Sexual selection has resulted in some of the most flamboyant and outrageous ornaments in the natural world. The flashy plumes of the peacock tail, regal fringe of the lion’s mane, and vibrant colors of the agamid lizard all advertise males’ merits as mates to females. Although sexually selected traits are regularly observed in mammals, birds, fish, and reptiles, it was unknown if sexually selected ornaments are present in social insects. Amanda Izzo reported one of the first examples of sexually selected traits in social insects, discovering that paper wasp males have yellow spots that females find sexy. By determining how much variation exists between spots, which signals males find intimidating, and what females find attractive, Izzo was able to show that social insects too engage in survival of the sexiest.
In order for an ornament to be sexually selected, it most often sends a signal of quality to potential mates and rivals. To do so, it should impose a fitness cost on the bearer to show that it is an honest signal of quality. For example, male peacocks are hampered in escaping predators by their huge tails, showing that the survivors with the most outrageous tails are the most fit. Izzo suspected paper wasps (Polistes dominulus) might experience sexual selection for several reasons. To start, they find mates through a system called “leks”. Male wasps ‘lek’ by clustering in groups, each individual fighting for the best place (usually the tallest area) to advertise to females. Females observe the lekking males, and appear to be actively choosing which they want to mate with. Males are not selected equitably; a few males mate with many females. These factors indicate that the males send some kind of signal to females that allows them to discriminate the hotties from the notties. A possible visual signal is located on the abdomens of male wasps – a conspicuous yellow spot with a high degree of variability between males, but not females. As female paper wasps can recognize female rival quality based off of facial patterns, Izzo suspected that these abdomen spots could convey information to them too. The yellow spots appeared both sexually dimorphic and highly variable, therefore making them good candidates for sexual selection. But how do you see whether female wasps love a guy with small oval spots?
To confirm that the spots were highly variable and to quantify the variation in males and females, Izzo photographed the spots of many males and females. She found that males have spots that are more irregular and larger than those of females. The next step was to determine if the spots send an attractive signal to females and a competitive signal to males. In lab trials, males were paired up in a small arena and their behaviors were recorded as they attempted to establish dominance through biting, grappling, and mounting their opponent. Dominance was considered established when males would accept the mount of their opponent. Males that had small, symmetrical oval shaped spots were more likely to win dominance contests with other males than males with large, irregular spots. After resting for a day, the pairs were reunited but with the addition of female to the arena. Female wasps preferred to mate with small oval spot males, rejecting the advances of their blobby spot rivals through stinging and biting.
In order to determine if the spots were really the signal causing these different responses, Izzo devised a sneaky new test: incognito males. Males with large irregular spots (ones females normally found unattractive) were painted to appear as though they had small oval spots, and then Izzo observed how other males and females responded to the disguised males. These disguised males beat unpainted males with less symmetrical spots in dominance contests. Females also found the altered males more attractive than males with less symmetrical spots, although the true spots of the imposters were no different than their rivals. Females even spent more time copulating with males wearing false oval spots than with blobby spotted males.
These experiments support the hypothesis that male wasp spots are sexually selected: they are a sexually dimorphic characteristic that influences female mate choice and are associated with male dominance over rivals. However, this study raises intriguing questions about the maintenance of sexually selected traits. Although spots may be a signal of male quality that affect female mate choice, it is often believed that sexually selected signals should impose a handicap of some kind to be a valuable signal of quality. The reasoning underlying this handicap assumption is that if a signal doesn’t impose a cost to males who demonstrate it, all males would start displaying the signal to reap the benefits, and signals would then become meaningless. These costs are usually thought of as physical ones, such as the male peacock’s tail. However, it is unclear what physical costs pigmentation might impose on male wasps–why would costs vary for producing symmetrical or asymmetrical spots? It seems more likely that other costs, such as social penalties to dishonest signaling, may play a role in maintaining these apparently “cost-free” signals.
Find Izzo’s paper here (warning, paywall): Izzo AS, Tibbetts EA. 2012. Spotting the top male: sexually selected signals in male Polistes dominulus wasps. Animal Behaviour 83(3):839-45.