Pages

▼

Monday, June 5, 2017

Flower Crawling, and Other Social Habits of Bees

Bees have been on earth for orders of magnitude longer than humans have; fossils of extinct bee lineages have been radiocarbon dated to hundreds and tens of millions of years ago. Consider, then, that this evolutionary time comes with mutations and adaptations that have made them successful species in many ways. Bees don’t just evolve as individuals – they have coevolved as pollinators and some have evolved as social insects. Most bees, like sweat bees and carpenter bees, don’t live in colonies, but the few species of honey bees and bumble bees that do are populous and successful. The bee is known, among other things, for its productive social order promoted as an ideal in tight communities. My own home state, Utah, features a beehive on its flag, and my high school featured a beehive on its seal. As such, I was surprised when I learned that most bees aren’t social insects, having grown up seeing their social behavior used to symbolize community ideals.

The Utah Flag (Wikimedia)


That being said, it’s interesting to consider the basis of their social behaviors. Why and how do they interact socially and with flowers, what advantages have they gained, and what is the best environment for a social bee? We’re trying to learn more about their social behavior in our field station research, but I’d like to also highlight other research about honey bees. These animals exhibit many surprising behaviors that you might not be aware of unless you observe them closely. We might gain more insight into their behavior by asking questions like these:

  1. What inflorescences – flower arrangements on a plant - do honey bees prefer?
  1. What incentivizes honey bees to support others socially under a system where they can’t pass along their own genes?
  1. Under what circumstances do honey bees risk leaving their hive to find pollen and nectar?

We’ll look at all of these in turn. Consider question one: what inflorescences might a bee prefer? This is one you can see for yourself quite easily! I observed big differences in the number of bees visiting flowers earlier this spring.

Consider the bush of Indian Hawthorn in front of Harvey Mudd's East Dorm which had, as an estimate, hundreds of bees foraging on it when this video was taken on April 1:



Then, consider this bee foraging on April 7 on Popcorn flower in our research area in the Bernard Field Station:

Knowing that there were hundreds of bees on a single small bush, compared to the small number of bees seen at the field station, you might guess that bees might prefer inflorescences that they can crawl over easily. It definitely seems preferable, considering that bees flap their wings nearly 200 times each second. The energy expenditure of their flight is around a half of a calorie per minute, and a similar energy expenditure is required in colder temperatures: to fly, they must raise and maintain a body temperature near 30 C. If they don’t have to take off and land on every flower, however, then they can greatly reduce their energy expenditure by not raising their temperature and flying as often. 

Bee researcher Bernd Heinrich notes that a bee which spends half of its time in flight expends approximately 0.27 calories per minute, and a bee spending a tenth of its time in flight expends about a fifth of that: 0.05 calories per minute. We see that foraging becomes much easier for bees when an inflorescence has many close, crawl-able flowers on it. The next time you see a flower patch, notice how flowers are arranged on a plant, and think about how a bee might move to drink nectar.

Now, worker bees might prefer certain flowers to forage on; once they do, many of their resources are brought back to feed the queen’s brood. In short, question two asks why they even bother to do this! From an evolutionary standpoint, it makes little sense for worker bees to provide for another queen’s brood and not their own, especially because they have working ovaries and can produce viable offspring. Basic evolutionary principles suggest that an organism’s primary objective is to pass on its own genes. So what gives?

As it turns out, the social behavior of honey bees comes from a few evolved traits. Firstly, a queen emits pheromones which inhibit workers from raising other queens – this is definitely beneficial for the queen. But what prevents the workers from raising their own brood? In short, this happens because workers share more genes with their mother, the queen, than any offspring of other workers. A worker’s relatedness to the queen, its mother, is 0.5, but due to different fathers, its relatedness to workers born by the same queen is 0.25. Because all worker bees are more closely related to queen’s offspring than other workers’ offspring, they selectively kill any of the few brood laid by other workers. Maybe this particular honey bee trait isn't a great example for creating well functioning societies... but it has evolved into a cooperative system where a colony has a queen whose offspring are all supported by each other.

In the Social Insect Behavior Lab, we’re researching a bee’s social behavior and its preferred environment, and how a bee’s social behavior allows it to better explore some of the environments it has adapted to. So, let’s look at question three, exploring further implications of their social behavior. Honey bees don’t just limit their own reproduction, they forage in fatal environments for their colony. Why?

Maybe it’s a little grim to think about a worker bee that goes out of its hive to forage and dies within a week, but that’s just what bees do. In Dynamic modelling of honey bee (Apis mellifera) colony growth and failure, the researchers constructed models of bee populations given certain environmental factors, studying what conditions might cause Colony Collapse Disorder (CCD). This mathematical model of CCD comes from both the social and environmental adaptations of bees. We need to understand them both to help create better conditions for our bees. Bees create clean, well-defended, structural hives which provide a dramatically low mortality rate, but a foraging worker bee has a 15% chance of dying once it leaves the hive. A worker bee spends most of her life working inside the hive; she is prevented from leaving the hive by signaling of the pheromone ethyl oleate, transferred to nest bees by foraging bees. If more food is available, more pheromones will inhibit bees from deciding to become foraging bees. However, if foragers die more frequently due to any number of factors (metabolic stress, predators, and in today’s human agriculture, chemical toxins), then more bees will need to forage for the colony. If the environment is extremely hostile, then this higher death rate creates a negative feedback loop, and bees will continue to throw themselves out into their environment, unable to find food, and causing the colony’s collapse.

An empty hive is sad to see; we recently found one of our own hives devastated. There were no bees living, the wax combs were filled with unhatched brood and honey reservoirs still left inside. We’re not sure whether the cause was disease, chemicals, inclement weather, or something else. In any case, bee behavior is important to study. As we learn more about their behaviors, we can help beekeepers and farmers who want to continue raising bees as pollinators and keep their population from declining. On a more optimistic note, we were able to reuse the old hive’s wax structure and honey for our new bee colonies, who might have an easier time this summer.

As we continue researching the social behavior of honey bees, we’re thinking about the many levels of social organization, from the individual to the colony. Those questions guided us to important bee behaviors, and our current research questions ask how an individual bee might choose to forage on one plant instead of another nearby plant of the same species. This individual bee might choose based on the plant’s resources, so one way we’re measuring this is by taking nectar samples. Yet a bee might make this decision based on what is best for the colony. We’re not yet sure how these priorities might affect a bee’s choice between one plant and another, but we’re continuing to think about their behavior as we take observations this summer.

Further Reading:



at

No comments:

Post a Comment