Masonry is serious business for mason bees. They build their brood cells inside cavities such as hollow reeds, rotting wood or cracks in buildings. They first fill these cells with pollen and nectar, to feed the larvae that will grow inside, before sealing them off with a mix of mud and sand. The mud partitions between the cells are like a shield: they prevent the food supply from spoiling by keep moist and microbes out of the door.
Relatives of the mason bee (their family is called Megachilidae) prefer different natural construction materials. Leafcutter bees line their cells with leaf disks that they cut from leaves with their mandibles, for example. And the carder bees scrape off the tiny hairs that grow on plant leaves to include in their nests. There is no end to the natural resources that the Megachilidae exploit. There are bees that specialize in collecting resin, animal hairs, plant hairs, leaves, mud, petals and pebbles. Confronted with this diverse family of artisan bees, the French entomologist Jean-Henri Fabre asked a simple question: “Why al these different trades?”*
The answer, ironically, comes from bees that have never learned such a trade: the Fidelia and the Neofidelia, which live in South-America and Africa, respectively. Jessica Litman and her colleagues recently published an updated family tree of Megachilidae. The Fidelia and Neofidelia are placed on the first two branches of this new tree (see below). The differences in their DNA suggest that they shared a common ancestor around 126 million years ago. While the Fidelia and Neofidelia now live on two different continents, separated by the Atlantic, South-America and Africa were still united in the supercontinent of Gondwana at this time. Given their current distribution, it is likely that the ancestor of these two groups lived in Gondwana, before the continents had split up.
Since the ancestor of the Fidelia and Neofidelia is also the ancestor of all the Megachilidae (again, see the family tree), every bee in this family can trace its ancestry back to Gondwana. Man might have come out of Africa, but the mason bee came out of Gondwana.
The Fidelia and Neofidelia have never strayed far from their ancestral territories, whereas their cousins can be found all over the planet. Why is that? Litman suggests that this has everything to do with their nesting behaviour. Unlike their leafcutting and cement mixing cousins, the Fidelia and Neofidelia don’t use any foreign materials in the construction of their nests. Their larvae grow up in underground burrows that lie naked in the sand. Such nests are vulnerable to moist and rain, which can make the mass of pollen rot. This behaviour severely limits a bee’s potential range, for it can only live and breed in environments where seasonal rainfall is low.
One lineage of bees circumvented this problem. It learned how to build nests using foreign materials that protect the larvae and its food. They then escaped the desert, diversified and colonized the wetter half of our planet. These builder bees became the ancestor of modern mason, leafcutter and carder bees. The story of their success can be measured in raw numbers. There are many more species of leafcutter bees and mason bees (3900) than there are Fidelia and Neofidelia (17, including the Pararhophites).
The Fidelia and Neofidelia were already out of the loop when their relatives evolved a more sophisticated nesting behaviour. They still follow the old ways and build their nests with unlined cells, like the ancestral Megachile bee in Gondwana must have done. Biologists call such an ancestral trait a ‘plesiomorphic‘ trait. This does not make the Fidelia and Neofidelia ‘primitive’. One of their traits just happens to resemble the ancestral form.
Further evidence that building simple, unlined nests is the ancestral condition for Megachile bees, comes from the apoid wasps, the wasps from which bees evolved. They also store their paralyzed prey in unlined burrows.
Litman finishes her article with some interesting speculation that the Fidelia and Neofidelia resemble their wasp-like ancestors in another way. They only collect pollen from a subset of flowers: they have to be large and have radial symmetry and stamens that are well exposed. Other Megachiles are not so picky: they happily collect pollen from bilaterally symmetrical flowers for example. Apoid wasps are specialized hunters that hunt for prey that have similar size and stature. They might be programmed to ‘respond to prey of a certain size and behaviour‘. The finicky nature of fideliini could be the heritage of this programmed hunting behaviour. If this is true, this would mean the shift from prey to pollen in the Megachilidae wasn’t the driving factor in the the diversification of bees, as has been suggested previously. Other behaviours evolved first, before bees started to exploit the wide range of flowers available to them.
While their ways might be ancient, they still serve the fideliini well. After all, they are still around in these modern times. And so, as the mason bee mixes her clay and the carder bee combs the leaves, another one still builds her simple nests, just like her ancestors have done for millions of years. Dig on, little friends.
* In his essay on Megachiles, Jean Henri Fabre also gives an answer to his question: “I foresee the answer: they are prescribed by the organization. An insect excellently equipped for gathering and felting cotton is ill-equipped for cutting leaves, kneading mud or mixing resin. The tool in its possession decides its trade.” While a true statement, this is not a great answer. It is akin to answering the question why lions hunt gazelles by saying that lions have claws and gazelles have hoofs. This is not the cause of their predator-prey relationship, it is a consequence.
Leafcutter bee image by Bernhard Plank.
Litman JR, Danforth BN, Eardley CD, & Praz CJ (2011). Why do leafcutter bees cut leaves? New insights into the early evolution of bees. Proceedings. Biological sciences / The Royal Society PMID: 21490010
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