Thirst level trajectories and influencing factors in critically ill patients based on a group-based trajectory model: A prospective observational study.

Sexual signals are widespread and play an important role in mate attraction and courtship. They often communicate essential information, such as species and sex of the sender (). Additionally, they can communicate a range of other traits that serve as quality indicators to the receiver of the courtship display (usually the female) (). This can include traits, such as body size (), metabolic state (), and age (; ), that allow the female to differentiate partner quality within a species. In order to communicate this information, animals use a wide array of sensory modalities. Because of our inherent human bias when observing animal behavior (), there are some modalities that are more intuitive to us than others. Signal modalities widespread in arthropods but often overlooked are, eg vibrational signals (; ) and chemical signals (). Reflecting the variety of information that is communicated, most courtship displays are composed of multiple signals, often across different sensory modalities (; ). This concept of multimodal communication can be traced back to . However, research on signal evolution has long focused on single signals in isolation or signal specialists (). As multimodal signals can be more costly to produce and increase the risk of predation (), they raise interesting questions on the evolution of more complex signals over cheaper, simpler signals. This has resulted in a growing interest in multimodal signals in recent years. A variety of hypotheses have been proposed to explain the evolution of multimodal courtship despite its additional costs (; ). Broadly, hypotheses about multimodal courtship can be split into content-based and efficacy-based hypotheses (). In efficacy-based hypotheses, the selection pressure is on the successful transmission of the signal, which can lead to redundant signals in different modalities (). For example, in the courtship of males, which uses visual, acoustic, chemical, and tactile cues, the suppression of 1 modality does not alter copulation success (; ). This suggests that in , all 4 modalities redundantly contain the same signal. In contrast, in content-based hypotheses, the multimodal signal is used to transfer different information in the different modalities (). Content-based hypothesis can be further split into 2 categories: the species specificity hypothesis, where 1 modality communicates species identity and the other communicates quality, and the quality hypothesis, where the different modalities communicate different aspects of the sender's quality ().