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United States Department of Agriculture

Agricultural Research Service

Research Project: BEE DIVERSITY AND THE DEVELOPMENT OF HEALTHY, SUSTAINABLE BEE POLLINATION SYSTEMS

Location: Pollinating Insects-- Biology, Management and Systematics Research

Title: Sexual Harassment by Males Reduces Female Fecundity in the Alfalfa Leafcutting Bee (Megachile rotundata)

Authors
item Rossi, Benjamin -
item Nonacs, Peter -
item Pitts Singer, Theresa

Submitted to: Animal Behaviour
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 22, 2010
Publication Date: December 15, 2010
Citation: Rossi, B.H., Nonacs, P., Pitts Singer, T. 2010. Sexual harassment by males reduces female fecundity in the alfalfa leafcutting bee (Megachile rotundata). Animal Behaviour. 79:165-171.

Interpretive Summary: Under theories of sexual conflict in animals, males evolve traits to increase their mating and reproductive success that often impose costs on females. Females evolve adaptations to counter males’ mating attempts and their related costs. Females’ abilities to resist some males may give them a chance to be choosy about their mates and thus the traits given to their offspring. Sexual harassment is a form of sexual conflict in which males make repeated, costly attempts to mate. Costs to females in food-gathering or in risking the chance of being eaten have been measured in several insect species, but measurements of direct costs are rare. In the alfalfa leafcutting bee, males harass females, and females resist all mating attempts. We placed bees in large, outdoor cages with differing numbers of males. Harassment rate, nest-building, offspring production, temperature and food availability were measured daily for seven days. When more males were present, there were higher harassment rates. Harassment reduced the number of food-gathering trips and increased the duration of those trips made by females. Females produced offspring at a slower rate when subjected to higher rates of harassment. This demonstrates a direct link from male harassment to female fitness under natural conditions.

Technical Abstract: Under sexual conflict, males evolve traits to increase their mating and reproductive success that impose costs on females. Females evolve counter-adaptations to resist males and reduce those costs. Female resistance may instead serve as a mechanism for mate choice if the male-imposed costs are outweighed by indirect benefits of screening out low quality males and producing “sexy sons.” Sexual harassment is a form of sexual conflict in which males make repeated, costly attempts to mate. Costs to female foraging or predation risk have been measured in several species, but measurements of direct fitness costs are rare. In the alfalfa leafcutting bee (Megachile rotundata Fabricius; Hymenoptera: Megachilidae), males harass females, and females resist all mating attempts. We placed bees in large, outdoor cages with different male-biased sex ratios. Harassment rate, nest progression, offspring production, temperature and food availability were measured daily for seven days. More male-biased sex ratios resulted in higher harassment rates. Harassment reduced the number of foraging trips and increased the duration of foraging trips made by females. Females produced offspring at a slower rate when subjected to higher rates of harassment. This demonstrates a direct link from sex ratio to harassment to female fitness under natural conditions. Now that the costs of harassment are confirmed, the haplodiploid genetic system of this species provides a unique opportunity to measure indirect benefits when females cannot receive “sexy son” benefits, but could instead receive benefits through daughters or grandoffspring.

Last Modified: 8/30/2014
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