New World monkey olfactory evolution follows separate lineage paths rather than one shared rule
Variation in olfactory receptor repertoires across primates has often been linked to common ecological pressures such as diet, activity pattern, and vision. In this preprint, a broader view of New World monkeys tells a different story. Across 80 species, the size and structure of olfactory repertoires do not line up neatly with those shared drivers, and the same broad outcome — fewer receptors — emerges through very different genomic routes in different lineages.
The central message is plain: olfactory evolution in these monkeys looks shaped more by history than by a single universal ecological pressure.
That conclusion matters because primate studies have often leaned on a small number of lineages to infer how sensory systems evolve. Here, the broader sampling weakens simple rules. Even howler monkeys, which are routinely trichromatic and leaf-eating, do not show an especially large olfactory repertoire. The usual expectation — that a visually oriented, folivorous primate would have a diminished need for smell, or that a more olfaction-dependent species would show a dramatic expansion — does not hold in any straightforward way across the radiation.
Ecological and sensory traits explain little of the genomic variation
The authors combined genomic data, CT-based olfactory anatomy, and ecological information to examine what drives repertoire variation. Across the dataset, ecological and sensory variables account for little of the diversity in gene repertoire size. Diet and diurnality, long treated as plausible correlates of olfactory investment, fail to explain the broad pattern on their own.
Instead of a clean relationship between behavior and genome architecture, the pattern is patchy. Some lineages retain large repertoires despite major shifts in ecology, while others lose genes through different kinds of genomic erosion. This is the kind of result that argues against a single adaptive script. It also suggests that the ancestral starting point, followed by lineage-specific constraints and chance events, has left a stronger imprint than a shared response to the same environment.
Receptor loss came from different genomic mechanisms in separate lineages
The most striking contrast concerns how convergent reductions in repertoire size actually arose. In titi monkeys, the reduction reflects extensive gene inactivation. In squirrel monkeys, the same broad outcome comes from large structural deletions. The phenotype converges; the genomic route does not.
This distinction is biologically important. Gene inactivation and structural deletion do not represent the same evolutionary process, even if both end with fewer functional receptors. One mechanism leaves behind a remnant of the ancestral gene family; the other removes large pieces of sequence entirely. In sensory evolution, that difference signals different mutational histories and different constraints on how a receptor family can shrink over time.
The data therefore support a model of convergent evolution with deep mechanistic heterogeneity. The same endpoint can arise through unrelated genomic trajectories, which makes a simple “same environment, same solution” explanation too blunt for this system.
Owl monkeys kept the ancestral repertoire and expanded the bulb instead
Owl monkeys introduce a separate twist. They are the only anthropoids to have reverted to nocturnality, yet they did not respond by expanding their olfactory receptor repertoire. Instead, they retained a large ancestral repertoire shared with diurnal capuchins and enlarged their olfactory bulb.
A single sentence captures the surprise: nocturnality did not trigger receptor expansion.
The implication is that olfactory adaptation can shift at different biological levels. Repertoire size and neural processing capacity do not necessarily move in lockstep. A larger bulb may support increased central processing of sensory input without requiring a matching increase in receptor number, or it may reflect altered wiring and integration demands rather than raw receptor diversity alone. In this case, the sensory system seems to have changed by emphasizing anatomy over gene family expansion.
Genomic and anatomical change correlate across the radiation, but not uniformly within lineages
Across the broader radiation, repertoire size covaries with olfactory bulb volume. Species with larger repertoires tend to have larger bulbs, which is consistent with some degree of coordinated sensory evolution. Yet the relationship weakens within lineages. Squirrel monkeys are the clearest exception: they lose receptors without a matching reduction in bulb size.
That partial decoupling is one of the most informative findings in the preprint. It suggests that genome evolution and neuroanatomical evolution can proceed on different schedules. A receptor family may shrink rapidly while brain structure remains comparatively stable, at least for a time. In practical evolutionary terms, this means that anatomy can preserve a footprint of ancestral sensory demands even after the underlying gene repertoire has shifted.
The pattern also cautions against using a single morphological measure as a proxy for the entire olfactory system. Bulb volume carries meaningful information, but it does not fully mirror the genomic state in every lineage. The mismatch in squirrel monkeys shows that receptor loss and bulb remodeling are not obligatorily synchronized.
Historical contingency better fits the data than shared ecological pressure
What emerges from the study is a story of contingency. The same broad trait — olfactory repertoire reduction — appears in multiple lineages, but the route to that outcome differs, and the expected ecological explanations only weakly predict the pattern. Evolution here is not following one shared playbook across New World monkeys.
That matters for how primate sensory evolution gets interpreted. When a trait appears repeatedly, it is tempting to attribute the repetition to the same ecological pressure. These data argue for caution. Shared phenotype does not guarantee shared mechanism, and shared mechanism is not required for shared phenotype. The lineage history itself can channel the direction of change, leaving closely related species with different genomic solutions to superficially similar sensory demands.
The study is also a reminder that olfactory evolution cannot be reduced to a single axis of gain or loss. Receptor repertoires, gene inactivation, structural deletion, and bulb size each capture a different layer of the system. The preprint shows that those layers can line up in one lineage and diverge in another, producing a mosaic rather than a uniform evolutionary trend.
Because this is a preprint, the findings have not yet undergone peer review. Even so, the combination of broad species sampling and parallel genomic and anatomical data gives the analysis unusual force. The remaining question is not whether olfactory systems changed in New World monkeys — they clearly did — but how often similar sensory outcomes in primates are built from the same biology rather than from different histories arriving at the same end point.