Synchronous waving in fiddler crabs: a review

Emergent collective behavior evolves more rapidly than individual behavior among acorn ant species

Emergence is a fundamental concept in biology and other disciplines, but whether emergent phenotypes evolve similarly to nonemergent phenotypes is unclear. The hypothesized process of emergent evolution posits that evolutionary change in at least some collective behaviors will differ from evolutionary change in the corresponding intrinsic behaviors of isolated individuals. As a result, collective behavior might evolve more rapidly and diversify more between populations compared to individual behavior. To test whether collective behavior evolves emergently, we conducted a large comparative study using 22 ant species and gathered over 1,500 behavioral rhythm time series from hundreds of colonies and isolated individuals, totaling over 1.5 y of behavioral data. We show that analogous traits measured at individual and collective levels exhibit distinct evolutionary patterns. The estimated rates of phenotypic evolution for the rhythmicity of activity in ant colonies were faster than the evolutionary rates of the same behavior measured in isolated individual ants, and total variation across species in collective behavior was higher than variation in individual behavior. We hypothesize that more rapid evolution and higher variation is a widespread feature of emergent phenotypes relative to lower-level phenotypes across complex biological systems.

Collective synchrony of mating signals modulated by ecological cues and social signals in bioluminescent sea fireflies

Individuals often employ simple rules that can emergently synchronize behaviour. Some collective behaviours are intuitively beneficial, but others like mate signalling in leks occur across taxa despite theoretical individual costs. Whether disparate instances of synchronous signalling are similarly organized is unknown, largely due to challenges observing many individuals simultaneously. Recording field collectives and ex situ playback experiments, we describe principles of synchronous bioluminescent signals produced by marine ostracods (Crustacea; Luxorina) that seem behaviorally convergent with terrestrial fireflies, and with whom they last shared a common ancestor over 500 Mya. Like synchronous fireflies, groups of signalling males use visual cues (intensity and duration of light) to decide when to signal. Individual ostracods also modulate their signal based on the distance to nearest neighbours. During peak darkness, luminescent 'waves' of synchronous displays emerge and ripple across the sea floor approximately every 60 s, but such periodicity decays within and between nights after the full moon. Our data reveal these bioluminescent aggregations are sensitive to both ecological and social light sources. Because the function of collective signals is difficult to dissect, evolutionary convergence, like in the synchronous visual displays of diverse arthropods, provides natural replicates to understand the generalities that produce emergent group behaviour.

Changes in the activity budget of the fiddler crab Leptuca uruguayensis throughout the reproductive period in temperate estuaries

Animal reproductive success implies the performance of several behaviours, such as courting, mate searching, copulation, offspring production and care. These behaviours usually have high energetic and ecological costs. Therefore, to maximise their reproductive success, animals should make choices throughout their lives, such as deciding how much energy to invest in different activities, according to their conditions and needs. In temperate estuaries, the fiddler crab L. uruguayensis has a short reproductive period, with two synchronous spawning events. Considering that reproductive behaviours incur high energetic cost to fiddler crabs, we estimated how this species manages its activity budget throughout the reproductive period, to quantify trade-offs between the time spent on reproductive behaviours versus time spent on other activities. By analysing videos of females and males recorded in the field at different moments of the reproductive period, we observed that pre-copulatory behaviours, such as female wandering and male waving were more intense at the beginning of the reproductive period, suggesting that most matings occurred before the first spawning event but not before the second one. The ecological conditions during the breeding season and the individual strategies adopted by males and females mostly determine when and how much time to spend on courtship behaviours, and behavioural plasticity can be expected whenever the conditions change. The strategy used by L. uruguayensis for energy management, females' ability to store male gametes and environmental temperatures might have been the main factors determining the relative time spent in courtship behaviours during the reproductive period.

Systematics and Biogeography of Fiddler Crabs -A Special Issue in Zoological Studies

Fiddler crabs are a fantastic group of intertidal brachyuran crabs, and the research fields focused on their biodiversity, phylogeography, phylogenomics, and larval biology are still in developing stages. In this special issue, seven articles are included focusing on the diversity, phylogeography, mitogenome phylogeny and larval morphology of fiddler crabs, covering the regions of the Indo-West Pacific and Americas. Results from this special issue open up further opportunities to study new species identification based on an integrative taxonomy approach, genomic-level phylogeny and larval morphology, especially in regards to the mitogenomes in the genera Cranuca, Gelasimus, Paraleptuca, and Uca for filling up the knowledge gap of fiddler crabs in the world.

