Ants Crossing


I found this colony of migrating ants on my way to work. They’re abandoning an anthill that’s situated on a small patch of grass surrounded by pools of rainwater. They formed a bridge of dead ants interlocked with half-drowned ones to cross the puddle, probably an ocean to the ants. I imagine these worker ants, enamored by a chemical signal of sorts, “martyring” themselves so the rest of the colony can march on to drier earth.

This behavior is described in evolutionary literature as part of a complex of behaviors in eusocial species. Behaviorists suggest that ant colonies have an altruistic worker caste that forego reproduction in order for fertile ants to have better chances of survival. These sterile ants take care of the larvae and fight off predators and competing colonies, often sacrificing themselves in the process. E.O. Wilson’s book, Sociobiology, presents three traits common among eusocial species  (1971:398-399):

(1) individuals of the same species cooperate in caring for the young, (2) there is a reproductive division of labor, with more or less sterile individuals working on behalf of fecund nestmates, (3) and there is an overlap of at least two generations in life stages capable of contributing to colony labor, so that offspring assist parents during some period of their life.

Self-sacrifice appears counter-intuitive at the individual level (I mean, who would want to be one of those ants forming the bridge, right?). Sociobiologists argue that this altruistic behavior seems to be a consequence of haplodiploidy, the mode of sex determination among ants (i.e., males only have only one copy of each chromosome while females have two):

Working from the traditional axioms of population genetics, Hamilton first deduced the following principle that applies to any genotype: in order for an altruistic trait to evolve, the sacrifice of fitness by an individual must be compensated for by an increase in fitness in some group of relatives by a factor greater than the  reciprocal of the coefficient of relationship (r) to that group…the coefficient of relationship (also called the degree of relatedness) is the equivalent of the average fraction of genes shared by common descent; thus, in sisters r is 1/2; in half-sisters, 1/4; in first cousins, 1/8; and so on. The following example should make the relation intuitively clearer: if an individual sacrifices its life or is sterilized by some inherited trait, in order for that trait to be fixed in evolution it must cause the reproductive rate of sisters to be more than doubled, or that of half-sisters to be more than quadrupled, and so on. (Wilson 1971:415)

Female ants are more sisters than regular sisters.  The sterile worker ants share 75% of their genes with their sister queen, who does all the reproducing by herself. Inclusive fitness theorists (basic idea outlined in the quote above), believe that this peculiar genetics is responsible for the altruistic behavior in ants–a sterile female worker caste devoting all their lives in ensuring the queen’s reproduction. In addition, Dawid Nowak et al (2007) also observed that life expectancy influences the ants’ engaging in risky foraging behavior; the shorter the life expectancy (or older), the greater the self-sacrifice for the benefit of the colony.

I came back a few hours later to check on the ant bridge. This time, the water has receded. There was a thin line of dead ants left. I thought only a few died in the crossing of the “great” puddle. But when i looked closer, the worker ants, using their mandibles, were picking their dead up. I followed the marching ants and, about a couple of feet away, on a dry space between grasses is an inch-high pile of dead ants, likely sterile females and probably older.


For a nice blog review on the debates surrounding the evolution of altruism, please click:

The Good Fight


Division of labour among workers with different life expectancy in the ant Myrmica scabrinodis
Sociobiology: The New Synthesis, Twenty-Fifth Anniversary Edition