QUESTIONS AND ANSWERS

Selection and fitness in harvester ants.
Blaine Cole, Univ. of Houston

Q: What percent of the foragers never return to the nest, and does this vary with nest size? 

A: I don't really know. My gut feeling is that no one knows the answer to this question for any ant species. Actually we could figure this out since we could count the numbers of individuals leaving and returning to the nest. Various people have obtained 'foraging curves' for species of ants--basically a count of the number of workers leaving and the number returning during some interval (say 2 minutes) over the course of a day. If the data were collected with sufficient time density, then the area under these two curves would measure the total number of foragers leaving and returning and the difference would reflect loss. We, and others, have done this casually without any systematic attempt to see whether it is a function of anything. 

Q: When you say that longer foraging times result in more foragers, is that a count of the actual individual ants or simply the number that are seen leaving the nest?  If the former, how do you mark the ants to be sure you don't count any twice? 

A: Actually, what I should have and meant to say was that increased time of foraging results in increased number of foraging trips. The data are a count of the workers returning to the nest from a foraging trip. My statement was about what happens when a colony forages for a longer period of time--the number of foraging trips goes up. I did not mean to convey the impression that there are more foragers, or a greater proportion of foragers in a particular colony. This could be true, we just have no information about it.

Q: Do colonies differ in proportions of late and early foragers? 

A: Let me give a weasel-ly answer. Yes, but no. When you measure early and late as absolute time (which seems reasonable), colonies differ in the proportion of early and late foragers because some colonies begin foraging earlier than others. Some colonies begin foraging when the surface temperature is markedly cooler than other colonies. However, if you measure early and late relative to foraging in a particular colony, then of course, there are equal proportions of early workers in all colonies. This is a sleazy response (which makes me a sleazy weasel), but it is relevant because the data I presented on the activity differences between early and late foragers as a function of temperature are taken from the perspective of a single colony. Early and late workers were with respect to timing of foraging activity in a particular colony. When we are able to, we will be analyzing the data to try to take into account the timing of forager activity from a particular colony. I think this will only make sense when we have identified every worker to patriline (see below), so stay tuned for a full answer.

Q: Is there any evidence that foragers from different patrilines have different foraging preferences?  That is, that some preferentially bring certain materials back to the colony?

A: We don't know the answer to that question. Do we have the ant equivalent of pollen, nectar and water foragers? We elected to try first to find out whether early and late foragers differed by patriline because we already had some information that colonies differed in the timing of foraging in relation to genetic diversity, however, it is not the last question we would like to ask. We one intriguing bit of information that seems to relate. These are basically seed harvesters, but they take a significant amount of insect material as well. They are not hunters--they seem to be opportunistic scavengers. There is a relation between time and the proportion of insect stuff in the material returned to the nest. Now this could be because insects become available overnight and are taken by the first ants that forager, but it could also relate to patrilines. 

Q: If mispackaging queens is not a big deal what do you think is keeping variance in queen size down relative to variance in male sizes? 

A: This question gets to the heart of a variety of interesting issues all of which I cannot cover. My interpretation is this: When we try to measure selection on body size of queens we find that there is a nonlinear effect of body size and therefore a great premium on making queens the 'correct' body size. [Actually what we mean is that investment will be preferentially in queens.]  Variance in queen size is low because the penalties for making a mistake with queen size are so great.  Selection operates strongly to buffer queen size with the consequence that there is not as much realized variation left in queen size among (or within) colonies.  Any extra benefit to a colony from packaging queens properly is likely to be small because selection has already changed what/how variation can occur in queen size. The most analogous process that I can think of is the evolution of canalization. I think that the general solution to the various dilemmas that this line of reasoning poses is that it is important to connect the observations with experiments. If we increase food to colonies do they change the sizes of reproductives?   For example, in our food supplement experiments we have found that we can only perturb the size of males--not the size of queens. We interpret this as indication that queen size is already highly buffered.   

Q: Have you looked at colony mortality after exposure to pathogens other than fungi (e.g., internal parasites)?

A: No. We don't know anything about the effect that other pathogens might have. [Incidentally, the fungi are internal, they burst through the cuticle for sporulation.]  The only we have done is to look for microsporidia (currently considered a degenerate fungus, anyway). We have seen what we take to be microsporidia, although they don't seem too common and it is difficult to believe that they have much of an effect on the population in general.