Although I set an ambitious goal of Chapters 13-16, as you could probably gather, it took more time then I expected to digest the information in Chapters 13-15. Let’s take a look at those chapters and save Chapter 16 for next week.
(For those of you jumping in late, we are discussing The Fire Ants by Dr. Walter Tschinkel by going over a few chapters per week. Click “The Fire Ants Book Discussion” category for related posts.)
Chapter 13. Another way for a queen to take over a colony
In addition to the drama of incipient colony foundation by independent queens discussed in the last post, Solenopsis invicta colonies have yet another way to gain reproductive success. Mature colonies produce a second set of reproductives in the fall, which overwinter in the nest. The overwintered females fly in early spring, under much different conditions than reproductives reared in the spring. The overwintered females try to take over established colonies that have lost their own queen, or have become orphaned. If they are not successful, the overwintered females are incapable of starting a colony of their own and simply perish. Tschinkel calls this type of behavior “dependent colony founding,” apparently because the queen depends on the presence of orphaned mature colonies to be successful (?).
The overwintering females may also take over their own natal nest if their mother dies. In this case they do not mate, and produce only males. In orphaned colonies with overwintering females, 86% had unmated sisters for their queen, whereas 14% had queens that had stolen in from other colonies. When no overwintering females were present in the orphaned colonies, 62% had unrelated queens and 38% percent had sisters (top of page 189). I did wonder where those sisters came from if there were no overwintering females. Any thoughts?
Tschinkel reaches the conclusion that Solenopsis invicta is a “weedy species” because independently founding queens are much more successful in sites lacking competing mature colonies. Only 3% of colonies showed evidence of dependent founding. In contrast, Solenopsis geminata had a much higher success rate of dependent queen founding, with up to a third of colonies showing evidence of that type of founding.
What do you think of the technique of using match-mated females to determine the success of dependent founding queens?
Chapter 14. Colony Growth
How do you keep track of how an ant colony grows? In the case of Solenopsis invicta, you can start with even-aged cohorts by opening a new area to colonization. To get a count of number of workers, Markin drove a large cylinder into the soil around a fire ant colony and then slowly flooded out the workers. Tschinkel developed a method for sampling soil and ants from a colony during excavation. It is tedious and difficult work, which explains why so few of these types of studies have been undertaken.
The results suggest a colony on rough average reaches maturity around four years (and lives up to eight years), with around 200,000 workers.
As with individual organisms, temperature and the amount/quality of food available determined how fast a colony grew in laboratory conditions.
Did you find any points particularly interesting for this chapter? Anything I skipped over that you would like to discuss?
Chapter 15. Another aspect of colony growth: What types of ants are produced and when?
When a colony grows, it changes in composition as well as in worker number. The incipient colony starts out with tiny minims. Over time, workers of a range of sizes are added. Eventually the colony begins to produce reproductives.
How much energy does a colony allocate to each type of ant? Tschinkel estimates, based on worker mortality, that a average fire ant queen produces 2.74 million workers during her lifetime. The colony’s reproductive success depends on optimally allocating resources between workers and reproductives. Too many workers, and the colony loses opportunities to reproduce. If there are too many alates, there won’t be enough workers to feed them all.
As a colony grows and becomes mature, a higher percentage of the workers are the large or major workers. He only briefly mentions juvenile hormone as a possible mechanism in the switch to larger workers, as well as level of food.
Do you know of any recent research that sheds more light on this topic?
Tschinkel also disscusses how the nest grows with colony size and how the size of the colony’s territory changes as well. He ends with a summary of how the colony ends, which he describes as resulting from the queen likely running out of sperm and thus being no longer able to produce replacement workers or female alates.
Do you have any comments on Chapter 15?
Interlude: The Porter Wedge Micrometer
A device for measuring 200 head widths of ants in one hour? Sounds like a dream come true. What do you think of Tschinkel’s idea that it has been slow to catch on due to “cultural viscosity?” I admit I did it the old-fashioned way, but I pinned all my specimens first. Now that was tedious!
Have you tried a wedge micrometer? How do you measure your ant head widths?
Let’s try to cover Chapters 16 and 17 this week, with the Interludes (up to page 271). Is anyone further along than that? Let us know, and I’ll try to move ahead more quickly.
Some chapters/text of The Fire Ants is available online at Google Books.