Imperfect synchrony in animal displays: why does it occur and what is the true role of leadership?

Synchrony can be defined as the precise coordination between independent individuals, and this behaviour is more enigmatic when it is imperfect. The traditional theoretical explanation for imperfect synchronous courtship is that it arises as a by-product of the competition between males to broadcast leading signals to attract female attention. This competition is considered an evolutionary stable strategy maintained through sexual selection. However, previous studies have revealed that leading signals are not honest indicators of male quality. We studied imperfect courtship synchrony in fiddler crabs to mainly test whether (i) signal leadership and rate are defined by male quality and (ii) signal leadership generates synchrony. Fiddler crab males wave their enlarged claws during courtship, and females prefer leading males-displaying ahead of their neighbour(s). We filmed groups of waving males in the field to detect how often individuals were leaders and if they engaged in synchrony. Overall, we found that courtship effort is not directly related to male size, a general proxy for quality. Contrary to the long-standing assumption, we also revealed that leadership is not directly related to group synchrony, but faster wave rate correlates with both leadership and synchrony. This article is part of the theme issue 'Synchrony and rhythm interaction: from the brain to behavioural ecology'.

An ecological approach to measuring synchronization abilities across the animal kingdom

In this perspective paper, we focus on the study of synchronization abilities across the animal kingdom. We propose an ecological approach to studying nonhuman animal synchronization that begins from observations about when, how and why an animal might synchronize spontaneously with natural environmental rhythms. We discuss what we consider to be the most important, but thus far largely understudied, temporal, physical, perceptual and motivational constraints that must be taken into account when designing experiments to test synchronization in nonhuman animals. First and foremost, different species are likely to be sensitive to and therefore capable of synchronizing at different timescales. We also argue that it is fruitful to consider the latent flexibility of animal synchronization. Finally, we discuss the importance of an animal's motivational state for showcasing synchronization abilities. We demonstrate that the likelihood that an animal can successfully synchronize with an environmental rhythm is context-dependent and suggest that the list of species capable of synchronization is likely to grow when tested with ecologically honest, species-tuned experiments.

This article is part of the theme issue ‘Synchrony and rhythm interaction: from the brain to behavioural ecology’.

Why signal if you are not attractive? Courtship synchrony in a fiddler crab

Synchronized male courtship signals are puzzling because males generally compete with each other for females. Male Austruca mjoebergi fiddler crabs wave in synchrony to attract females, but, all else being equal, females have a strong preference for “leader” males that can produce waves before other males (“followers”). So why do followers participate in synchrony? Here, we experimentally investigate three explanations for why followers might wave in synchrony: 1) followers obtain a small proportion of matings, 2) followers are more likely than a leader to attract females if they are positioned closer to her than is the leader, and 3) synchrony functions as a long-distance visual signal that attracts females so there is a net benefit to synchrony for all males. Using robotic male crabs, we found that females show a strong preference for leading males, but followers obtain a “better-than-nothing” proportion of mates. We also showed that closer proximity of a follower to the female did not affect her preference for leaders, although being a leader increased a male’s success when he was further from the female than were rival males. Finally, females were more likely to approach a distant group if there was a leader present, suggesting that followers do benefit from participating in synchrony.

Impacts of pollution, sex, and tide on the time allocations to behaviours of Uca arcuata in mangroves

Fiddler crabs (Uca) are ecosystem engineers in coastal ecosystems. Many anthropogenic and natural factors can affect the time allocated to various behaviours in Uca. However, the behaviour of U. arcuata, a widely distributed fiddler crab in Asia, has not been studied in mainland China. Here, we used binoculars to record the time budget of ten behaviours of U. arcuata to investigate the potential effects of sex, tides, and pollution on these behaviours. We found that the crabs spent 42.3%, 27.0%, and 10.6% of their time on feeding, feeding while walking, and stationary respectively. The crabs spent <1.5% of their time on copulation and grooming. The total foraging time (feeding + feeding while walking) did not differ among the three polluted sites. However, crabs spent more time on feeding but less time on feeding while walking. The feeding rate and probability of burrowing and grooming decreased while the possibility of locomotion and stay in burrow increased with increasing nutrient concentration. Females spent 13.9% more time on feeding and fed 54.9% faster than males. Males had a higher tendency to grooming and combat while they were less likely to walk than females. Regarding to the influence of tide, fiddler crabs fed 11.2% faster at ebb tides than at flood tides, and they were more likely to walk and stay in burrows at flood tides than at ebb tides. Our results indicated that nutrient pollution had stronger impacts on the behaviours of crabs than sex and tide. In polluted mangroves, increasing nutrient concentration reduced the quantity of sediment processed by fiddler crabs due to their smaller feeding area, slower feeding rate, and reduced frequency of burrowing activities. These results imply that mitigating nutrient pollution in mangroves may benefit the restoration and management of coastal ecosystems through the enhanced engineering functions of fiddler crabs.

