In eusocial insects, many individuals share a nesting area but only very few
produce fertilized eggs. Most ants, for example, direct behavior toward maintaining
the nest structure, retrieving food, and caring for larvae. These behaviors
define "eusociality". Sociality comes in many different degrees, from
the tendency for reproducing females to nest in close proximity, to groups of
reproductives sharing resources, to colonies largely made up of non-reproductive
workers (eusociality). This range of sociality has evolved independently many
times (ants, at least three clades of bees, wasps, termites). What factors promote
the evolution of individuals that forgo reproduction? Selection for behaviors
that increase the fitness of genetically similar individuals is one factor,
of course ("kin selection"). This is not a sufficient explanation,
however; all hymenoptera have high relatedness among sisters but only a small
percentage of species show the most extreme adaptation of eusociality. Other
factors must be important to have such a range of sociality.
One hypothesis for the evolution of eusociality is that a colony with specialized
foragers can respond more quickly to new food resources. Also, large numbers
of individuals can potentially monopolize food resources, either by removing
food items faster or by excluding other species from the food. Both these potential
advantages require that communication occurs after a resource is located. Workers
in the nest must learn about the presence of a rich food source, and for maximum
efficiency the location, identity, and amount of resource should be communicated.
With this information, an appropriate number of workers can move quickly to
harvest the food. This process of communication is called recruitment.
Today we will conduct a manipulative experiment in which we test the hypothesis
that recruitment by worker ants enables ants to more quickly exploit a temporary
food source. We will test the prediction from this hypothesis that if we allow
ants to visit a particular food source and return to their nest (the RETURN
treatment), more ants will visit that food source than if we did not allow the
visiting workers to return to their nests (the NO RETURN treatment). A second
prediction is that the rate of visitation by ants to the RETURN food source
will increase with time, whereas the visitation rate to the NO-RETURN food source
should not increase with time.
In this lab we will test the hypothesis that ants recruit to new food resources,
and that the occurrence of recruitment is related to the quality of food available.
We will attempt to see whether recruitment results in a nest having exclusive
use of a food source. The basic procedure involves placing pairs of baits on
the ground and allowing ants to return to their nest from only one of the baits.
An especially attractive bait is quarter chunk of a chocolate-chip cookie. In
lab, we will talk about the details of the experimental design, but the general
procedures are described below.
You will work in pairs. Each pair should set up two observation stations. Look
for sites where you will be able to see ants move (e.g. away from dense shrubs).
Each observation station should be at least two meters from the nearest one.
Mark each station with a plastic flag. For each food type, place two baits (cookies)
at each station, about a half meter apart. Flip a coin to decide which bait
will be the "return" and "no-retum" treatments. For the
first part of the experiment, you will need to move between the stations to
see which baits are discovered by ants. Once a bait at one station is discovered
this marks the start of your experiment, pick up the bait and flags from the
other station - you will then work only with the first station that ants discover.
[You might delay retrieving other baits in case one of your colleagues has less
luck than you.] Note the time that you observed the first ant on a bait.
Watch the "no-return" bait especially carefully. Pick up any ant that
encounters this bait, and place it in a vial. (To pick up the ant, you may try
using your fingers - local ants cannot sting or seriously bite.) A frightened
ant can move quickly - do your best to prevent her return to her nest. At 15-minute
intervals after the start of your experiment, record the number (approximate
if there are too many individuals to careftilly count) and kinds of ant on each
bait. "Kinds" will be based primarily on size and color. For the "no
return" treatment, you also should note the appearance of individuals and
different kinds of ants as they are picked up. Also note any potential aggressive
encounters between ants. These are marked by biting and carrying. Ants that
touch antennae without further interaction are probably from the same nest.
You will need to retain specimens from each bait; different species of ants
might have different behaviors.