Why do some animals have extravagant, showy ornaments — think elk and deer antlers, peacock feathers and horns on dung beetles — that can be a liability to survival? Charles Darwin couldn’t figure it out, but now a Northwestern University research team has a possible explanation for this puzzling phenomenon of evolution.
The researchers developed a mathematical model that made a surprising prediction: In animals with ornamentation, males will evolve out of the tension between natural selection and sexual selection into two distinct subspecies, one with flashy, “costly” ornaments for attracting mates and one with subdued, “low-cost” ornaments.
“Ornamentation does persist in nature, and our quantitative model reveals that a species can split into two subspecies as a result of the ornamentation battle that occurs over time,” said Daniel M. Abrams, an associate professor of engineering sciences and applied mathematics in the McCormick School of Engineering.
Evidence from nature agrees. The researchers studied available data on animal ornaments, such as deer antlers, peacock feathers, brightness of certain fish and tail length of some birds, from 15 species. They found the same distribution pattern of ornament sizes across many of the species: The animals often split into the two subgroups predicted by the model, one showy and one subdued, with very few in the middle.
It was deer antlers that first made Abrams, an applied mathematician, wonder why some animals spend precious energy to grow and carry around something that could compromise life. It’s not unusual for male deer and their antlers to get stuck in trees or fences -- or to each other in a fight -- and die.
“Animals with extravagant ornaments are showing just how fit and strong they are -- that they can overcome the costs of these ornaments -- and this attracts the opposite sex,” said Abrams, who led the study.
n fact, the subdued subgroup’s existence is a factor in the ability of individuals in the flashy subgroup to pass on their genes. The contrast gives the more ostentatious individuals physical distinction and cachet, helping to woo mates and propagate themselves.
The study, which aids our understanding of how life has evolved on Earth, was published today (Nov. 30 in the U.K.) by the biological sciences journal Royal Society Proceedings B.
The study’s other authors are Sara M. Clifton, a graduate student in Abrams’ group, and Rosemary I. Braun, a computational biologist and assistant professor of preventive medicine at Northwestern University Feinberg School of Medicine.
“This is a study of evolution using mathematical biology -- how sexual selection and natural selection play off each other and produce some of the strange things we see in the animal world,” said Clifton, the paper’s first author. “The horned dung beetle from our study is a good example of how large horns really handicap the animal, yet they exist.”