Social evolution – living with the family or a single life
The subject of this month’s series may seem a bit abstract at a first glance – social evolution. What’s that? In a nutshell, it means the evolution of species in which individuals instead of living on their own, come together in colonies. This has a number of implications, for reproduction, communication, behaviour and nutrition. Between the two extremes – a solitary or social life – there is a continuum, many shades of grey between the black and white. The diversity we see today shows us different stages of this.
The first thought most people have when thinking of bees is usually a honey bee colony in a box. This immediate association between “bees” and the species that is an exception in so many ways is quite surprising. I already explained in the bee diversity series what makes a bee. It’s not the honey. It’s the feeding on flowers for all nutrients. Again, in a nutshell.
At the beginning, there was a solitary bee
The very first bees, about 100 millions of years ago, were probably solitary. This means that these bees didn’t live in colonies, but as single individuals. In solitary bees, every female is fertile and does all the work of nest building, providing food to her offspring and all the other tasks of a bee life all alone. No one helping out. With the big advantage though, that every female has her own offspring and doesn’t need to work for someone else’s children.
When a solitary bee female emerges, there are usually already some males waiting for her. After the mating (we may talk about that in a separate post), she begins to provision the nest with food for her offspring. Lots of pollen, they need protein for their development. In solitary bees, the mother will never see her sons and daughters. She builds a nest that is save and full of food for them. By this, they have all they need to grow, but they have to feed on their own, the adults don’t feed the larvae.
A step further in social evolution – shared accommodation
I already mentioned that solitary and social life style are the two extremes of a continuum. There are several steps in social evolution, which we still can observe today. In some species, the females are all fertile, but they nest in aggregations.
In these aggregations, the single females seem not to take much notice from each other, they are still providing all their own nests. All busy building brood cells, providing them with pollen and a bit of nectar, laying eggs and finally closing the nest.
This may be just because the “right” spots are a limited resource. But, nesting in aggregations also has the advantage that it may be a protection against parasites. If there are so many nests together, the probability that a single nest gets infested may be lower. In addition, there’s always “somebody at home”, who may disturb parasites. Some species share even a common nest entrance, though the single females still care only for their own brood cells only.
In some species several females care together for the brood, but still are all fertile (quasisocial species). Or they even have division of labour, there is one “dominant” female that reproduces, while the others “help out”. But, these “helpers” are still fertile and could also lay viable eggs.
What does it mean to be social?
If a bee species is defined as “social”, this implies several characteristics. I already mentioned one, indirectly. In social bees, there is a pronounced division of labour, including also reproduction. There is one fertile queen and many infertile workers. The queen has the monopole of reproduction, the workers – as their name suggests – do all the rest of the work. There is also an overlap between the generations, the queen “meets” or “knows” her offspring (the workers and males). The workers on the other hand, actively feed the brood, they work together in building the nest and guard it from parasites and predators. It even may depend on the age of a worker, what job she does, like in honey bees.
But not all social species fulfill all characteristics of “true social” lifestyle. The scientific term for this would be “eusocial”, eu- meaning “true” or “real” in Greek. Bumblebee colonies, for instance, in spring pass a “subsocial” phase. The queen is still alone, but she actively cares for the brood. Later on, bumblebees are “primitively eusocial”, as the queen is the only one who is able to reproduce and the generations overlap. But the fertile females (young queens) overwinter all alone and found a new colony in the following year without help of workers.
Social evolution isn’t linear
In highly eusocial species, like the well-known Western Honey Bees, finally all characteristics of social lifestyle come together. The workers are not fertile, they (usually…) don’t lay eggs. They totally gave up their chances for own offspring. This actually is one of the most amazing aspects of social evolution. If the sense of life – like my professor of evolutionary biology put it – is to survive and reproduce, why should thousands of workers give up their reproductive abilities? Already Darwin wondered about this question. I will address some aspects of this in the following posts of this series.
As you may have noticed from this introduction, social evolution isn’t linear. There are clear definitions for the two extremes – solitary and highly eusocial. But, in between every combination seems possible. We can still see some of them, while others may got lost in the course of the past 100 million years. Evolution isn’t target-oriented, the definitions of “true”, “primitive” and “highly social” are human classifications. They help to understand, but don’t mean that one species has a higher “value” because it has a “higher level” of social development. Every single species has its own ecological niche and a value for itself. We need all 20,000 bee species, none of them can do the pollination job all alone. Nor do bees in general, they also need and get help from many other pollinators. However, several questions raise from all these different life-histories. And I hope, you will follow me this month while I try to answer – or at least discuss – some of them.