Host taxonomy and temperature differentially shape gut microbiome assembly in tropical moths across an Andean-Amazonian Mountain gradient

In the hidden world inside an insect's gut, a bustling community of bacteria lives, helping its host digest food and navigate its environment. For decades, scientists have known that these microscopic residents are not random; they are shaped by who the host is and where it lives. But for the vast majority of moth species, which make up the bulk of the insect world's diversity, we know very little about these internal communities. Unlike the well-studied honeybee, which relies on a specific set of bacteria passed down through its hive, moths are solitary creatures. Their larvae eat leaves, and their adults sip nectar, moving freely through the air. It has been unclear whether the bacteria inside them are strictly determined by the moth's family tree or if the changing weather around them plays a bigger role. This question matters because as the planet warms, understanding how these tiny partnerships hold together could tell us how entire ecosystems might survive or struggle in a hotter future.

To answer this, researchers set out to explore the gut contents of moths across a dramatic slice of the Peruvian Andes. They chose a location where the landscape rises steeply from the humid lowlands to the cold, high peaks, covering a vertical distance of over 3,300 meters. This natural staircase of elevation creates a perfect laboratory for studying temperature, as the air gets steadily colder the higher one goes. The team focused on six different groups of moths, representing two large families that are common throughout the region. They collected hundreds of adult moths at twenty-eight different sites, ranging from the warm valleys to the frigid heights. Using light traps to catch the insects at night, the scientists carefully extracted the gut from each moth, preserving the bacterial DNA before it could degrade. They then sequenced the genetic material of these bacteria to identify exactly which species were present, comparing the results against the specific location and temperature where each moth was caught.

The findings revealed a clear hierarchy of influence. The most powerful factor determining which bacteria lived inside a moth was the moth itself. The specific family and genus of the insect acted as a strong filter, creating distinct bacterial communities that stayed consistent regardless of the altitude. For instance, the bacteria found in one group of moths were consistently different from those in another group, suggesting that the host's biology sets the stage for its microbial guests. However, the environment was not powerless. While the main cast of characters was determined by the host, the temperature of the habitat influenced how diverse these communities were. Contrary to the expectation that life becomes simpler and less diverse in colder, high-altitude environments, the researchers found that microbial diversity actually increased as the moths climbed higher into the cooler air. This trend was not uniform across all moth groups, but the general pattern showed that the cold, high elevations harbored a richer variety of bacteria than the warm lowlands.

Temperature also played a specific role in how different these communities were from one another. When the researchers compared moths caught far apart in elevation, the bacterial communities became increasingly different the greater the distance between them. This effect was particularly strong in warmer conditions, where small changes in temperature led to rapid shifts in the types of bacteria present. In the cooler, high-altitude zones, the communities were more stable and similar to each other. This suggests that while the host moth decides the general type of bacteria it carries, the surrounding temperature fine-tunes the mix, especially at the lower levels of the bacterial family tree. The study also noted that some moth groups, like those that feed on lichens, shared more similar bacterial profiles with each other than with other groups, further highlighting the link between diet, host identity, and microbial life.

These results challenge the simple idea that environmental stress always reduces biological complexity. Instead, they suggest that in the high Andes, the cooler temperatures may support a wider array of bacterial life within the moths. The researchers emphasize that this is a snapshot of wild moths in their natural habitat, a perspective often missing from laboratory studies. The work shows that while a moth's evolutionary history is the primary architect of its gut microbiome, the climate acts as a dynamic sculptor, shaping the diversity and turnover of these invisible communities. As global temperatures rise, these delicate interactions could shift, potentially altering how moths function in their ecosystems. The study does not predict a specific collapse or boom, but it establishes a baseline for understanding how these partnerships respond to the changing world, reminding us that even the smallest creatures carry complex histories written in their genes and their guts.