Acoustic coordination by allied male dolphins in a cooperative context

Synchronous displays are hallmarks of many animal societies, ranging from the pulsing flashes of fireflies, to military marching in humans. Such displays are known to facilitate mate attraction or signal relationship quality. Across many taxa, synchronous male displays appear to be driven by competition, while synchronous displays in humans are thought to be unique in that they serve a cooperative function. Indeed, it is well established that human synchrony promotes cooperative endeavours and increases success in joint action tasks. We examine another system in which synchrony is tightly linked to cooperative behaviour. Male bottlenose dolphins form long-lasting, multi-level, cooperative alliances in which they engage in coordinated efforts to coerce single oestrus females. Previous work has revealed the importance of motor synchrony in dolphin alliance behaviour. Here, we demonstrate that allied dolphins also engage in acoustic coordination whereby males will actively match the tempo and, in some cases, synchronize the production of their threat vocalization when coercing females. This finding demonstrates that male dolphins are capable of acoustic coordination in a cooperative context and, moreover, suggests that both motor and acoustic coordination are features of coalitionary behaviour that are not limited to humans.

Digest: Ecology and signal structure drive the evolution of synchronous displays in fiddler crabs

Behaviors are often influenced by both ecology and genetics. Perez et al. tested whether display patterns and the ecology of different species of fiddler crabs influence synchronous waving and whether this a phylogenetic phenomenon. They found that there was no phylogenetic signal in wave display synchronicity, and suggested that signal structure, microhabitat complexity, and different mating systems might instead be the cause. These results support the non-phylogenetic nature of synchronicity that can be observed across animal taxa.

Interactive rhythms across species: the evolutionary biology of animal chorusing and turn-taking

The study of human language is progressively moving toward comparative and interactive frameworks, extending the concept of turn-taking to animal communication. While such an endeavor will help us understand the interactive origins of language, any theoretical account for cross-species turn-taking should consider three key points. First, animal turn-taking must incorporate biological studies on animal chorusing, namely how different species coordinate their signals over time. Second, while concepts employed in human communication and turn-taking, such as intentionality, are still debated in animal behavior, lower level mechanisms with clear neurobiological bases can explain much of animal interactive behavior. Third, social behavior, interactivity, and cooperation can be orthogonal, and the alternation of animal signals need not be cooperative. Considering turn-taking a subset of chorusing in the rhythmic dimension may avoid overinterpretation and enhance the comparability of future empirical work.

Ecology and signal structure drive the evolution of synchronous displays

Animal synchrony is found in phylogenetically distant animal groups, indicating behavioral adaptations to different selective pressures and in different signaling modalities. A notable example of synchronous display is found in fiddler crabs in that males wave their single enlarged claw during courtship. They present species-specific signals, which are composed of distinctive movement signatures. Given that synchronous waving has been reported for several fiddler crab species, the display pattern could influence the ability of a given species to sufficiently adjust wave timing to allow for synchrony. In this study, we quantified the wave displays of fiddler crabs to predict their synchronous behavior. We combined this information with the group's phylogenetic relationships to trace the evolution of display synchrony in an animal taxon. We found no phylogenetic signal in interspecific variation in predicted wave synchrony, which mirrors the general nonphylogenetic pattern of synchrony across animal taxa. Interestingly, our analyses show that the phenomenon of synchronization stems from the peculiarities of display pattern, mating systems, and the complexity of microhabitats. This is the first study to combine mathematical simulations and phylogenetic comparative methods to reveal how ecological factors and the mechanics of animal signals affect the evolution of the synchronous phenomena.