The family Fabaceae (Leguminosae) represents the third-largest group of flowering plants, comprising approximately 22,500 species worldwide (Hughes et al., 2025). The family includes small herbs to large trees, many of which hold significant cultural, ecological, and economic importance (Acharya et al., 2004). The family Leguminosae is traditionally divided into three subfamilies: Papilionoideae, Caesalpinioideae, and Mimosoideae (Lewis et al., 2005). However, the Legume Phylogeny Working Group (LPWG) proposed a revised classification in 2017, recognising six subfamilies—Caesalpinioideae, Cercidoideae, Detarioideae, Dialioideae, Duparquetioideae, and Papilionoideae—based on a comprehensive phylogenetic analysis of plastid matK gene sequences (Azani et al., 2017). Among these, Detarioideae represents a tropical, monophyletic lineage exhibiting remarkable morphological diversity (Azani et al., 2017; de la Estrella et al., 2018). Within this subfamily, Humboldtia Vahl stands out as an ecologically significant and endemic tree genus of the Western Ghats–Sri Lanka Biodiversity Hotspot (Vahl, 1794; Sanjappa, 1986) (Fig. 1).

The Humboldtia genus comprises only eight species, seven of which are endemic to the southern Western Ghats across Kerala, Karnataka, and Tamil Nadu, while Humboldtia laurifolia is confined to Sri Lanka (Sanjappa, 1986; Jose and Anuraj, 2023) (Fig. 1A). All recognised species face a high risk of extinction and are represented by limited populations with very few mature individuals (Table 1). The genus Humboldtia was first described by the Danish–Norwegian botanist Martin Vahl in 1794 (Vahl, 1794). Initially named Batschia, it was later renamed Humboldtia in honour of the eminent Prussian naturalist and explorer Friedrich Wilhelm Heinrich Alexander von Humboldt. This genus remains under-researched and under-utilised, with all its species confined to a single biogeographic region. Members of Humboldtia typically occur in evergreen forests at elevations ranging from 200 to 1250 m, often along riverbanks and in interfluvial zones. The genus is distinguished by its persistent foliaceous stipules, which often bear basal appendages (Sasidharan and Sujanapal, 2007). Three of the species in this genus are myrmecophytes: H. brunonis, H. decurrens and H. laurifolia. Myrmecophytes are plants that possess specialised structures, called (myrmeco) domatia, that facilitate ant nesting (Beattie, 1985) and thereby promote ant fidelity. The genus Humboldtia includes species that are non-myrmecophytic, semi-myrmecophytic (H. brunonis), and true myrmecophytic (H. laurifolia). However, because the phylogeny of the genus Humboldtia remains unclear, it is unknown how the domatium feature evolved in this genus (Chanam and Borges, 2017). The genus also exhibits cauliflory, an ecological adaptation in tropical forests. Due to its ecological, biogeographical, and evolutionary significance, Humboldtia deserves more attention in research and conservation areas.

Humboldtia is distinctive for its unique ant–plant symbiotic relationships, representing a rare case of myrmecophily in woody, non-neotropical trees (Fig. 1G and I). Several Humboldtia species exhibit specialised morphological traits, such as domatia (swollen and hollow stem internodes or branch bases), that provide nesting habitats for ant colonies (Ambika and Ganesh, 1990; Gaume et al., 1998) (Fig. 1). These domatia provide a secure and humid microhabitat, offering protection from predators and environmental stress, while simultaneously granting ants access to extrafloral nectaries and other secretory tissues that produce carbohydrate-rich food rewards (Gaume et al., 1998). For the host plant, the benefits are substantial. Ants act as defensive mutualists, deterring herbivores, reducing fungal growth, and preventing insect oviposition (Gaume and McKey, 1999). This activity enhances leaf longevity, photosynthetic capacity, and overall plant health. The Humboldtia–ant relationship exemplifies a facultative mutualism for the plants but is often obligate for the ants. The dominant associated ant species, Technomyrmex albipes, is highly dependent on Humboldtia domatia for nesting and brood development (Gaume and McKey, 1999). From an evolutionary perspective, the Humboldtia–ant symbiosis offers an exceptional model for examining coevolutionary dynamics in tropical trees. The evolution of domatia and extrafloral nectaries in Humboldtia likely reflects adaptation to persistent herbivory in the wet evergreen forests of the Western Ghats (Gaume et al., 1998). This indicates an independent evolutionary origin of myrmecophily, likely driven by region-specific ecological pressures.

Humboldtia also exhibits cauliflory-production of flowers and fruits directly from the main trunk, or woody stems, rather than from new shoots. From an ecological perspective, cauliflory facilitates pollination and seed dispersal by animals that do not typically forage in the forest canopy. Similarly, fruits developing near the trunk are within reach of ground-dwelling dispersers, such as rodents, civets, and primates, thereby enhancing seed dispersal efficiency and genetic connectivity among populations (Diniz et al., 2019). Evolutionarily, cauliflory represents a functional adaptation to dense forest environments, where low light conditions and complex vertical structure may limit visibility and access to canopy flowers. By positioning reproductive structures along the trunk, plants can maximise reproductive success even under shaded understory conditions (Warren and Emamdie, 1997).

