The basic building blocks of communication are signals, assembled in various sequences and combinations, and used in virtually all inter-and intra-specific interactions. While signal evolution has long been a focus of study, there has been a recent resurgence of interest and research in the complexity of animal displays. Much past research on signal evolution has focused on sensory specialists, or on single signals in isolation, but many animal displays involve complex signaling, or the combination of more than one signal or related component, often serially and overlapping, frequently across multiple sensory modalities. Here, we build a framework of functional hypotheses of complex signal evolution based on content-driven (ultimate) and efficacy-driven (proximate) selection pressures (sensu Guilford and Dawkins 1991). We point out key predictions for various hypotheses and discuss different approaches to uncovering complex signal function. We also differentiate a category of hypotheses based on inter-signal interactions. Throughout our review, we hope to make three points: (1) a complex signal is a functional unit upon which selection can act, (2) both content and efficacy-driven selection pressures must be considered when studying the evolution of complex signaling, and (3) individual signals or components do not necessarily contribute to complex signal function independently, but may interact in a functional way.

Oviposition behavior in herbivorous and frugivorous insects and parasitoids is dynamic at the level of the individual, responding to variation in host quality and availability. Patterns of variation in egg load in response to host presence and quality suggest that ovarian development also responds to variation in the host environment. Ovarian dynamics are mediated by feedback from oviposition, by host feeding, and by sensory input from the host. The last of these mechanisms, host sensory cuing, is known to occur in three major orders and provides strong evidence that ovarian dynamics are adaptive by design. Conditions favoring host effects on ovarian development include trade-offs between egg production and either survival or dispersal, uncertainty in the host environment, and a correlation in host conditions between the time that oogenesis is initiated and the time that eggs are laid. Some host defenses block ovarian development, suggesting that ovarian dynamics in host-specific insects should be viewed from a coevolutionary perspective.

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Bees collect food from flowers that differ in morphology, color, and scent. Nectar‐seeking foragers can rapidly associate a flower's cues with its profitability, measured as caloric value or ‘net energy gain,’ and generally develop preferences for more profitable species. If two flower types are equally easy to discover and feed from, differences in profitability will arise from differences in the volume or the sugar concentration of their nectar crops. Although there has been much study of how bees respond to one or the other of these two kinds of nectar variation, few studies have considered both at once. We presented free‐foraging bumblebees with two different types of equally rewarding artificial flowers. After a period of familiarization, we made one type more rewarding than the other by increasing its nectar concentration, volume, or both. Bees responded more rapidly to a change in the reward's sugar concentration than to a change in its volume, even if the profitability differences were approximately equal. Sucrose concentration differences (40% vs. 13%) caused bees to virtually abandon the more dilute flower type, whether both types offered the same volume (2 μl) or the less concentrated reward offered higher volume (7 μl vs. 0.85 μl). When the two types of flower differed only in nectar volume (7 μl vs. 0.85 μl), the less rewarding type continued to receive 22% of the visits. We propose three different hypotheses to explain the stronger response of the bees to changes in sugar concentration: (i) their response threshold to sucrose concentration might change; (ii) less time is needed to assess the concentration of a reward than its volume; and (iii) a smaller sample size may be needed for reliable estimation of profitability when flowers differ in concentration.

Summary1. Many floral displays are visually complex, transmitting multi-coloured patterns that are thought to direct pollinators to nectar rewards. These 'nectar guides' may be mutually beneficial, if they reduce pollinators' handling time, leading to an increased visitation rate and promoting pollen transfer. Yet, many details regarding how floral patterns influence foraging efficiency are unknown, as is the potential for pollinator learning to alter this relationship. 2. We compared the responses of bumblebee (Bombus impatiens Cresson) foragers to artificial flowers that either possessed or lacked star-like patterns. By presenting each bee with two different foraging scenarios (patterned flowers rewarding ⁄ plain flowers unrewarding, plain flowers rewarding ⁄ patterned flowers unrewarding) on different days, we were able to assess both short-and long-term effects of patterns on bee foraging behaviour. 3. Bees discovered rewards more quickly on patterned flowers and were less likely to miss the reward, regardless of whether corollas were circular or had petals. Nectar guides' effect on nectar discovery was immediate (innate) and persisted even after experience, although nectar discovery itself also had a learned component. We also found that bees departed patterned flowers sooner after feeding. Finally, when conditions changed such that flowers no longer provided a reward, bees visited the now-unrewarding flowers more persistently when they were patterned. 4. On the time-scale of a single foraging bout, our results provide some of the first data on how pollinators learn to forage efficiently using this common floral trait. Our bees' persistent response to patterned flowers even after rewards ceased suggests that, rather than being consistently mutually beneficial to plant and pollinator, nectar guide patterns can at times promote pollen transfer for the plant at the expense of a bee's foraging success.