and Europe highlights the shortage of journals from TRCs dedicated to tiger research. We found that the highest publisher PLoS ONE (published from USA) may be skewed towards native authors, as there were second highest lead authors from the same country.

As expected universities and research institutes dominate the production of tiger research, and publications attributed to government organizations accounted for less than 10%, reflecting a lower emphasis on research production within government.

The extensive researchers from many nationality, along with good connections of authors or co-authors at the country where research occurred, signifies the robustness of Bengal tigers research. Though, Indian authors demonstrates a strong culture of collaboration among its domestic authors, they should further enhance collaborative efforts with authors from other countries to create stronger links in research endeavors. Despite non-TRCs, the USA holds significant position in terms of authorship and organizational involved. This prominence could likely be attributed to the resource availability and substantial number of academic institutes in the USA.

The topics such as animal behavior, estimation methods, human-wildlife conflict, wildlife trade, ecotourism, economics have been relatively underexplored. It is imperative to focus on these underrepresented topics to advance the understanding on these areas.

Conclusions

We found that the number of publications on Bengal tigers is on upward trajectory every year with wide range of publication outlets. We found that the highest number of research were conducted on biology-theme and lowest on poaching-theme. Our findings highlight a hopeful trend in both wild and captive Bengal tigers research, both within and beyond its geographical range, signifying a positive expansion of knowledge on charismatic species. Based on our analysis, the following key research gaps emerge, necessitating attention and action:

  1. 1.

    To ensure the long-term survival of Bengal tigers, it is essential to address the gaps in transboundary research, mainly at the interconnected forest and protected areas spanning across its range countries. Conducting comprehensive, long-term studies throughout their habitat is essential.

  2. 2.

    A significant gap exists in the behavior study of Bengal tigers within Bangladesh, Bhutan and Nepal, especially long term research to know the behavior of the species in distinct habitats. Furthermore, expanding protected areas for conservation of tigers demands in-depth studies to support future decisions. The interaction of tiger with other species should also be investigated more closely.

  3. 3.

    Genetic research, along with capacity enhancement is a dire need in Bangladesh, Bhutan and Nepal. These endeavors are crucial for a comprehensive understanding of the species and its long term viability.

  4. 4.

    Addressing the gaps in human-tiger conflict management, particularly through the lens of coexistence and infrastructure development aspects is essential. Likewise, a compelling need for intensified research on dispersal of Bengal tigers from their prime habitats in the context of climate change is warranted.

  5. 5.

    The scarcity of research focused on poaching-theme need to be prioritized in the future, especially in the illegal trade networks analysis though found to be difficult task. Similarly, anti-poaching strategy could be bridged through the technological innovation and studies.

  6. 6.

    Emerging areas such as tourism, economic analysis, and veterinary aspects remain relatively unexplored, and should be prioritized.

  7. 7.

    Governments, functioning as policy-maker and field implementing agencies should escalate Bengal tigers research integrating insights from academia for effective conservation of the species. It is important to encourage collaborative research among multinational researchers to promote and expand the scope and knowledge of the tigers across their range.