Termites are among the most dominant animals on the planet, forming enormous colonies that can contain millions of individuals. Their highly organized societies raise an obvious question: how did insects with such advanced social systems evolve from solitary ancestors that closely resembled modern cockroaches?
New research from the University of Sydney points to an unexpected explanation. Rather than becoming more complex by adding new genes, termites evolved advanced social behavior by shedding genes, including those associated with sperm competition. The findings provide new insight into a long-standing scientific debate over whether monogamy is a critical step in the evolution of complex insect societies.
The international study, published on January 29 in Science, traces termites back to ordinary cockroaches, including the ancestors of today's 'domestic' cockroaches, that began feeding on dead wood. That shift in diet set off a series of genetic and behavioral changes that eventually gave rise to termites and their tightly structured colonies.
The research was carried out by an international team that included scientists from China, Denmark, and Colombia.
"Termites evolved from cockroach ancestors that started living inside and eating wood," said Professor Nathan Lo from the University of Sydney's School of Life and Environmental Sciences, a senior author on the paper. "Our study shows how their DNA changed first as they specialized on this poor-quality diet and then changed again as they became social insects."
To understand how these changes unfolded, the researchers analyzed and compared high-quality genomes from cockroaches, woodroaches, and several termite species with varying levels of social organization. Woodroaches are close relatives of termites and live in small family groups, making them an important evolutionary link.
One of the clearest patterns to emerge was that termite and woodroach genomes are smaller and less complex than cockroach genomes. As termites became more dependent on cooperation and food sharing within their colonies, they lost many genes involved in metabolism, digestion, and reproduction.
"The surprising result is that termites increased their social complexity by losing genetic complexity," Professor Lo said. "That goes against a common assumption that more complex animal societies require more complex genomes."
Some of the most revealing genetic losses involved genes responsible for forming the tail, or flagellum, of sperm. Unlike cockroaches and most animals, termite sperm lack tails and are unable to swim.
"This loss doesn't cause monogamy," Professor Lo said. "Instead, it's a strong indicator that monogamy had already evolved."
In many animals, including cockroaches, females mate with multiple males. This leads to intense sperm competition, favoring sperm that can swim quickly using tails. Once termite ancestors became monogamous, that competition disappeared. Without sperm competition, there was no longer an advantage to maintaining genes that support sperm movement.
"Our results indicate that the ancestors of termites were strictly monogamous," Professor Lo said. "Once monogamy was locked in, there was no longer any evolutionary pressure to maintain genes involved in sperm motility."
These findings speak directly to a broader scientific debate about whether close genetic relatedness is necessary for complex social systems to evolve. While some researchers have argued that high relatedness is not required, the new evidence suggests that monogamy and strong genetic ties were essential for termite societies.
The study also explains how termite colonies organize themselves internally. Experiments showed that whether a young termite becomes a worker or a future king or queen depends largely on nutrition during early development.
Larvae that receive abundant food from older siblings develop high energy metabolism and become workers, which do not reproduce. Larvae that receive less food grow more slowly at first and retain the ability to become reproductives later in life, meaning kings or queens.
"These food-sharing feedback loops allow colonies to fine-tune their workforce," Professor Lo said. "They help explain how termites maintain stable, highly efficient societies over long periods."
When a termite king or queen dies, monogamy usually continues. In many cases, one of their offspring takes over the reproductive role, which results in widespread inbreeding within colonies.
"From an evolutionary perspective, that reinforces relatedness even further," said Professor Lo, who is part of a dynamic and growing insect research group in the School of Life and Environmental Sciences at the University of Sydney.
By combining genomic data with physiological and behavioral studies, the researchers present one of the most detailed explanations so far of how termites transitioned from solitary, cockroach-like ancestors to some of the most socially complex organisms on Earth.
"This work shows that understanding social evolution isn't just about adding new traits," Professor Lo said. "Sometimes, it's about what evolution chooses to let go."
Funding was received from the National Natural Science Foundation of China, the Department of Science and Technology of Guangdong Province and the Australian Research Council.
