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A Comparison of Queen Number and Reproductive Allocation in Two Southeastern Populations of the Ant Leptothorax curvispinosus

Kenneth Howard, Valerie Banschbach, and Patricia A. Peroni

Department of Biology, Davidson College

Abstract

We investigated variations in queen number and patterns of reproduction due to varying environmental effects on Leptothorax ant populations at a mountain site near Mountain Lake Biological Station (MLBS) in Virginia and a piedmont site (ERW) near Davidson, North Carolina. The mountain population of L. curvispinosus is facultatively polygynous whereas the piedmont population is a rare case of a strictly monogynous population of this species. The piedmont population produced a more female-biased male allocation ratio than the mountain population. At the mountain site, L. curvispinosus coexists with an extensive, facultatively polygynous population of L. longispinosus, a species not found at the piedmont site. Differences in social structure of the L. curvispinosus populations may relate to differences in competitive regimes at the two study sites.

Introduction

Ant colonies harbor a great amount of internal conflict. Due to the system of haplodiploidy, in which females are diploid and males are haploid, worker offspring, which are female, are more related to their sisters than to their brothers. These relatedness issues suggest that workers should profit most from a female biased sex ratio in the colony's offspring. The queens, however, are equally related to their male and female offspring, and so should profit greatest by producing equal numbers of males and females. Since the workers outnumber the queen and raise the brood, they can control the sex ratio by starving or even eating male larvae. This is only the case in monogynous (one queen) colonies. In polygynous (more than one queen) colonies, the added queens act as a cost to the workers because these queens mate with males unrelated to their nestmates and produce offspring which are less related to the workers than their sister siblings. As more mated females re-enter the nest, the workers favor a male biased sex ratio in the offspring. In the ant species Leptothorax curvispinosus, populations may only have monogynous colonies or may have both monogynous and polygynous colonies. Ecological conditions such as nest-site limitation and cold winters may affect the queen number per nest in a population.

In this study we asked:

How does geographic variation affect the social structure and therefore the allocation ratios to males and females in Leptothorax curvispinosus ?

Methods

1. Collected and censused nests from MLBS and ERW in early June and from MLBS in early August of 1996.

2. June nests were raised in the lab and censused in early August.

3. Samples from each caste for each population were dried in an oven and weighed on a microbalance.

Extracted nest of Leptothorax sp. in the lab.

Typical nest of Leptothorax curvispinosus. Several larvae and workers are visible.

Figure 1: Distribution of queens in L. curvispinosus nests for three different populations. (queen number in nest at time of collection)

Figure 2: Male allocation ratios for L. curvispinosus populations at the two sites. The male allocation ratio is calculated by dividing the total mass of males produced by the total mass of reproductives in the population.

Figure 3: Proportion of the population producing either males or females for each queen number category.

Conclusions

The male allocation ratios for each population are what has been predicted. A monogynous population is predicted to produce a more female-biased male allocation ratio than a polygynous population. The piedmont population, which is monogynous, was more female-biased than the mountain population, which is polygynous.

The presence of polygyny at MLBS and not at ERW suggests pressure from an ecological factor, such as nest site limitation and colder winters at MLBS.

At each site, the male allocation ratios for each queen number category were not significantly different, probably due to small sample sizes within each category.