Exploring the Importance of Environmental Complexity for Newly Hatched Zebrafish

The effects of an early impoverished social or physical environment on vertebrate neural development and cognition has been known for decades. While existing studies have focused on the long-term effects, measuring adult cognitive phenotypes, studies on the effects of environmental complexity on the early stages of development are lacking. Zebrafish (Danio rerio) hatchlings are assumed to have minimal interaction with their environment and are routinely reared in small, bare containers. To investigate the effects of being raised under such conditions on development of behaviour and cognition, hatchlings housed for 10 days in either an enriched or a standard environment underwent two cognitive tasks. The results were mixed. Subjects of the two treatments did not differ in performance when required to discriminate two areas. Conversely, we found a significant effect in a number discrimination task, with subjects from impoverished condition performing significantly worse. In both experiments, larvae reared in impoverished environment showed a reduced locomotor activity. Given the effects that enrichment appears to exert on larvae, a third experiment explored whether hatchlings exhibit a spontaneous preference for more complex environments. When offered a choice between a bare setting and one with objects of different shapes and colors, larvae spent over 70% of time in the enriched sector. Deepening these effects of an early impoverished environment on cognitive development is crucial for the welfare of captive zebrafish populations and for enhancing the quality and reliability of studies on larval zebrafish.

Shortest path choice in zebrafish (Danio rerio)

Many animals regularly move between different locations within their home range. During these journeys, individuals are expected to use the shortest path, because this strategy minimizes energy expenditure and reduces exposure to adverse conditions, such as predation. The ability to find the shortest distance route has been demonstrated in ants, migrating birds and a few mammals. We investigated whether a freshwater fish, Danio rerio, exhibits this ability. Small groups of zebrafish were allowed to move between the two compartments of their tank using two paths differing in length. They developed a preference for the shorter path gradually over the six days of the experiment. Subjects' accuracy in choosing the shorter path varied from below 60%, with a 20% length disparity, to 80% when one path was twice as long as the other. In a second experiment, zebrafish were initially allowed to practice in groups and then tested individually. We found evidence of individual and sex differences in performance, with males performing more accurately than females. However, due to our experimental design, we cannot conclusively determine whether these differences are indeed cognitive or influenced by confounding factors during the group phase of the experiment.

Some like it "local": A review of hierarchical processing in non-human animals

When seeing a visual image, humans prioritize the perception of global features, which is followed by the assessment of the local ones. This global precedence has been investigated using hierarchical stimuli that consist of a large, global shape formed by the spatial arrangement of small local shapes. Comparing non-human animals to humans, research on global and local processing has revealed a heterogeneous pattern of results with some species exhibiting a local precedence and others a global one. Many factors have been proposed to influence the global and local processing: internal factors (e.g., age, sex) and external elements or perceptual field variables (e.g., stimulus size, visual angle, eccentricity, sparsity). In this review, studies showing that different non-human species process hierarchical stimuli in the same (global precedence) or reverse (local precedence) direction as humans are first collated. Different ecological, perceptual, and anatomical features that may influence global and local processing are subsequently proposed based on a detailed analysis of these studies. This information is likely to improve our understanding of the mechanisms behind the perceptual organization and visual processing, and could explain the observed differences in hierarchical processing between species.

Female reproductive fluid attracts more and better sperm: implications for within-ejaculate cryptic female choice

Mounting evidence shows that the female reproductive fluid (FRF) can differently affect sperm performance of different males by biasing paternity share among competing males. Here, we tested for the first time the potential of 'within-ejaculate cryptic female choice' mediated by the FRF in the zebrafish (Danio rerio). Using a recently developed sperm selection chamber, we separated and collected FRF-selected from non-selected sperm to compare the two subpopulations of sperm in terms of sperm number, viability, DNA integrity and fertilizing ability. We showed that the sperm attracted by FRF are more numerous, more viable and with higher DNA integrity. In addition, FRF-selected sperm fertilized more eggs, but if this is due to fertilization ability per se or numerical advantage remains to be tested. Our results suggest that FRF can select sperm with a better phenotype, highlighting the crucial and impactful role that FRF might play in the process of fertilization and post-mating sexual selection dynamics, along with the potential implications for sperm selection in assisted reproductive techniques.

