Research: Cockroaches gather in groups to keep from drying out

When conditions get too dry, Madagascar hissing cockroaches like to cuddle.

Technically, the term is aggregation. Under certain conditions, the large insects gather in groups, with many participants in physical contact with one another. According to recent research from ��������’s Biological Sciences program, this strategy may prevent the cockroaches from drying out.

“The main takeaway from our study is that Madagascar hissing cockroaches actively adjust their social behavior based on humidity, showing that even large adult insects rely on behavioral plasticity to cope with environmental stressors,” explained Assistant Professor of Biological Sciences Lindsey Swierk.

“Plastic Behavioral Responses to Ambient Relative Humidity Influence Aggregation in a Large Gregarious Insect” recently appeared in the journal Ethology. The first four co-authors — Alison Kryger, Kayla Huang, Kayhan Tabrizifard and Justin Yang — were students in Swierk’s fall Animal Behavior 2024 undergraduate course. The course has an associated laboratory in which students form research groups, conduct experiments, and analyze and write up their results in the style of a scientific manuscript.

“The students’ research projects are generally outstanding — and many of these student groups are motivated to submit their research for publication following the end of the course,” said Swierk, the paper’s senior author.

This is the fourth student group from Swierk’s Animal Behavior class to have their work published in a peer-reviewed journal since 2022; another paper from a different group is currently undergoing peer review. The paper’s fifth author, Alexandra Martin, PhD ’25, is a recent graduate of Swierk’s lab and offered guidance to the undergraduates on experimental design and data interpretation.

Adjusting to climate change

Madagascar hissing cockroaches are around 2 to 3 inches long and widely used in educational settings. Surprisingly, many aspects of their natural history aren’t well understood, Swierk said.

In their native Madagascar, these insects form large social groups. The African island experiences both a wet season and a dry season, and Swierk’s Animal Behavior students found that the roaches’ tendency to aggregate is connected with those swings in humidity.

When the humidity is low, cockroaches in the experiment were significantly more likely to gather into groups, with more of them in physical contact with one another. Aggregation was less common under high humidity, and groups that did form included fewer individuals, the researchers discovered.

“In general, insects can lose water pretty quickly because of their high surface area to volume ratios, and so humidity really affects their abilities to retain moisture,” Swierk explained. “Our research shows that even larger adult insects, that may in theory be more resistant to low humidity than smaller or larval insects, still use aggregation as a flexible behavioral adaptation to reduce the risk of water loss.”

When the air is dry, insects lose water through their cuticle and through breathing. Physiological adaptations can help them fend off desiccation, including a layer of hydrocarbons on their exoskeletons and changes in metabolic rates. Aggregation is another such strategy, which has been previously observed in small insects and insect larvae.

By grouping together, the cockroaches create a type of micro-climate that increases humidity in their immediate area. But this strategy comes with costs; aggregation could increase social competition or make the insects easier prey for other animals.

Hissing cockroaches, who dwell on the forest floor, play an important role in the environment as decomposers and prey for other animals. Changes in their behavior could end up influencing the larger ecosystem, which is already experiencing the impact of climate change; Madagascar’s dry seasons are becoming longer and more severe, with the humidity in some regions dropping to very low levels, Swierk said.

“Because we now know that these cockroaches adjust their aggregation behavior in response to humidity, more frequent or extreme dry periods could push them to aggregate more often to conserve water, which could potentially affect their foraging, reproduction, or broader ecological roles as decomposers,” she said.

Most likely, humidity is only one of many factors that influence aggregation among Madagascar cockroaches. Whether the hissing cockroach findings apply more widely to other large insect species remains to be seen.

“Our study suggests that such aggregation behavior under low humidity levels could more generally apply to larger insects, as well as small or larval insects, but of course this will also depend on species’ natural history and social behaviors,” Swierk said.