Wednesday, April 23, 2014

You socialites!

a gaggle of naked mole rats: nude dudes stay together to stay warm
photo: BBC - The Independent
In this blog, I've discussed how our biological fitness is measured by the number of genes we pass on to children, grandchildren, and so on.
Our progeny.

It's actually slightly more nuanced than that, since we share genes with close relatives.

The last post discussed how many individuals of a social animal group (in this case, bees) contribute to the well-being of not only their herd or colony but also of young that are not their own.

Why, you may ask, don't all the adult females in the hive just lay and care for their own eggs?


In the case of bees, female worker bees are rendered sterile by chemicals (pheromones) emitted by the queen, so any eggs they lay are unfertilized. This living in and defending a tight-knit colony and supporting the offspring of somebody else instead of one's own is just part of being a Eusocial species.

Most eusocialites are insects (e.g. bees, wasps, ants, termites), although two species of African mole rats also follow this strict communal lifestyle. These species are super organized and share three characteristics:

leafcutter ants working together can
move a plant, piece by piece, to their nest
photo: Bandwagonman,Wikimedia
1. overlapping generations: different generations (eggs, immatures, and adults) live together in the colony so older generations can care for young ones.

2. non-reproducing (often sterile) worker caste: the colony has one dominant breeding female and a number of smaller-bodied workers that do not reproduce.

3. cooperative care of young ones: members of the colony share responsibility for raising the young of a dominant female.

Typically, a group of eusocial animals also has a single nest (or other home) that they both want to and are able to defend.

So now that we are all expert eusocial species biologists, tell me this: does this mean that worker bees have no measurable biological fitness?

If the workers are helping the dominant female's genes carry on and they aren't reproducing themselves, how and why do their traits continue to get passed to subsequent generations?

For example, all those worker bees have a particular set of genes that give them their shape, size, ability to fly, gather nectar and pollen, and convert these into honey and royal jelly, etc. These traits make for a successful species, yet how do they get passed on to future generations?


Mother's little helper


Even though some animals (such as worker bees) don't reproduce, by being helpful, they can still gain biological fitness in two ways:

(1) First, the helper gains "indirect fitness" by increasing the breeding success of close relatives (sisters, cousins, etc), who share at least part of his/her genetic makeup. The same "stock," as Charles Darwin called it.
image: Doug Savage

This is called kin selection," and we do it too: we give presents and help out members of our family more than non-family members (and more presents to our kids than to our nieces and nephews). Those of us with no kids of our own (might include DINKs, homosexual couples, monks, nuns, and a heap of conservation biologists) will be likely to help out nieces and nephews more than the average kid.

Research has shown that when individuals are more closely related, they are more likely to act altruistically, which helps their kin to survive and (their genes hope) reproduce.

Scientists measure relatedness using the percentage of genes that are shared by two individuals. Zero indicates no relation among the two individuals. Full siblings share 50% of the other's genes, as does a single parent with his/her offspring (the other parent has the other half of the offspring's genes, right?). Grandparents thus share 25% of a given grandchild's genes, and cousins share 12.5% .

So if you help your sister raise her kids, you help 1/2 of your genes pass down to the next generation. Not a bad deal.

ants communally caring for larvae
photo: Entomomania
Eusocial bee colonies actually produce many, many young bees, most or all of whom are hatched from eggs of the same mother (the queen), so they are related, sisters and half-sisters.

Thus, even if each new bee has only a slight relatedness to a given female worker (which would translate into low indirect fitness), the large number of eggs laid by a queen may pass on more of the worker's genes than if she each tried to reproduce independently.

If you think about it, straightforward natural selection (the continuation of traits that enhance survival and reproduction) will favor altruistic action, like helping care for your naughty nephew at a cost to you, if the benefit of your actions (in terms of your inclusive fitness) exceeds the cost of the activity (in terms of your direct fitness = survival and reproduction).

helpers of all ages.  photo: Cede Prudente, WWF Malaysia

Basically, your fitness is enhanced when more copies of your genes are passed on, and helping close family members contributes to that.

And, for many more social species, there are actually several direct fitness benefits to be had by being helpful, the second way that non-breeding individuals enhance their fitness. Coming up in the next post!

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