Communicating and learning from each other are part of
the success of insect societies, but some of the most complex, self-organizing
societies function on the basis of very simple processes of information
transfer between individuals.
For social insects with larger colonies, queen dominance
is often replaced by other forms of control. Indeed, usually workers of
many species of ant, bee and wasp do not lay eggs, despite having functional
ovaries. However, under certain circumstances, given an opportunity, workers
will lay eggs. In a large colony, the queen could not successfully police
all such behavior and often ignores it. Instead, other workers do the
policing, destroying the eggs of their co-workers.
A comparative study of ten policing insect species revealed
that altruism is modulated more by constraints on worker reproduction
than by relatedness. The species with the highest fraction of fully committed
altruistic workers - those that do not lay eggs - tend to be those with
lower relatedness, contrary to simple expectation. Instead, more workers
are fully committed when policing is most effective, as measured by the
fraction of worker-laid eggs eaten by either queens or other workers.
Workers are not leaping at every opportunity to be altruistic; they are
coerced into that role, often by their fellow workers.
Thus, cooperation among workers and their seeming altruism
result from strict policing by nest mates. With honey bees, workers refrain
from laying eggs in the presence of a fecund queen. The mechanism by which
workers switch off their ovaries is cued by pheromones, but there are
genetically-based variations among individuals: some workers have high
thresholds for ovary activation while for others the response threshold
is lower.
Genetic variation for threshold response by workers to
ovary-suppressing cues is most evident in 'anarchist' colonies in which
mutant patrilines have a proportion of workers that activate their ovaries
and lay eggs despite the presence of a queen. As long as other workers
police and eat these eggs, there is no problem. However, if this process
breaks down, the colony is usually headed to unsuccessful queen supercedure,
resulting in the demise of the colony.
Many social conflicts create winners and losers. But
only kinship allows evolution to make creative use of the social losers,
turning them into non-reproductive police, exquisite communicators and
heroic defenders. The honey bee will not hasten to sting unless danger
is imminent and definite. The stinging honey bee worker commits suicide
when her sting is torn out, but this saves her kin. She is not 'making
an escape from outrageous fortune' (like Shakespeare's Hamlet), but making
the best of it - not fearful of what dreams may come, but hopeful for
what genes may come. However socially constrained her life may have been,
her last action makes her own clear statement: long live the kin!
In the 1932 novel Brave New World, Aldous Huxley created
a society where fetuses develop in bottles and are treated with chemicals
to modify their bodies and mentalities. Later, children are sleep-conditioned
to their future task in society. This procedure creates people who have
clear roles, putting them in castes, ranging from alphas (the leaders)
to epsilons (the drones). Among other things, lower castes are programmed
not to be aggressive against higher caste members. A treatment with neurotoxic
chemicals during development leads to the appropriate brain changes.
Within the animal kingdom, social insects have evolved
the most stable caste societies. Many ant species have a wide range of
castes, from workers to foragers, from groomers to soldiers. Individuals
all develop from eggs laid by the same mother--the colony's queen. Generally,
the food supplied to each egg is the biological signal that leads the
embryo to develop into one caste or another, a situation reminiscent of
Huxley's fictional world. Thus, if we take as an example the leaf-cutter
ant, Atta texana, small individuals tend to the fungus garden within the
nest, intermediate-sized individuals search and collect leaves from the
forest to feed the fungus, and large individuals with strong mandibles
defend the colony.
Honey bees have evolved a different caste system. The
individual worker bees are always females because male drones do not contribute
to social life apart from mating with queens during nuptial flights. The
workers perform different tasks in the course of their lives and start
off as nurses tending to the hive right after emergence, continue with
tasks such as nest-building and hive defense, and then end their lives
as foragers by collecting nectar and pollen to make honey and feed their
sisters.
How is this developmental polyethism organized? Age is the main factor
that determines the task that an individual will perform, mediated by
regulation with juvenile hormones, but feedback from the hive is also
important. Much information is delivered by pheromones. For example, when
a hive loses the queen, her queen mandibular pheromone (QMP) will also
disappear, leading to rapid changes in behavior among the worker bees
who start raising new queens to replace her.
However, the life of a honey bee is not hard-wired. Bees
are amazingly intelligent animals and learn a lot about their environment.
Their context adaptation beyond age-related caste is phenomenal. Recent
results have created an unprecedented link from molecules all the way
to complex behavior, and some of the molecular effects of QMP are now
understood. It turns out that QMP directly influences the chemistry of
the brain in an age-dependent manner, contributing to developmental polyethism.
One major component of QMP is homovanillyl alcohol (HVA),
a substance with a striking chemical similarity to the biogenic amine
dopamine, the neurotransmitter that mediates aversive learning. Indeed,
QMP acts directly on the dopamine pathway: Levels of dopamine in the brain
are reduced in young bees exposed to QMP, and this effect is amplified
by a concurrent reduction in the levels of dopamine-sensitive receptors.
Thus, exposure of a nurse bee to the queen's odor down-regulates the brain's
dopamine networks and reduces activity levels. Indeed, the fact that neurotransmitters
affect human behavior and the role of hormones and environment in eliciting
such behavioral modification is well known to all parents dealing with
teenagers.
Thus, the presence of the queen, through her pheromone,
influences the behavior, and indeed the brain pharmacology, of her hive.
Young workers remain in the hive, are docile, and display less motor activity,
whereas older guards and foragers leave the hive and become more motile
and aggressive.
Why would aversive learning be blocked in a young nurse
bee? Within the hive, the sting reflex can only have negative effects.
Thus, preventing nurse bees from developing aversive memories against
the odors in the hive, which include the queen's own odor, makes the colony
more secure. With increasing age, bees start to leave the colony, fly
to distant foraging sites, and perform tasks outside the hive where they
need to learn not only about sweet nectar but also about nasty dangers.
It is useful, under these circumstances, that the effect of QMP blocks
aversive learning wanes. This is a wonderful example in which the effect
of a releaser pheromone can be followed all the way to the neurons that
are being modulated, and then to behavioral modifications. It also goes
a long way to explain how behavioral context is governed at the molecular
level.
Thus, honey bees differ substantially from the beings in Huxley's world because individuals are not trapped within their castes for their entire life span. The manipulation of brain activity by the queen, modulating learning capacity in young bees to make them more docile, is a different view of parenthood. As the saying goes: "When children are young, give them roots; when they grow, give them wings." The bee mother seems to have evolved to appreciate and execute exactly this strategy for her family.
