Canales, R. | Search bioRxiv

bioRxiv2026

Beyond Terrestrial Carnivores: Olfactory Bulb Anatomy in a Semi-Aquatic Mammal

The organization of the olfactory systems remains poorly understood in mustelids, particularly in lutrines adapted to semiaquatic environments. This study provides the first detailed anatomical, histological, lectin-histochemical, and immunohistochemical characterization of the main (MOB) and accessory (AOB) olfactory bulbs of the Asian small-clawed otter (Aonyx cinereus). Serial transverse and sagittal sections were examined using Nissl and Tolivia staining, a panel of lectins, and neuronal and olfactory immunohistochemical markers. Macroscopically, the olfactory bulbs were prominent and elongated, although no distinct AOB could be identified. Histologically, the MOB displayed a well-developed laminar organization, abundant mitral and tufted cells, and a clearly defined lateral olfactory tract. By contrast, the AOB was small, located dorsocaudally within the olfactory peduncle, and lacked the canonical lamination observed in many other mammals. Its identification was supported by the selective binding of Ulex europaeus agglutinin and strong Gi2 immunoreactivity in glomerular-like structures, whereas Go labeling was weak. LEA and STA labeled the superficial layers of both bulbs, while VVA and SBA recognized only scattered MOB glomeruli. Neuronal markers, including NCAM, NSE, NeuN, PGP 9.5, and {beta}-tubulin III, confirmed the neuronal nature of the AOB, while calretinin strongly labeled its glomerular structures and projection neurons. These findings reveal a marked contrast between a highly developed MOB and a small, poorly laminated but neurochemically differentiated AOB. This organization indicates that the vomeronasal pathway is retained in A. cinereus, although in a structurally reduced form, and provides a comparative basis for investigating chemosensory evolution in semiaquatic carnivorans.

neuroscience↗

bioRxiv2026

Otters, but not apes, prepare for mutually exclusive possibilities.

The ability to prepare for mutually exclusive outcomes is often considered uniquely human. Solving such problems requires anticipating alternative futures before acting. In the classic forked-tube task, the optimal strategy is to block both exits to secure a reward: children under four years and great apes typically fail, whereas older children succeed. Using this paradigm, we tested three Asian small-clawed otters (Aonyx cinereus) and one Eurasian otter (Lutra lutra) and compared their performance with chimpanzee data. Otters covered both exits significantly more often than chimpanzees, with all individuals succeeding within their first trials. Initial inconsistency in maintaining the strategy appeared linked to anatomical constraints that limited reward success. When retested two months later with an apparatus better suited to otter morphology, individuals adopted and maintained dual coverage as success increased, indicating that the behavior tracked the payoff structure of the task rather than reflecting low-level mechanisms such as trial-and-error learning. Together, these findings indicate that blocking both exits is an adaptive response to the tasks causal structure, supporting the ecological intelligence hypothesis: cognition evolves in response to ecological demands, particularly foraging challenges that place recurrent pressures on memory, decision-making, and executive control, rather than being driven solely by social complexity.

animal behavior and cognition↗