Sex Ratios and Caste Allocation

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Trivers and Hare's Hymenopteran Sex Ratio theory (1976) began an important area of research with ants that investigates fundamental ideas in evolution and how eusociality potentially operates. The theory combines elements of sex-ratio theory, kin selection and parent-offspring conflict. From Fishers sex-ratio theory we know that selection should act to produce a sex ratio that provides an equal evolutionary return (=genes passed to future generations) for investment in new reproductive queens or males. The theory has yielded models that predicts the population-level proportional investment in reproductive females and males. This is different from simply considering the proportion of individuals, i.e., the ratio of how many males:females, since in ants queens and males are typically very different in size and provisioning. A large claustrally founding queen of a monogynous ant species, full of fat stores just prior to her mating flight, is much more costly to produce than a single male. Parent-offspring conflict is also an important consideration in ant colonies. A reproductive queen and her non-reproductive workers are differentially related to new reproductives that are collectively raised within their nest. Estimates of genetic relatedness between these parties of interest and new reproductives yields different optimal sex-ratios for a queen versus her workers. The simplest case to consider is that of a monogynous, single mated queen that produces workers that are all her daughters. Queens interests are realized through the production of her daughters and sons. Workers interests are indirectly realized through raising what are, in this simplest case, their sisters and brothers. There are ready means available for queens and workers to adjust colony-level investment in new reproductives. A queen can control the fertilization of the eggs she produces, altering the number of females versus males. Workers on the other hand care for the brood and can raise more of less females as queens versus workers. They may also be able to cull males. In the decades since the publication of Trivers and Hare's theory, models have been developed to predict sex ratio evolution across a range of mating systems. The consensus suggested by theory and the data used to test modeled predictions suggest that workers win the conflict over sex allocation. The sex ratio is thus thought to be selected for the worker optimum, i.e., the sex ratio match predictions of models with details that include worker relatedness relative to new queens and males.

Trivers and Hare used sex ratio data gathered from a variety of published studies to investigate their theory. They concluded that the sex-ratio data in ants supported their theory and showed workers were in control of sex allocation.

Figure 4 from Trivers and Hare plotted the male/femala sex ratio as a function of the adult dry weight ratio (female/male). Data from the following species and sources were used to make this plot that supported the idea that ant sex ratios were close to 3:1 than 1:1, therefore sex allocation was controlled by workers and not queens.

  • 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 Mycetomoellerius 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 Mycetomoellerius holmgreni: (a) sex investment ratios; (b) intranidal population; and (c) external and internal nest architecture. In April 2007, we excavated ten nests of M. 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 M. holmgreni, which is a member of the Mycetomoellerius 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.

Related Topics

Split Sex-Ratios

References

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