Sex Ratios and Caste Allocation

Trivers and Hare's Hymenopteran Sex Ratio (1976) theory began an important area of ant research aiming to understand evolutionary dynamics of eusociality. The theory combines elements of Fishers sex-ratio theory, kin selection and parent-offspring conflict. From Fishers theory, we know that selection should act to produce a sex ratio that produces the best evolutionary outcome, i.e., passes the most genes to future generations. The relative investment in reproductive females and males, not their numbers, are the theoretical targeted by selection. For ants, there is a parent-offspring conflict between the reproductive queen (or queens) and their non-reproductive workers. A queen has a different optimal sex ration than the workers, and both queens and workers are able to adjust the relative investment in new reproductives. The consensus is the workers win this conflict in most cases as they raise the brood. Queens interests, here genes are passed on through her offspring. Workers interests are indirectly realized through raising their close relatives. Differences in relatedness among and between queens and workers, and the reproductives produced in a colony, lead to different predicted optimal sex ratios depending on whose interests are being modeled.

Trivers and Hare used sex ratio data gathered from a variety of published studies to investigate their theoretical ideas, concluding in ants that sex ratio data nicely supported their theory and showed workers were in control of sex allocation.

The data they gathered included:


 * Camponotus ferrugineus (=Camponotus chromaiodes) Pricer, 1908
 * Camponotus herculeanus
 * Camponotus pennsylvanicus
 * Formica pallidefulva
 * Prenolepis imparis
 * Acromyrmex octospinosus
 * Aphaenogaster rudis
 * Aphaenogaster picea
 * Aphaenogaster treatae
 * Atta bisphaerica

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 * Albuquerque, E. Z., E. Diehl-Fleig, E. Diehl, and A. J. Mayhe-Nunes. 2018. Sex investment ratios and natural history observations in a population of Trachymyrmex holmgreni (Formicidae) in southern Brazil. Insectes Sociaux. 65:297-303.

Nest architecture studies provide important information about the natural history of ants, such as number of workers in a colony and presence/absence of winged forms, larvae, pupae, and food resources. These aspects of the population biology may help answer questions related to sex ratio, kin selection, and parent-offspring conflict and to the impact of environmental conditions on the colony. Here, we describe the following aspects of the natural history of the fungus-farming ant Trachymyrmex holmgreni: (a) sex investment ratios; (b) intranidal population; and (c) external and internal nest architecture. In April 2007, we excavated ten nests of T. holmgreni in Itapeva beach in southern Brazil. In four of the ten nests, we measured height, width, and depth of all nest chambers found and counted and weighed all ant individuals in the laboratory. We found gynes and males in all four colonies, and larvae and pupae in three of them. The numerical sex ratio and the estimated sex investment in the colonies examined here suggest a higher expenditure of energy for the production of females. Our study provides new data for understanding the biology of T. holmgreni, which is a member of the Trachymyrmex iheringi group, a rather diverse group of fungus-farming ants that are closely related to leaf-cutting ants. -


 * Warner, M. R., J. Lipponen, and T. A. Linksvayer. 2018. Pharaoh ant colonies dynamically regulate reproductive allocation based on colony demography. Behavioral Ecology and Sociobiology. 72:13. doi:10.1007/s00265-017-2430-1

Abstract The success of social insect colonies is dependent upon efficient and dynamic allocation of resources to alternate queen and worker castes. The developmental and molecular mechanisms regulating the caste fate of individual larvae in response to environmental cues have been the focus of intense study. However, the mechanisms regulating colony-level resource allocation into alternate castes (i.e., caste allocation ratios) are less well studied. Here, we systematically manipulate colony demography to elucidate the social regulatory mechanisms of caste allocation in the ant Monomorium pharaonis. By measuring the effects of demographic manipulation on colony productivity, we infer that caste allocation results from differences in timing and efficiency of culling of very young reproductive-destined larvae, which are always present in colonies. Based on our results, we develop a conceptual model depicting how colonies integrate numerous individual-level caste determination decisions to regulate colonylevel caste allocation. We propose that adult workers make decisions about culling larvae based on the ratio of the number of workers to the number of eggs contained in colonies, likely signaled by pheromone present on eggs. This strategy enables the dynamic alteration of colony demography in response to internal and external conditions, which is likely key to the ability of M. pharaonis and similar ants to thrive in disturbed habitats and to become widespread invasive species.

Significance statement The defining feature of social insect societies is the presence of alternate queen (reproductive) and worker (non-reproductive) castes of individuals. The fitness of social insect colonies is dependent upon efficient allocation of resources to alternate castes, particularly in the case of highly polygynous (multiqueen) societies. However, the mechanisms by which such societies regulate caste allocation are largely unknown. In this study, we perform a range of manipulative studies to disentangle social mechanisms of caste allocation in polygynous ant societies. Based on our results, we develop a model in which colonies manipulate their production of queens (and also males) versus workers according to the present density of eggs in the colony, a reliable indicator of queens’ fertility. Provided egg density is high, colonies kill queen- and male-destined larvae; when egg density falls, colonies begin to rear queens and males. This flexible resource allocation strategy is key to the ability of highly polygynous species to thrive in marginal (often human-associated) habitats.