Citation: Xu M, Kalra L (2026) Do delta waves in the receiver’s brain explain slow tempos of animal communication signals? PLoS Biol 24(6): e3003814. https://doi.org/10.1371/journal.pbio.3003814

Published: June 10, 2026

Copyright: © 2026 Xu, Kalra. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: The authors received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Abbreviation:: OU, Ornstein–Uhlenbeck

Like humans‌, other animals routinely communicate. Signals used in communication are diverse and elaborate. What drives the evolution of these signals remains a central question in evolutionary biology, for which numerous hypotheses have been proposed [1,2]. In the 1990s, a group of related ideas emerged, offering an explanation from the receiver’s side [3–5]. The central philosophy is that neurological biases in the sensory and/or central processing systems of receivers represent a key selective pressure shaping the evolution of signals. An iconic example comes from the African cichlid fishes. As water depth increases, the ambient spectrum becomes redder. Accordingly, color sensitivity of cichlids shows a concerted red shift, which in turn drives the body coloration of males to shift towards red [6]. In other cases, such as some swordtail fishes, the receiver bias lies in not the sensory but the central processing systems [3]. Although this idea was first proposed in the context of sexual selection, the principle transcends communication contexts. Over three decades after the birth of this hypothesis, evidence has mounted [7], but the neurological mechanisms generating the biases, especially those in central processing, remain less well understood.

Many animal signals are temporally repetitive: the songs of birds, frogs, and insects, the flashing of fireflies, and the waving of the major chelae of fiddler crabs all contain elements repeated at relatively regular tempos (Fig 1a). Two recent Discovery Reports in PLOS Biology from Amichay and colleagues [8] and Piette and colleagues [9] focused on the tempo of these signals. Inspired by cricket singing and firefly flashing, and after surveying publications and databases, Amichay and colleagues identified a hotspot of tempos in communication signals between 0.5 and 4 Hz across modalities and taxonomic groups (Fig 1b) [8]. Independently, by analyzing existing recordings, Piette and colleagues demonstrated a similar concentration of slow tempos below 4 Hz among acoustic signals (Fig 1b) [9]. These convergent findings raise an intriguing question: what could have driven the evolution of such a conserved pattern? Amichay and colleagues [8] and Piette and colleagues [9] approached this question from the perspective of computational neurobiology and comparative phylogeny, respectively. In doing so, the two studies have independently arrived at a similar hypothesis: biases arising from slow oscillators or circuits in the receiver’s brain may, at least in part, drive the evolution of signals with slow repetition tempos (Fig 1).

In a comparative phylogenetic study published in the current issue, Piette and colleagues [9] tested the hypothesis that species-level factors including body weight, whether food is chewed, and living environment explain the observed pattern. Phylogenetically controlled Bayesian models failed to reject the null, suggesting that these species-level factors are unlikely to be major contributors to the observed pattern. The authors subsequently examined the dynamics of signal tempo evolution across a phylogeny containing 58 birds and 28 mammals. They tested two alternative hypotheses: that below 20 Hz, signal tempos have evolved randomly, approximated by a Brownian motion model, or that signal tempos have evolved toward an optimum, approximated by an Ornstein–Uhlenbeck (OU) model. Results supported an OU process, with an estimated optimum signal tempo at 2.7 Hz, providing evidence for stabilizing selection for the conserved slow tempos (Fig 1c). The robustness of this conclusion could be improved by increasing both sample size and species representation in future research. The authors hypothesized that the evolution of this pattern could be driven by delta oscillations in the receivers’ brains, a deeply conserved brain wave that matches the observed tempo pattern and has been implicated in sensory processing [10–12].

In another Discovery Report published earlier, Amichay and colleagues proposed a similar hypothesis that the hotspot of slow signal tempos may be an adaptation to the behaviors of small circuits in the receivers’ brains [8]. Is there any evidence that such a mechanism could be plausible? Amichay and colleagues [8] provided an initial test in two computational experiments based on simulated circuits comprising five oscillators. In the first experiment, the authors asked whether the emergent circuit response to a driving signal of 2 Hz depends on circuit topology (i.e., how neurons are connected within a circuit). Results indicated little dependence (Fig 1d). In the second experiment, the authors asked whether the circuit can selectively entrain to a forcing tempo in the delta band. Results showed that entrainment was the strongest when the forcing tempo matched the intrinsic tempo of the circuit itself but not its harmonics, providing initial evidence that circuits consisting of delta-band oscillators can selectively respond to external tempo inputs in the 0.5–4 Hz range (Fig 1e). Additionally, by increasing variability in intrinsic tempo of the neurons and strength of coupling to the driving signal, the circuit can respond to a slightly broader range of forcing tempos. Such flexibility and the robustness to circuit topology may both facilitate evolution and persistence of this mechanism throughout deep evolutionary history. This work using small, simple circuits offers an early proof-of-concept support for the hypothesis. As the authors noted, more complex computational models are needed to obtain sophisticated insights. Meanwhile, identifying a system where this idea can be tested empirically would be a challenging but fruitful endeavor.

Taken together, these two studies converged on a similar idea and support an intriguing hypothesis that biases in the receiver’s brain may, at least in part, drive the evolution of signals with slow repetition tempos [8,9]. This hypothesis transcends signal modalities and taxa and proposes a neuronal mechanism for how conserved biases in central processing of receivers can emerge (Fig 1).

These two studies are an excellent example showcasing the value of integrating evolutionary thinking into neurobiological research. Signals with slow tempos have long been recognized in behavioral ecology, and delta oscillations were discovered in neurobiology over a century ago. Yet only recently with increased cross-disciplinary integration has a potential connection between these phenomena begun to emerge. Together, these two studies help pave the way toward an integrative framework and underscore the importance of collaboration across discipline boundaries as a promising approach for ultimately explaining diverse animal signals.