Giant pandas lose smell genes just like seals and whales, study finds

When a giant panda wanders through a bamboo forest, it is navigating one of the most sensory-deprived landscapes a mammal can occupy. A new genomic study now suggests that this lifestyle has left a measurable signature in the panda’s DNA—one that looks strikingly similar to the genetic decay seen in seals, sea lions, and other carnivores that abandoned the land for the sea. The research, published in the journal iScience, compared olfactory receptor genes across 25 carnivoran species and found that the bamboo-eating panda has shed its sense of smell at a pace comparable to fully aquatic predators.

Olfactory receptors are the molecular gatekeepers of smell. Expressed in the olfactory epithelium of the nasal cavity, these G protein-coupled receptors each carry seven alpha-helical transmembrane domains and span roughly 310 amino acids. When odorant molecules drift into the nose, they bind to these receptors in a combinatorial fashion: a single odorant can activate multiple receptors, and each receptor can respond to several odorants. The brain then reads the resulting pattern of neural activity as a unique odor code, allowing animals to distinguish an enormous range of chemical cues in their environment—from the scent of ripe fruit to the chemical signature of an approaching predator.

The size of this receptor repertoire varies enormously across mammals, and those differences often track ecological niches. The African elephant holds the record with nearly 2,000 putatively functional olfactory receptor genes, while cetaceans—the whales and dolphins—retain only a few dozen. During the evolutionary transition from land to water, marine mammals lost vast numbers of these genes, and toothed whales compensated by evolving echolocation. The platypus, with roughly 350 functional receptors, traded olfaction for electroreception in its semi-aquatic lifestyle. Primates, including humans, also experienced accelerated olfactory gene loss, likely tied to their greater reliance on vision and, in some lineages, a dietary shift from fruit foraging to leaf eating.

But the giant panda is a terrestrial mammal that never left the land. So why would its olfactory genome look degraded? The research team, led by Liwen Kang of China, hypothesized that habitat, dietary specialization, and habitat breadth are the key drivers of olfactory receptor evolution within Carnivora—the order that includes cats, dogs, bears, seals, and civets. Their prediction was straightforward: species with narrow ecological niches and specialized diets, whether marine carnivorans or bamboo-eating pandas, should possess fewer intact olfactory receptor genes and a higher proportion of pseudogenes compared with generalist terrestrial carnivorans.

To test this, the researchers annotated olfactory receptor genes from the whole-genome sequences of 25 carnivoran mammals spanning nine families, from big cats like the leopard and tiger to pinnipeds like the ringed seal and Steller sea lion. Using the Genome2OR pipeline—an automated annotation tool that outperforms older methods and maps 92.3 to 99.4 percent of its sequences to the curated HORDE database—they identified a total of 20,685 intact olfactory receptor genes. The results were unambiguous: marine carnivorans carried only 333 to 367 intact genes, while terrestrial carnivorans carried 702 to 1,393. The proportion of pseudogenes—genes rendered nonfunctional by frameshift mutations, premature stop codons, or severe structural disruptions—was significantly higher in marine species, ranging from 49.31 to 59.82 percent, compared with 24.15 to 49.75 percent in terrestrial ones.

The giant panda stood out immediately. Among terrestrial carnivorans, it possessed the lowest number of intact olfactory receptor genes at 702—considerably fewer than other bear species, which ranged from 813 to 1,276. Its pseudogene proportion was exceptionally high at 49.75 percent, reaching levels comparable to fully marine carnivorans. This finding challenges a previous study of 23 mammalian species that included the panda but reported no evidence of olfactory degeneration. The discrepancy likely stems from differences in genome assembly versions and annotation pipelines: earlier studies reported 611, 645, or even 408 functional genes for the panda depending on the method used, while the current work, with updated chromosome-level assemblies and standardized full-length prediction algorithms, recovered 702.

To trace the evolutionary history of these genes, the researchers organized all intact genes into 374 orthologous gene groups—clusters of genes descended from a single ancestral gene in the carnivoran common ancestor. Most groups contained relatively few genes, but the number of intact genes per group varied considerably among species. In one large group, the American black bear carried 124 intact genes while the ringed seal carried only 16. Marine carnivorans had significantly fewer orthologous groups overall, ranging from 190 to 200, compared with 259 to 333 in terrestrial species. The giant panda had the fewest among terrestrial carnivorans at 259.

Phylogenetic reconstruction revealed that every one of the 25 species has lost dozens to more than one hundred complete olfactory receptor genes that were present in their most recent common ancestor. Marine carnivorans lost far more ancestral groups—174 to 184—than terrestrial carnivorans, which lost 41 to 115. The giant panda lost the largest number among terrestrial carnivorans at 115 ancestral groups. Meanwhile, selective pressure analysis showed that all carnivorans operate under purifying selection, with omega values below 1, meaning deleterious mutations are still preferentially eliminated. But the intensity of that selection varied: orthologous groups from marine carnivorans and the giant panda consistently displayed elevated omega values relative to other terrestrial carnivorans, indicating relaxed purifying selection and accelerated accumulation of loss-of-function variants.

To identify the ecological factors driving these patterns, the researchers constructed generalized linear models quantifying the independent effects of habitat type, diet type, and habitat breadth on olfactory receptor copy number, pseudogene proportion, and selective pressure. For intact gene counts, habitat type was the primary predictor: terrestrial species averaged 1.79 times more intact genes than aquatic species, and generalist diets conferred an additional 1.54-fold advantage. For pseudogene proportion, diet type dominated—generalist species had an odds ratio of only 45.2 percent relative to specialists, meaning they were far less likely to carry nonfunctional receptor genes. Habitat breadth showed no significant effect on gene counts but was positively correlated with pseudogene proportion.

The convergent loss of olfactory receptor genes in marine carnivorans likely reflects their aquatic sensory environment, where most environmental odorants are unavailable and olfactory cues carry little survival value. In the giant panda, the researchers argue, the driving factors are different but equally powerful: pandas inhabit forests overwhelmingly covered by bamboo, where diverse environmental odorants are less critical for survival, and their nearly exclusive bamboo diet creates a stable, uniform food resource that lowers the ecological demand for broad-spectrum odor detection. Together, these factors weaken the purifying selection that normally maintains full olfactory sensitivity, allowing disruptive mutations to accumulate and pseudogenes to proliferate.

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Subject of Research: Genetics

Article Title: Similar degenerative patterns of olfactory receptor genes in the giant panda and marine carnivores

Article References: Kang, L., Chen, J., Hong, M., Wei, W., Xiang, Z., & Zhang, Z. (2026). Similar degenerative patterns of olfactory receptor genes in the giant panda and marine carnivores. iScience, 29(11), Article 117936. https://doi.org/10.1016/j.isci.2026.117936

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117936

Keywords: Similar, degenerative, patterns, olfactory, receptor, genes, giant, panda, marine, carnivores, scientific research