Ethnobotanical information on the Humboldtia genus remains limited, with most records derived from indigenous knowledge. Traditionally, the bark of several Humboldtia species has been used as an anticonvulsant and in the treatment of biliousness, ulcers, leprosy, and epilepsy (Asirvatham and Yesudanam, 2018). Decoctions prepared from bark powder are also believed to possess blood-purifying properties (Sanjappa, 1986). Among these, Humboldtia unijuga—locally known as “Palakan” by the Kani tribes of the Agasthyamala region—has been used to treat headaches, chickenpox, and snakebites (Asirvatham and Yesudanam, 2018; Nair et al., 2023). The roots of this species exhibit notable anti-inflammatory and anticancer activities, while the bark and leaves of H. unijuga var. trijuga are used by traditional healers to treat skin ailments (Arun Vijayan et al., 2007). Similarly, Humboldtia brunonis has been used in folk medicine to manage urinary disorders and menstrual irregularities (Nisbet and Moore, 1997). Its bark and leaves are used to control excessive menstrual bleeding (Nagabhushan and Raveesha, 2015), and local communities in the Shiradi and Bisle Ghats of Karnataka utilise the same plant parts to alleviate symptoms of diabetes and arthritis (Trivedi, 2009; Prasad and Kumar, 2013). Humboldtia vahliana has also been reported in traditional medicine as a remedy for biliousness, leprosy, ulcers, and epilepsy (Leela and Pillai, 2005).

Humboldtia species are characterised by their narrow endemism and highly restricted geographic ranges. They exhibit specific habitat preferences and thrive only under particular climatic conditions. Factors such as deforestation, habitat fragmentation, climate change, and poor natural regeneration have placed nearly all species of the genus under severe threat of extinction. According to IUCN assessments, among the eight recognised species, one is classified as Critically Endangered, two as Endangered, three as Vulnerable, one as Near Threatened, and one remains unassessed. However, inaccuracies exist within the current IUCN evaluations, particularly concerning H. bourdillonii (Jose and Anuraj, 2023; Jose, 2026). The Western Ghats–Sri Lanka biodiversity hotspot, which harbours the entire genus, is increasingly impacted by anthropogenic pressures and climate-induced changes. Climate projections suggest that the Western Ghats could lose up to 33% of its biodiversity by 2050 due to extreme climatic conditions (Kulkarni, 2021). Endemic genera, such as Humboldtia, are especially vulnerable to these changes compared to other taxa (Manes et al., 2021), and field observations show that Humboldtia populations are already exhibiting signs of climate-related stress. Climate change can lead to range contractions and shifts in suitable habitats, ultimately resulting in population decline or extinction (Franklin, 2013; Khanal et al., 2022). Furthermore, since Humboldtia species maintain complex ecological interactions—such as domatia-mediated ant–plant mutualisms—any disruption to these relationships or to forest structure could further increase their extinction risk. There are limited conservation efforts for this endangered, endemic tree genus.

Given that Humboldtia is an endemic genus, opportunities for funding and collaborative research are limited, and plant conservation initiatives in India already face significant financial constraints. Although both in situ and ex situ conservation approaches can be adopted, in situ methods are generally preferred, as Humboldtia species maintain ecological associations with several other organisms, such as the Malabar giant squirrel. Molecular and genetic research on the Humboldtia genus remains scarce. One of the few available studies assessed the genetic diversity of H. brunonis using ISSR markers (Dev et al., 2010). Restoration or reintroduction programmes could also be developed for this genus. Furthermore, ecological niche modelling combined with genetic studies can provide critical insights to guide conservation-oriented restoration strategies for Humboldtia species.

The genus Humboldtia represents an ecologically and evolutionarily important lineage endemic to the Western Ghats–Sri Lanka biodiversity hotspot. Its narrow distribution and ecological adaptations render it an excellent model system for investigating speciation, biogeographic history, and ecological specialisation within tropical forest ecosystems. This review underscores that Humboldtia remains a scientifically valuable yet underexplored component of the Western Ghats flora. The genus exhibits pronounced endemism, habitat specificity, and complex ecological interactions, all of which reinforce its significance for both research and conservation. Despite this, substantial gaps persist in understanding its ecology, genomics, and phylogeny. Furthermore, field-based investigations are often constrained by the rarity of populations and their occurrence in remote forest habitats. Advancing the conservation of Humboldtia requires an integrated approach that combines molecular and ecological research with in situ and ex situ conservation measures, implemented through collaborations among researchers, local communities, forest management agencies, and policy-makers.

Acknowledgements

This work was supported by the Council of Scientific and Industrial Research - University Grants Commission (CSIR-UGC) fellowship [221610031949] and the Mohamed Bin Zayed Conservation (MBZ) Fund [240535253]. The MBZ grant is funded as part of the Fonseca Leadership Program, which was established by the Global Environment Facility (GEF). The authors are grateful to the CSIR-UGC and the University of Hyderabad for providing facilities to carry out the research work. They also thank the forest departments of Kerala and Karnataka for granting permission for the fieldwork. The second author would like to thank the CSIR Pool scientist scheme for the support and facilities. We are grateful for the support of the ORG.one project of Oxford Nanopore Technologies (ONT) (Kara Dicks), Rufford Grants (45249-1), Idea Wild Grants (Project ID- KJOSINDI0125-00), and Mohamed Bin Zayed Species Conservation (MBZ) Funds in our efforts to conserve threatened trees in the Western Ghats Biodiversity Hotspot Forest regions. Thankful to K.M.G. Gehan Jayasuriya, K Murugesan, Joseph Mappilacherry, Vinayaraj, and Karthikeyan S for providing the photos for the manuscript. The photo plate was made by Amal Thankachan (Lamadesigns).

Data availability statement

All data present in the manuscript.

Ethical statement

All ethical practices have been followed in relation to the development, writing, and publication of the article. All authors agreed on the ethics policy of the Journal.

Credit authorship contribution statement

Jithu K Jose: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Validation, Visualization, Writing – original draft, Writing – review & editing.Saranya KRL: Conceptualization, Formal analysis, Resources, Supervision, Validation; Visualization, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.pld.2025.12.014.