The influence of visual illusion perception on numerosity estimation could be evolutionarily conserved: exploring the numerical Delboeuf illusion in humans (Homo sapiens) and fish (Poecilia reticulata)

Discriminating between different quantities is an essential ability in daily life that has been demonstrated in a variety of non-human vertebrates. Nonetheless, what drives the estimation of numerosity is not fully understood, as numerosity intrinsically covaries with several other physical characteristics. There is wide debate as to whether the numerical and spatial abilities of vertebrates are processed by a single magnitude system or two different cognitive systems. Adopting a novel approach, we aimed to investigate this issue by assessing the interaction between area size and numerosity, which has never been conceptualized with consideration for subjective experience in non-human animals. We examined whether the same perceptual biases underlying one of the best-known size illusions, the Delboeuf illusion, can be also identified in numerical estimation tasks. We instructed or trained human participants and guppies, small teleost fish, to select a target numerosity (larger or smaller) of squares between two sets that actually differed in their numerosity. Subjects were also presented with illusory trials in which the same numerosity was presented in two different contexts, against a large and a small background, resembling the Delboeuf illusion. In these trials, both humans and fish demonstrated numerical biases in agreement with the perception of the classical version of the Delboeuf illusion, with the array perceived as larger appearing more numerous. Thus, our results support the hypothesis of a single magnitude system, as perceptual biases that influence spatial decisions seem to affect numerosity judgements in the same way.

Timing matters: female receptivity and mate choice in the zebrafish (Danio rerio)

Abstract

Female choice has been documented in many animal taxa, and how we test it has been refined through years of studies on the topic. However, when designing mate choice experiments some variables, surprisingly, often remain overlooked, including receptivity and reproductive stage. Here, we aimed to assess whether the female reproductive stage influences strength and direction of mate choice in the zebrafish, Danio rerio. Females were offered a choice between two males differing in body size. We found that female choice in our experimental setup was significantly repeatable and that females preferred larger males. Nonetheless, the level of choosiness of females was affected by the time since the last spawning. Females spent more time choosing when tested 7 and 10 days after spawning rather than 4 days, indicating a higher receptivity to males from one week after the last spawning. Moreover, females preferred larger males only when tested 7 and 10 days after spawning. Our results suggest that female mate choice should take female receptivity into account, by standardizing time since the last spawning across females. More broadly, this suggests that 7–10 days since the previous spawning is the ideal time interval for zebrafish female receptivity to peak, with implications for facilities and researchers to increase egg production in natural spawning events and manual egg collection.

Significance statement

The role of pre-copulatory female mate preference has long been recognized in sexual selection. Nonetheless, female receptivity often remains overlooked in mate choice experiments especially in external fertilizing species. In the present study, we investigated if the female reproductive stage affects the strength and direction of female mate choice in an external fertilizing fish, the zebrafish, Danio rerio. We found that, when tested 7 and 10 days after spawning rather than 4 days, females spent more time choosing, demonstrating an increased receptivity to males from 1 week following the last spawning. Furthermore, only at 7 and 10 days after spawning females exibith a clear preference for the bigger males. Our study highlights the importance of considering the female receptivity in future studies assessing mate choices in this and other externally fertilizing species, and also for zebrafish facilities to increase egg production in natural spawning events and manual egg collection.

Learning and visual discrimination in newly hatched zebrafish

With the exception of humans, early cognitive development has been thoroughly investigated only in precocial species, well developed at birth and with a broad behavioral and cognitive repertoire. We investigated another highly altricial species, the zebrafish, Danio rerio, whose embryonic development is very rapid (< 72 h). The hatchlings’ nervous system is poorly developed, and their cognitive capacities are largely unknown. Larvae trained at 8 days post fertilization rapidly learned to associate a visual pattern with a food reward, showing significant performance at 10 days post fertilization. We exploited this ability to study hatchlings’ discrimination learning capacities. Larvae rapidly and accurately learned color and shape discriminations. They also discriminated a figure from its mirror image and from its 90°-rotated version, although with lower performance. Our study revealed impressive similarities in learning and visual discrimination capacities between newborn and adult zebrafish, despite their enormous differences in brain size and degree of development.

