Elephants utilize their trunks almost constantly, frequently contacting their environment with the trunk tip. Behavioral studies have demonstrated high sensitivity of the trunk tip to sensory stimuli, indicating a dense tactile innervation. The infraorbital nerve, responsible for tactile sensation in the trunk, is significantly larger than the optic and vestibulocochlear nerves, highlighting the importance of touch. Despite this behavioral relevance, detailed knowledge about the peripheral tactile specializations of the elephant trunk remains limited. The elephant trunk, a fusion of the elongated nose and upper lip, is a derived facial structure. Its immense size and weight necessitate an enlarged skull to support the extensive musculature (~40,000 muscles compared to 600-700 in humans), which functions as a muscular hydrostat. Trunk actions are highly skillful and lateralized. Morphological differences exist between African and Asian elephants, specifically in trunk tip morphology (two fingers in African vs. one in Asian), which correlates with species-specific trunk use. Previous research touched upon trunk whiskers, describing their presence and general follicle structure but lacked detailed anatomical and functional characterization. This study aims to address this gap by investigating several key aspects of elephant trunk whiskers across African and Asian elephant species.

Early descriptions of elephant trunk whiskers date back to 1890, distinguishing them from typical hairs as bristles with larger follicles. Subsequent studies confirmed the whisker-like nature of certain hairs on the lower lip and trunk. Sprinz (1952) traced nerves terminating in ventral trunk-tip whisker follicles, suggesting their crucial role in tactile stimulus transmission, noting a higher whisker count in African elephants. Rasmussen and Munger (1996) described the sensory neurohistology of an Asian elephant trunk finger. Research on related species like hyraxes and manatees detailed their whisker characteristics. However, a comprehensive study specifically focusing on elephant trunk whiskers has been lacking, prompting this investigation.

The study used specimens from zoo elephants that died of natural causes or were humanely euthanized, adhering to CITES regulations. Trunk tips were photographed using a high-resolution camera, and whisker counts were performed manually using ImageJ. Whisker thickness was measured using ImageJ. Behavioral observations of Asian elephants retrieving fruit from a closed box were conducted to study whisker movement during haptic tasks, using high-speed videography and subsequent whisker tracking using ImageJ and Python. Follicle-sinus complexes (FSCs) were dissected, and tissue sections were stained with hematoxylin-eosin and immunohistochemistry (Neurofilament H) for anatomical and innervation analysis. Micro-computed tomography (microCT) scans were performed on iodine-stained FSCs and whiskers to visualize follicle structure, whisker shape, and overall whisker patterns. Statistical analyses included t-tests, ANOVA, and Fisher's exact test using Python's NumPy, SciPy, and Scikit-posthocs packages. Sample sizes varied depending on the specific analysis.

African elephants possess significantly more trunk tip whiskers (621 ± 91) than Asian elephants (367 ± 44) (p = 0.0007, Welch's t-test). Adult elephants exhibit marked lateralized whisker abrasion, likely due to lateralized trunk use. Whisker length is longer in newborns and becomes significantly lateralized in adults. Elephant whiskers are thick, cylindrical, and show minimal tapering compared to rat whiskers. Elephant whisker follicles are exceptionally large, lacking a ring sinus, and vary in size and shape across trunk regions and species. Follicles are innervated by multiple nerves (~90 axons on average). High-speed video analysis revealed no active whisking, indicating trunk movements dictate whisker-object contact. Ventral trunk ridge whisker arrays, organized in two distinct rows, are densely packed and are hypothesized to facilitate object balancing. The overall number of whiskers on the trunk was considerably larger in the newborn African elephant (~1220) compared to the Asian elephant (~986).

The substantial differences in whisker number and lateralization between African and Asian elephants suggest adaptations related to their distinct feeding and trunk-use behaviors. The absence of dual whisker follicle replacement, as seen in rodents, may contribute to the observed lateralization of whisker length in adult elephants. The large size of the elephant whisker follicles, coupled with the lack of a ring sinus, may indicate specialization for sensing coarser tactile stimuli rather than fine-grained deflections. The extensive innervation of each follicle by multiple nerves suggests robust tactile input. The lack of whisking, along with the rigid skin embedding of the whiskers and absence of capsular whisker musculature, points to the trunk's high mobility rendering independent whisker movement redundant. The ventral trunk ridge whisker arrays represent a unique specialization likely involved in object manipulation and balance.

This study provides a comprehensive anatomical and functional characterization of elephant trunk whiskers, revealing unique features shaped by their specialized trunk behaviors. The marked differences between African and Asian elephants, particularly in whisker number and lateralization, highlight species-specific adaptations. The absence of whisking underscores the trunk's dominant role in tactile exploration. Future research should investigate the precise sensory roles of different whisker arrays and explore the relationship between whisker morphology, innervation, and specific trunk behaviors in greater detail.

The study relied on zoo elephant specimens, limiting the generalizability of findings to wild populations. The limited number of whole trunks available restricted certain analyses, particularly concerning complete whisker counts. While behavioral observations suggested a link between whisker abrasion and trunk laterality, more direct measures of trunk usage are needed to further strengthen this association. The study focuses predominantly on Asian and African savanna elephants and so more studies are needed to generalize the conclusions further.