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What Developmental Mechanism Leads to Extreme Trait Size?

Exaggerated, sexually selected traits, like the horns of male rhinoceros beetles or the antlers of male elks, are more sensitive to insulin/insulin-like growth factor (IGF) than other body parts, according to biologists from the University of Montana, Washington State University and Michigan State University. Growth of such traits may depend on the overall physiological condition of the male.

These findings provide a developmental mechanism for the rapid growth that leads to extreme trait size. The molecular basis for exaggerated animal structures reinforces their position as reliable signals of male quality. This could explain why exaggerated structures have evolved so many times in the context of male assessment of rivals or female choice of mates.  

The exaggerated sizes of these traits and their highly diverse forms have fueled hundreds of theoretical and empirical studies examining their overall evolution. However, little was known about the underlying mechanisms responsible for extreme trait size.

By reducing the expression of the insulin receptor, the researchers showed that the beetle horns were more sensitive to this signaling pathway than other structures such as genitalia and wings. These findings, along with recent evidence that this same pathway plays a role in red deer antler and crustacean claw development, point to the existence of a general mechanism for the evolution of exaggerated trait size. 

It also hints at why so many sexually selected structures attain extreme sizes: Poor quality males cannot fake their status--insulin-sensitive traits are especially useful in assessing rival males or mate choices. As a trait becomes more sensitive, it becomes more conspicuous and informative. This leads to the evolution of ever-greater sensitivity to these physiological signals, and larger and larger trait sizes in the best-conditioned adults.

The findings were highlighted in Science.

Images (1 of )

  • head and thorax of a male rhinoceros beetle with a branched horn
  • a variety of animals exhibit exaggerated growth of weapons and ornaments of sexual selection
A male rhinoceros beetle.
William Freihofer and Douglas Emlen
Exaggerated growth of weapons and ornaments of sexual selection.
Images used under license from Shutterstock.com (NH; Simon_g; Wesley Aston; Henrik Larsson; Manamana)

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