When it comes to mating, it seems that you're damned if you do, and damned if you don't. Mating can transmit diseases, and mating partners might even injure or kill one another. Yet, for most animals, mating is necessary for reproduction, and mating with multiple partners can increase the number or quality of offspring. In this complicated world, what is the best strategy for maximizing the benefits while minimizing the costs of mating?
As it turns out, the situation is even more complicated for female red flour beetles (Tribolium castaneum): the costs and benefits of different mating strategies are dependent on environmental conditions. Red flour beetles have been maintained in the laboratory at 30°C for over 30 years, which amounts to over 350 beetle generations, so these beetles are well adapted to life at 30°C. However, life at 34°C is significantly more stressful for the beetles, so how do the relative costs and benefits of different mating strategies change at a stressful temperature? Vera Grazer and Oliver Martin, from the Swiss Federal Institute of Technology Zurich, decided to address this question by measuring the survival and reproductive success of female red flour beetles at different temperatures.
The researchers placed individual female beetles into small enclosures alone (virgin females that did not invest in reproduction), with a single male beetle (monogamous females that invested in reproduction with a single mate), or with multiple male beetles (polyandrous females that invested in reproduction with multiple mates) at both 30 and 34°C. After 1 week, the researchers removed the males from the enclosures, and monitored the females for an additional 9 weeks to assess their long-term survival, and to count the number of larvae that hatched for each female. Using this method, Grazer and Martin were able to quantify both the survival and reproductive success of females at both temperatures.
The duo found that at the standard 30°C temperature, the virgin red flour beetles had the highest survival, with intermediate survival in the monogamous beetles, and the lowest survival in the polyandrous beetles. In other words, at the red flour beetle's adapted temperature, reproduction became increasingly costly for the females as the number of mates rose. The team also found that at 30°C there was no difference in the number of larvae produced by monogamous and polyandrous beetles. Therefore, at the standard temperature, it is clear that mating with multiple males carries a survival cost and does not confer any reproductive benefits for females.
However, at 34°C, the survival differences disappeared. All of the ‘hot’ females survived for a reduced length of time relative to the beetles held at 30°C, and all of the ‘hot’ females survived for the same length of time, regardless of whether they were virgins, monogamous or polyandrous. What was even more exciting was that at 34°C the polyandrous beetles produced more larvae than monogamous beetles. Therefore, mating with multiple males does not carry an additional survival cost at higher temperatures, and results in increased reproductive success for female red flour beetles.
This experiment elegantly demonstrates that both the costs and the benefits of polyandry are dependent on environmental conditions for female red flour beetles. As climate change and anthropogenic activity increases, wild populations are faced with rapid environmental change and environmental conditions that are quite different from those that populations are adapted to. The results of Grazer and Martin's experiment therefore have wide-reaching implications for wild populations across the globe. In this changing world, mating strategies will need to adapt.