Would you rather fight a horse-sized duck or 100 duck-sized horses?
The famous question, though implausible, reflects a ubiquitous tradeoff between quantity and quality. Now, a new study shows that this dilemma operates in biology at the evolutionary scale.
Research published in the journal Science Advances found that certain ant species structure their colonies by favoring quantity over quality. These species invest less into each individual's cuticle-the protective layer of the exoskeleton-which liberates nutritional resources to produce more workers. The strategy of producing less protected but more numerous workers was evolutionarily successful, the study found. The findings shed light on what happens to individuals when complex societies, like those seen in humans, evolve.
"There's this question in biology of what happens to individuals as societies they are in get more complex. For example, the individuals may themselves become simpler because tasks that a solitary organism would need to complete can be handled by a collective," said senior author Professor Evan Economo, chair of the Department of Entomology at the University of Maryland and head of the Biodiversity and Biocomplexity Unit at the Okinawa Institute of Science and Technology (OIST).
Part of this is that individuals may become "cheaper," meaning they are easier to produce in greater numbers but individually less robust.
"That idea hasn't been explicitly tested with large-scale analyses of social insects until now," said Prof. Economo.
Ants provide an ideal system to test how complex societies arise because colonies range from tens to millions of individuals.
"Ants are everywhere," said the study's lead author Arthur Matte, Ph.D. student in zoology at the University of Cambridge. "Yet the fundamental biological strategies which enabled their massive colonies and extraordinary diversification remain unclear."
The authors hypothesized that there may be a tradeoff between colony size and investment in the cuticle. The cuticle protects ants from predation, desiccation and disease. It also forms structural support for the muscles. However, it is nutritionally expensive, requiring scarce elements such as nitrogen and various minerals to build. A thicker cuticle demands more nutrients, which could limit how many individuals a colony can support.
Using a large dataset of 3D X-ray scans, the authors measured the cuticle and body volumes for more than 500 species of ants. Cuticle investment varied widely from 6% to 35% of an ant's total body volume. The team incorporated those data into evolutionary models and found that ants with less cuticle investment tended to have larger colony sizes.
Although thinner cuticles provide less protection, they may have facilitated the formation of large colonies. The authors think weaker armor may be part of a syndrome of beneficial social traits, including collective foraging, nest defense and division of labor that evolve with greater colony size.
"Ants reduce per-worker investment in one of the most nutritionally expensive tissues for the good of the collective," Matte explained. "They're shifting from self-investment toward a distributed workforce, resulting in more complex societies. It's a pattern that echoes the evolution of multicellularity, where cooperative units can be individually simpler than a solitary cell, yet collectively capable of far greater complexity."
Moreover, lower cuticle investment was associated with higher diversification rates-a measure of speciation events that biologists often use as a proxy for evolutionary success. Not many traits have been found to be associated with diversification in ants, Economo said, which makes the finding particularly intriguing.
It's not yet known why reducing cuticle investment leads to speciation, but one hypothesis is that it allows ants to occupy new habitats with scarce nutrients.
"Requiring less nitrogen could make them more versatile and able to conquer new environments," said Matte, who started this research during his master's program as an intern at OIST.
The authors propose that collective nest defense, disease control and other traits associated with complex societies reduced the need for formidable individual armor. This may have kicked off a positive feedback loop: Reduced cuticle investment facilitated larger colonies, which could have circumvented the need to build formidable armor.
"I think of this as the evolution of squishability," laughed Prof. Economo. "Many kids have discovered that insects aren't all equally robust."
Other social organisms, such as termites, may have trekked similar evolutionary paths, but this remains to be tested.
There are also implications for humans. The study invites analogies to military strategies. In early warfare, heavily armored knights gave way to specialized archers and crossbowmen. Prof. Economo also mentioned Lanchester's Laws-a set of mathematical equations devised during World War I that explore the advantages of outnumbering opponents with weak fighters versus outmuscling them one-on-one with fewer, stronger units.
"The tradeoff between quantity and quality is all around. It's in the food you eat, the books you read, the offspring you want to raise," Matte said. "It was fascinating to retrace how ants handled it through their long evolution. We could see lineages taking different directions, being shaped by different constraints and environments, and ultimately giving rise to the extraordinary diversity we observe today."
Original press release issued by the University of Maryland, available here.
