The carpenter bee (Xylocopa violacea) is now more common in cities than in its original environment. It’s really a bee, but so big and black that humans have enhanced it. This size seems to have helped him to colonize an environment so different from his own and full of new possibilities but also dangers: the city. New work with dozens of species of bees and bumblebees now shows that species with larger heads are more common in urban areas. This finding would confirm in insects what has already been observed in other creatures, starting with humans, namely that a larger brain offers additional adaptive abilities.
Cities with 10,000 years of history are something new for species that have lived on the planet for thousands, if not millions, of years. Some knew how to exploit them immediately, such as pets, rats or cockroaches. In general, these are ambivalent habitats for animals. They can be a dangerous place, especially for predators that competed with humans. This has not prevented many creatures from taking advantage of the advantages they also offer, such as the absence of enemies and competitors, the availability of food and even better climatic conditions. For some birds and mammals, cities have become their last refuge. But why have some creatures adapted to cities and others not? The answer could lie in the size of your head.
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A group of Spanish scientists asked the same question, but about the family of species to which bees and bumblebees belong, the Apidae. They examined the occurrence of specimens of 89 aphid species from North America and Europe in three environments: natural, agricultural and urban. At the same time, they measured their average size, that of their head and the ratio between body and head. The results of their work, published in the journal Biology Letters, show that bees generally dislike the city. In particular, the discovery of specimens of 56 species in city parks and gardens was rare or exceptional, always falling below 20% of observations of this species. But there are 28 others that frequent the cities. Even some, like the carpenter bee mentioned above or the European woolly cardia bee (Anthidium manicatum), already feed more on urban flowers and pollen than on rural or natural pollen. Of the latter, for example, they collected 2,800 urban observations compared to 350 collected in natural systems. “Just because they are good in urban environments does not mean that they are not present in natural environments,” explains José B. Lanueza, first author of this research when he worked at the Doñana Biological Station (EBD-CSIC).
“The presence or absence of bees in urban environments is not a coincidence, we observe a connection: species with larger heads tend to concentrate a larger number of specimens in cities,” emphasizes Lanueza, now at the German Research Center for Integrated Biodiversity Halle-Jena- Leipzig. There may be other factors that explain this, such as diet. But they saw that among urban bees there were both generalists, or polylectians (feeding on pollen and nectar from multiple families), and specialists, feeding from only one family of flowers. Furthermore, the larger the head-to-body ratio, the more consistent the relationship between head size and urbanity. It is logical and expected that large species such as the carpenter bee thrive in cities. Its size makes it easy to move from one park to another, which may be too far away for smaller aphids. But there are species like A. manicatum or the hairy-legged bee (Anthophora plumipes) that have a relatively larger head and are among the most urban of all. They also observed the opposite: species with smaller heads or lower head-to-body ratios tend to be less likely to be seen in the city.
Although most bee species avoid cities, there are others that occur in both urban and agricultural or natural environments, such as the mining bee “Andrena pilipe”. Curro Molina
In 1993, evolutionary biologists proposed the idea of the cognitive buffer. This hypothesis suggests that greater cognition protects animals from environmental changes by helping them decide where to live, what to eat, or what risks to avoid. And if one accepts that brain size is related to cognitive abilities, one can conclude that species with larger heads have greater plasticity, something that would fit into such changing environments as cities. This connection has been demonstrated in birds and mammals, particularly primates. “Especially in humans,” says Daniel Sol, researcher at the Center for Ecological Research and Forest Applications at the Autonomous University of Barcelona (CREAF/CSIC).
“This cognitive protector helps us decide what to do when faced with changes and new environments. It gives us the opportunity to choose,” explains Sol. “Humans are the best example of the cognitive buffer with which we have conquered the planet,” adds Sol, who has studied it in various animal groups, such as birds. “There are species that don’t need to change their behavior. The pigeons continue to eat in the city what they ate in the country. But others, such as herons and corvids, have changed their diet,” says Sol. “The cognitive buffer hypothesis has been demonstrated in animals that we consider to be intelligent, such as crows, parrots and primates,” adds the CREAF researcher. They are species with a relatively large brain, with a collection of neurons in the cerebral cortex (in mammals) or in the pallium (in birds). “But insect brains are very small. It was thought that they could not change complex behaviors, but a series of experiments disproved this,” concludes Sol.
“Humans are the best example of the cognitive buffer with which we have conquered the planet”
Daniel Sol, researcher at the Center for Ecological Research and Forest Applications at the Autonomous University of Barcelona
In one of these experiments, some bumblebees showed that they were able to learn from others. In a study with bees that were prevented from sleeping, they observed that they showed poorer retention. In 2021, work led by Doñana Biological Station ecologist Ignasi Bartomeus confirmed that bees have the ability to learn in new environments. “It was a very simple test in which they had to remember where the sugary reward was,” says the scientist. They saw that species with larger heads were more successful. This experiment led them to wonder how bees fared in cities and discovered that the more unruly species fit in better. Bartomeus warns against going beyond the results achieved. The authors acknowledge that the work has its limitations. The first is twofold: it is believed that a larger head always means a larger brain, and that a larger brain also means larger cognitive abilities. “But intelligence is something very complex that we cannot reduce to size,” he remembers. In addition, the work is based on 89 species, of which there are about 20,000. Another caveat is that the relationship they observed (larger head means greater urbanity) could be in the opposite direction: that the challenges and opportunities presented by the city have exerted selection pressures favoring the more persistent insects.
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