Monday, December 15, 2008

Eusocialism

Eusocial organisms are species such as ants, bees, wasps and termites who organize themselves into "colonies." They practice the highest level of social organization which includes reproductive division of labor (often with sterile castes), overlapping generations, and cooperative care of young. The eusocial species we're all most familiar with are all insects, but did you know that there are actually two species of mammals that live in colonies like this? Let's explore eusociality in general a bit more before we take a look at the eusocial mole rats.

Eusocial Insects

As we all know eusocial insects live in colonies which generally have one queen who is the only reproductive female. Then there are sterile female workers and/or soldiers who care for the young (and the queen and the drones), forage for food, defend the colony and basically do all the work, and male drones whose only function is to mate with a new queen. But this is just the beginning: did you know that in many eusocial species the queen determines the sex of her offspring through a procees called haplodiploidism? In honeybees for example, after mating the queen stores the drone's sperm in an internal sac. The eggs which she fertilizes with this sperm will have 32 chromosones and develop into females, whereas males develop from unfertlized eggs (this is parthenogenesis) and have only 16 chromosones. When it mates, a drone transfers all its genes which means that female workers on average share 75% of their DNA with their sisters (100% of the chromosones which come from their father and 50% on average of those which come from their mother). Through the haplodipolid process the queen can maintain a desirable sex ratio among her offspring. Worker bees, then, determine whether a female larva will develop into a new queen by feeding the larva large quantities of royal jelly which cause the larva to develop with fully-formed ovaries.

As for ants: some ants are "ranchers" who keep aphids as livestock, protecting them from predators and harvesting the sweet honeydew they produce. And other ants have slaves! These ants raid colonies of other ant species, stealing the larvae which are brought back to their colony where they will grow up to serve as slaves who do all the work. This is called helotism after the people who were oppressed by the Spartans. Slaver ants are actually entirely dependent on their slaves for survival as their workers/soldiers have lost the ability to forage for food (all they can do is raid other ant colonies for slaves).

Termites have a winged reproductive caste called "alates" and their colonies can consist of multiple pairs of monogamous reproductive couples. I'm most familiar with the bothersome termites that live in and eat wood, but some like the spinifex termites of Australia eat grass and create spectacular cathedral mounds to house their colonies which can be 20 feet tall.

Natural Selection

Eusocial insects whose colonies consist mainly of sterile workers posed a particular problem for Charles Darwin (1809-1882). His theory of natural selection stated that over time favorable genetic traits (beneficial to survival and reproduction) should become more common in a given population and unfavorable genetic traits should disappear thus resulting in the survival of the fittest. But sterile organisms have 0 fitness given that they are incapable of reproducing and passing on their genes, thus under Darwin's theory this genetic trait should disappear. Later biologists have sought to explain this phenomenon by characterising the behavior of sterile workers as kin altruism. Especially given that sisters share 75% of their DNA (making them "supersisters" and even more closely related to each other than parents are to their children), by caring for their younger sisters (the larvae) workers are still indirectly ensuring the survival of their genes even though they will never themselves reproduce. Another way of looking at this is to view the colony as a superorganism where the individual ants are like cells in a single organism. A superorganism is defined as "a collection of agents which can act in concert to produce phenomena governed by the collective" and these phenoma carried out by the individual agents are "what the hives wants." All Power to the Soviets!

Stay tuned for the mole rats!


Bee hive image posted by Andrew Cot on travelblogs.com, herding ant image from Imperial College London found on Science Daily, Cathedral mound image posted on travelblog.org by Kate and Brian Lang.

1 comment:

Kevin said...

Awesome stuff, dude! I'd never heard of eusocialism before and based on the post title I thought it had to do with French politics or something. I think this and your piece on vampire bats are my favorite posts of yours so far. It's cool to learn new stuff.