Animal weapons like antlers, tusks and specialized limbs for fighting are pricey to produce and maintain. Because the leaf-footed bug, Leptoscelis tricolor, literally drops its legs, Ummat Somjee, now a Tupper fellow at the Smithsonian Tropical Research Institute (STRI) and colleagues could put a price tag on the hidden energetic cost of weapons’ maintenance.
Wild animals can spend up to 30 and 40 percent of their total energy budget while at rest. “Human athletes often burn more calories during their relatively long rest periods than during physical exercise itself,” said Somjee, who did this study as part of his doctoral dissertation at the University of Florida in Christine Miller’s lab group. “We calculated the metabolic cost of maintaining large hind legs in a leaf-footed bug and found that males invest more in weapons than females. Large males expend relatively less energy on their super-sized weapons than smaller males.” Their results are published online in Proceedings of the Royal Society B.
Male leaf-footed bugs hang out on bright orange and yellow Heliconia inflorescences, feeding on nectar and developing heliconia fruit. Males’ hind legs, covered with thorny structures are larger than females’ legs and serve as weapons in male-male duels.
“By dissecting legs and staining the tissue, we showed that the insects’ tough exoskeleton is filled with metabolically-active muscle tissue,” said Meghan Duell, Ph.D. student at Arizona State University and co-author. “We realized that because these bugs drop their hind legs when molting, wounded or trapped, we could simply measure carbon dioxide production rates before and after a bug shed a leg to see how much energy was being expended on maintaining it.”
The team discovered that the resting metabolic rate of one-legged bugs dropped by almost a quarter of the total (23.5 percent) in males and 7.9 percent in females. Heavy-weight males had less of a drop per gram of leg lost, indicating that the price per gram of a big weapon is energetically lower than it is for light-weight males.
Largest deer develop especially large antlers for their body size and large elephants have relatively larger tusks than smaller individuals, but we don’t really know how energetically costly these structures are in most organisms. Many researchers previous focused on the size of these structures, and not neccesarily of the energetic costs of maintaining them “Metabolic costs like these could be important in shaping the evolution of these diverse structures,” Somjee said.
Funding for the study was provided by a University of Florida Graduate Student Fellowship, a STRI short-term fellowship a Society for the study of Evolution Rosemary Grant Award, U.S. National Science Foundation Grant NSF IOS-1553100.
Somjee, U., Woods, A., Duell, M., and Miller, C.W. 2018 The hidden cost of sexually selected traits: the metabolic expense of maintaining a sexually-selected weapon. Proc. Roy. Soc. B 20181685. http://dx.doi.org/10.1098/rspb.2018.1685