COLUMBUS, Ohio – A lab study in crickets has revealed sex differences in how the insects direct their nutritional resources to increase chances of generating offspring, finding that females prepare for producing eggs while males prioritize growing bigger bodies and banking extra energy.
In insects that mated, the females' investment in reproductive organs was even greater, but minimal change was seen in males – a sign that males' reproductive success is related more strongly to winning the competition for mates, the research suggests.
Ensuring survival while distributing finite resources is a trade-off faced by all living creatures, said first author Madison Von Deylen , a PhD candidate in the Department of Evolution, Ecology and Organismal Biology at The Ohio State University .
"Any organism is going to face these trade-offs between allocating limited resources: Should I invest in growth? Should I build up fat stores? Or should I transition energy into some kind of reproductive output?" Von Deylen said.
"Female crickets demonstrate these trade-offs when they invest directly in reproductive success. They are putting resources into the creation of eggs instead of their own growth. Males, on the other hand, are indirectly investing in their reproductive success by trying to grow bigger and build up energy stores. This could allow them greater access to mating opportunities."
Von Deylen completed the study with Susan Gershman of Ohio State Marion and Agustí Muñoz-Garcia of Ohio State Mansfield, both associate professors in evolution, ecology and organismal biology. The study was published recently in the Journal of Experimental Biology .
Half of the 75 crickets reared for the study were randomly selected to be offered mating opportunities. Once they reached adulthood, crickets in the mating group were paired with a potential partner and monitored to ensure mating occurred.
The study was designed to test whether mating made a difference in resource allocation – with the understanding that in the lab setting, unlike in the wild, the males didn't have to compete with each other to win over the females.
The researchers measured the crickets' metabolic rate and total evaporative water loss during the living portion of the study to determine the rate at which they converted food to energy. Later in the study, the insects' digestive tube, reproductive organs, body fat and carcass (all remaining components) were dissected and measured.
Statistical analysis of the data helped explain how varied metabolic rates, body mass, organ mass and fat reserves were impacted by sex and mating. These patterns revealed how reproductive effort and physiological processes geared toward storing energy differed across mated and virgin males and females.
"The predictions that we posed, that females would show different patterns of resource allocations than the males, were teased apart by the modeling," Von Deylen said.
"We specifically predicted that because females tend to have higher direct investments in reproduction, we would probably see a big difference between mated and unmated females," she said. "Whereas for males, a lot of the traits that affect their reproductive success – like growth – are things that are developed before sexual maturity, so we're not going to see the same difference between mated and unmated males as we would see in females."
The analysis also showed the specific ways in which mated and unmated females' allocation of resources differed.
"We found a trade-off between the amount of fat that individuals had and the gonad size in mated females, which made a lot of sense because mated females will start taking that fat and using it for egg development," she said.
The study's findings align with previous research in this species of field crickets, Gryllus vocalis, that has shown females tend to select males as mates that have specific calling characteristics or that call more often. The calls are produced by males rubbing sections of their wings together – an energetically demanding task.
Similarly, earlier studies found males that have larger body sizes or larger weaponry tend to be more successful at accessing mates. Growing to a larger size will also be more energetically expensive, Von Deylen said.
Because this study took place in captive crickets, environmental influences on resource allocation were not a factor – food was plentiful and there were no threats from predators. While the environment can be an important factor in influencing life history strategies and patterns of resource allocation, this study clarifies sex differences in those strategies and the significant role mating plays in the female trade-offs when individuals are offered an even playing field.
"This gives us a lot of good information about what kind of pressures that these different life history strategies are evolving under," Von Deylen said. "If you are alive, you are going to have to go through the same process of considering how to divvy up resources."
This work was supported by Engie-AXIUM fellowship funds provided by Ohio State's Graduate School.
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