Life-histories – changing conditions depending on sociality
In the past two weeks, we clarified some definitions and examples of social evolution. You may wonder what this means if we consider different bee species and their life-histories. Actually, being a social or solitary species to some extent limits your choices. The most obvious may be the reproductive behaviour, but there are several other traits that are influenced by the degree of sociality of the species. And as it’s winter time, let’s begin with the most frequent question I get in this period: what do bees in winter?
One of the many silly things I often say is that I like bees because they need the same conditions as me: warm and sunny. This is not really true, because there are bumblebees that like it much cooler than me. But anyway, overall bees live under quite warm and dry conditions. However, in many regions of the world they have to survive a colder period, meaning the winter. How different bee species do this, largely depends on if they’re social or not.
Overwintering – surviving cold temperatures
In the picture above, you see a thermograph of a honey bee colony in winter. That bright white spot in the middle indicates the location of the winter cluster. This is a typical adaptation of honey bees in colder climates to low temperatures. They survive in the colony for several years, this is an important trait in the life-histories of highly eusocial bees. Therefore, they need to keep themselves warm, especially the queen. They do so in contracting to a winter cluster – they sit closely together forming a sort of ball on the combs. The queen is the most protected in the middle of this cluster. The workers on the other hand, go from the center to the periphery of the cluster and back to the center. By this, they also regulate the temperature and the center doesn’t overheat.
Their close relatives the bumblebees already have a completely different strategy. They don’t overwinter as a colony, but as solitary young queens. During the cold months, they seek for a frost-free, protected place and survive in a dormant stage. The founder queen of the colony they were born in, the workers, the drones – they are all dead when winter comes. At least this was the case before global warming and warm winters. But that’s another story. Bumblebees are kind of polar bears in bee form, they fly much earlier in spring than other bees and with cooler temperatures. If you’re interested in how they do this, I strongly recommend the book “Bumblebee economics” from Bernd Heinrich.
But let’s move on to the solitary bees. Like bumblebees, they overwinter alone, though… that’s not totally true, as they generally overwinter in their nests. Their siblings are near by, but this doesn’t mean that they warm each other. Though they’re not completely exposed to the external conditions (their mother chose the nest site carefully…), it gets cold within a solitary bee nest in winter. So they’re in a dormant stage, not active as honey bees.
Foraging – social bees are generalists
As a rule of thumb, social bees cannot “afford” to specialize on certain flowers. If we start again from the honey bees, a colony has thousands of adults and even more larvae that have to be fed. That means honey bee colonies need large amounts of food, and they need it during the whole active season. They have stores, that’s true, but these are used during bad weather periods or during winter. As long as the conditions allow it, they will be foraging to cover the demands of the colonies. Therefore, honey bees are polylectic, generalist foragers.
Bumblebees on the other hand, have smaller colonies. However, they are active throughout season which means that they cannot rely on single plant species. Most bumblebee species are polylectic, too and will forage on a wide variety of flowers. There are some exceptions on this, but let’s stay with the overall picture for now.
Solitary bees are different. The adults have a limited activity period, they don’t feed whole colonies but “only” provide a certain number of nests and themselves – this enables them to specialize on certain flowers. This is an important aspect of many solitary bee species’ life-histories. Oligolectic bees (specialists) exploit these very efficiently and pollinate them really well. In addition, there may be some other benefits from being a flower specialist. For instance, it may protect them from cleptoparasites, like it may be the case for bees specialized on flowers from the daisy family. Being a specialist, can have a number of advantages. But, specializing means also that you limit yourself and you must be able to survive in spite of this.
Reproduction – specialized on egg-laying
Finally, let’s come back to reproduction. We already discussed that a key characteristic of social bees is division of labour. Monopolizing reproduction like a honey bee queen; to an extent that they need workers to feed and tend them is really an amazing exception. Both queens and workers are female, but only the queen mates with the males and lays eggs. At the end, she is an egg-laying robot. On the other hand, she suppresses worker reproduction by pheromones (volatile substances for communication). As long as she is present and doing her job, the workers don’t have strange ideas. In colonies without a queen, some workers however begin to lay eggs. The eggs however, develop only into males, as they are not mated.
In bumblebees, the specialization isn’t as pronounced as in honey bees. The queen suppresses any egg-laying behaviour of the workers, but she is autonomous to a certain extent. In spring, she builds the nest, forages and feeds the larvae in addition to her egg-laying job. Something that would never pass through the head of her majesty the honey bee queen. Only when there are some workers, she specializes more and more on her reproductive job.
And finally, solitary bees do everything during their whole life-span: foraging, nest-building, egg-laying, provisioning of the cells for the offspring… everything. They may be specialized on food, but in this aspect of their life-histories, reproduction, they are generalists.
These are only some examples for the differences in the life-histories of solitary and social bees. Next week I will go more into the detail of bee health – also how bees cope with parasites and diseases partly depends on their lifestyle