American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Offspring size and reproductive allocation in harvester ants”

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Diane C. Wiernasz and Blaine J. Cole


Taking the Trivers-Willard hypothesis beyond mammals, researchers find that ant colonies invest differently in queens and males

Nest of <i>Pogonomyrmex occidentalis</i> surrounded by a halo of <i>Oenothera caespitosa</i>.<br/><br/><br />(Credit: Blaine J. Cole)
Nest of Pogonomyrmex occidentalis surrounded by a halo of Oenothera caespitosa.
(Credit: Blaine J. Cole)

When organisms reproduce, they must distribute resources among their offspring. How many offspring should be produced? How big should they be? Should the number of females be the same as the number of males? Should similar amounts of resources be invested in females and males or should one sex receive preferential investment? Diane Wiernasz and Blaine Cole (University of Houston, Houston, TX) address these questions in Western Harvester Ants, using information from an experiment on more than 200 ant colonies, part of their long-term study population near Grand Junction, CO. Harvester ants collect and store the seeds of local plants to use as food.

Both male and female offspring are more successful when they are larger, but small size has greater negative effects on daughters. In a field experiment, when colonies are given additional food that they can store, they make more of both male and females; colonies benefit the most from producing more offspring of both sexes. However, colonies given extra food that cannot be stored make larger males but not larger females. Colonies appear to have an "invest in females first" strategy, and always make females that are relatively large. When food is limited, resources are used preferentially to make large daughters and what is left is invested in sons. The mathematical model developed in this study successfully extends the classic work of Trivers and Willard to show how parents that produce multiple offspring should invest in females vs. males based not only on parental resources but on the benefits that increased investment will provide to offspring of each sex.