| This week, while you're enjoying a 4th of July picnic, give an appreciative thought to the ants, bees and wasps doing their best to get to your potato salad. For a holiday about banding together to form a more perfect union, let us celebrate social insects, which started living together in complex societies millions of years before us. Admiration for ants and other social insects as paragons of selflessness and industry goes back at least as far as Aesop, and modern ecologists have tallied our collective debt to them for their roles in pollination, aerating the soil, distributing seeds, controlling pests, nourishing other life and countless other ecosystem services. But what makes social insects intriguing for evolutionary biologists is that they live in cooperative nests where adults share in the burden of raising and protecting offspring. Often a single queen lays eggs and breeds with her squad of male drones, while other females dedicate their lives to the common good. On its face, such a "eusocial" arrangement sounds wildly contrary to the "me first" ethos of natural selection. For years, scientists thought they had found a resolution to this evolutionary paradox in landmark work published by William D. Hamilton in 1964, which formalized earlier speculations by J.B.S. Haldane. Ants, bees and wasps have the genetic quirk of haplodiploidy: The females develop from fertilized eggs, while males develop from unfertilized eggs, with half as many chromosomes. As a bizarre consequence, females have on average more genes in common with their sisters than with their daughters. Hamilton showed that under the tenets of kin selection and "inclusive fitness theory," female ants can get more of their DNA to the next generation by helping raise sisters than they can by mating and having babies of their own. Kin selection, however, fell short as a full solution for the mystery of social insects. It didn't explain anything about termites, for example, which aren't haplodiploid. It didn't explain why more than 90% of haplodiploid bees don't live in colonies. Worse, work by Robert Trivers and Hope Hare in the 1970s showed that the genetic relatedness of males to workers could offset the bias for siblings over offspring. In summer 2010, three researchers — including Edward O. Wilson, perhaps the world's most eminent ant scholar — set off fireworks with a paper that rejected the kin selection model: It's more likely, they argued, that other traits predisposed some insects to evolve increasingly cooperative behaviors, and that haplodiploidy at best enabled that strategy. More than 100 biologists promptly responded with fierce rebuttals, and most evolutionary biologists who study eusociality are probably still loyal to kin selection as an explanation. Beyond that debate, there's still much else to learn about the origins of insect eusociality. The ability of social insect communities to act with astonishing collective ingenuity never ceases to amaze both biologists and frustrated picnickers.
What's New and Noteworthy Some of the most fascinating recent work on social insects has focused on understanding what drives the queens, workers and others to behave so communally. Surprisingly, metabolic pathways involving the hormone insulin seem to be key. Several years ago, Daniel Kronauer and other researchers discovered that the larvae of some ant, honeybee and wasp species release manipulative chemical signals that decrease insulin production in nearby adult workers to make them drop other tasks and take care of the larvae. Evolution may have helped lock that behavioral mechanism in an "on" position, creating permanent nursemaid worker ants. When researchers experimentally raised the insulin levels of workers, the usually nonreproductive insects started making eggs. A difference in insulin response also seems to be what enables ant queens to live 10 times as long as their worker daughters. (A type of tapeworm that infests ants naturally exploits this life-extension mechanism in its hosts for its own purposes.) You might think that hundreds of millions of years of evolution would have irrevocably committed ants to a communitarian way of life, but evolution is never a closed project, which is why some parasitic ant species have evolved to cheat the system. Instead of making all the workers they need, for example, some ant species raid the colonies of other species, steal their larvae and chemically bond them to serve their new stepmother queen. Another type of parasitic ant chemically camouflages itself to sneak into a nest and masquerade as a queen. Recently, researchers discovered that although the behaviors that make this kind of parasitism possible are elaborate, the genetic changes that enable them can arise with surprising speed. Ants also astonish people with their ability to weave their bodies into a mass to create bridges, rafts and other structures to span gaps in their environment — without any central sharing of information or decision-making. "There is no leader, no architect ant saying 'We need to build here,'" said Simon Garnier, a researcher at the New Jersey Institute of Technology, to Kevin Hartnett in a 2018 Quanta article. Garnier's work showed, however, that many of these construction feats arise from a surprisingly simple behavioral algorithm. | 
