Honest Signals
There is a large body of theory on signal honesty in the field of animal communication, much of it having to do with conflicts of interest between signal senders and receivers, often in the context of courtship or competition for mates. Conflicts between sender and receiver are also common in communication among social animals, especially when there is well developed division of labor and reproductive skew, as in eusocial species. Recent PhD student Callum Kingwell devoted his thesis to the question of how queen bees in a tropical, facultative eusocial sweat bee, Megalopta genalis, use pheromones to convince their daughters to remain as helpers in the nest, rather than leave to establish their own nest. Callum used an elegant combination of endocrinological, chemical ecological and behavioral assays to demonstrate that one class of compounds present in queens is an honest indicator of fecundity and an accurate predictor of reproductive success. Callum showed that these compounds elicit both releaser and primer functions in daughter bees, and reasoned that selection for increased inclusive fitness could favor their attentiveness to such signals.
[Right, Callum Kingwell assembles an observation nest for pheromone bioassays with Megalopta genalis bees on Barro Colorado Island, Panama]
There is a large body of theory on signal honesty in the field of animal communication, much of it having to do with conflicts of interest between signal senders and receivers, often in the context of courtship or competition for mates. Conflicts between sender and receiver are also common in communication among social animals, especially when there is well developed division of labor and reproductive skew, as in eusocial species. Recent PhD student Callum Kingwell devoted his thesis to the question of how queen bees in a tropical, facultative eusocial sweat bee, Megalopta genalis, use pheromones to convince their daughters to remain as helpers in the nest, rather than leave to establish their own nest. Callum used an elegant combination of endocrinological, chemical ecological and behavioral assays to demonstrate that one class of compounds present in queens is an honest indicator of fecundity and an accurate predictor of reproductive success. Callum showed that these compounds elicit both releaser and primer functions in daughter bees, and reasoned that selection for increased inclusive fitness could favor their attentiveness to such signals.
[Right, Callum Kingwell assembles an observation nest for pheromone bioassays with Megalopta genalis bees on Barro Colorado Island, Panama]
Pollination can be thought of as mating competition among plants that is mediated by third parties, such that floral signals target the sensory biases and foraging logic of pollinators as receivers and surrogate mates. Previous research has identified several kinds of honest signals in flowers, such that plants attacked by herbivores make smaller flowers that offer less nectar (a quality signal), or flowers that produce acetylated glycerols as rewarding floral oils also emit volatile metabolites (diacetins) of those oils (an index signal). Two of my doctoral students have studied Oenothera flava plants as a model system for asking further questions about signal honesty. Holly Summers found that most of the floral scent blend emitted by O. flava consisted of volatile derivatives of the essential amino acids, leucine (LEU) and isoleucine (ILE), and that the enzymatic step responsible for converting LEU and ILE to aldoximes was encoded by a cytochrome P450 gene (cyp79) similar to those identified in mustard oil biosynthesis in Arabidopsis thaliana. She and Geoff Broadhead then showed that Hyles lineata, a widespread hawkmoth pollinator, is innately attracted to the aldoximes in a dosage-dependent manner. Geoff then showed that another hawkmoth, Manduca sexta, prefers artificial nectars with sugar and amino acids over those with only sugar, when sugar concentrations are high (25% w/v). One outstanding question remains whether nitrogenous scent compounds are specific indices of their biosynthetic source amino acids or represent more general indices of total nectar nitrogen.
[Right, Hyles lineata moth training to visit artificial flowers with different nectar sugar and amino acid composition, paired with different colors]
[Right, Hyles lineata moth training to visit artificial flowers with different nectar sugar and amino acid composition, paired with different colors]