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Newly hatched zebrafish can associate a visual stimulus to a food reward

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Learning occurs in just two training days and can be used to study perception

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Hatchlings compare to adults in color and shape discrimination

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Poorer performance was observed in discriminating rotated shapes and mirror images

Zebrafish excel in number discrimination under an operant conditioning paradigm

Numerical discrimination is widespread in vertebrates, but this capacity varies enormously between the different species examined. The guppy (Poecilia reticulata), the only teleost examined following procedures that allow a comparison with the other vertebrates, outperforms amphibians, reptiles and many warm-blooded vertebrates, but it is unclear whether this is a feature shared with the other teleosts or represents a peculiarity of this species. We trained zebrafish (Danio rerio) to discriminate between numbers differing by one unit, varying task difficulty from 2 versus 3 to 5 versus 6 items. Non-numerical variables that covary with number, such as density or area, did not affect performance. Most fish reached learning criterion on all tasks up to 4 versus 5 discrimination with no sex difference in accuracy. Although no individual reached learning criterion in the 5 versus 6 task, performance was significant at the group level, suggesting that this may represent the discrimination threshold for zebrafish. Numerosity discrimination abilities of zebrafish compare to those of guppy, being higher than in some warm-blooded vertebrates, such as dogs, horses and domestic fowl, though lower than in parrots, corvids and primates. Learning rate was similar in a control group trained to discriminate between different-sized shapes, but zebrafish were slightly more accurate when discriminating areas than numbers and males were more accurate than females. At the end of the experiment, fish trained on numbers and controls trained on areas generalized to the reciprocal set of stimuli, indicating they had used a relational strategy to solve these tasks.

Guppies (Poecilia reticulata) are deceived by visual illusions during obstacle negotiation

Animals travelling in their natural environment repeatedly encounter obstacles that they can either detour or go through. Gap negotiation requires an accurate estimation of the opening's size to avoid getting stuck or being injured. Research on visual illusions has revealed that in some circumstances, transformation rules used to generate a three-dimensional representation from bidimensional retinal images fail, leading to systematic errors in perception. In Ebbinghaus and Delboeuf illusions, the presence of task-irrelevant elements causes us to misjudge an object's size. Susceptibility to these illusions was observed in other animals, although with large intraspecific differences. In this study, we investigated whether fish can accurately estimate gap size and whether during this process they may be deceived by illusory patterns. Guppies were extremely accurate in gap negotiation, discriminating holes with a 10% diameter difference. When presented with two identical holes surrounded by inducers to produce Ebbinghaus and Delboeuf patterns, guppies misperceived gap size in the predicted direction. So far, researchers have principally considered illusions as useful tools for exploring the cognitive processing underlying vision. Our findings highlight the possibility that they have important ecological implications, affecting the everyday interactions of an animal with its physical world besides its intra- and interspecific relationships.

Stimulus characteristics, learning bias and visual discrimination in zebrafish (Danio rerio)

Zebrafish is an emerging model in the study of brain function; however, knowledge about its behaviour and cognition is incomplete. Previous studies suggest this species has limited ability in visual learning tasks compared to other teleosts. In this study, we systematically examined zebrafish's ability to learn to discriminate colour, shape, size, and orientation of figures using an appetitive conditioning paradigm. Contrary to earlier reports, the zebrafish successfully completed all tasks. Not all discriminations were learned with the same speed and accuracy. Subjects discriminated the size of objects better than their shape or colour. In all three tasks, they were faster and more accurate when required to discriminate between outlined figures than between filled figures. With stimuli consisting of outlines, the learning performance of zebrafish was comparable to that observed in higher vertebrates. Zebrafish easily learned a horizontal-vertical discrimination task, but like many other vertebrates, they had great difficulty discriminating a figure from its mirror image. Performance was more accurate for subjects reinforced on one stimulus (green over red, triangle over circle, large over small). Unexpectedly, these stimulus biases occurred only when zebrafish were tested with filled figures, suggesting some causal relationship between stimulus preference, learning bias and performance.

Red-footed tortoises (Chelonoidis carbonaria) do not perceive the Delboeuf illusion

Visual illusions have been widely used as a tool to study animal visual perception. In many cases, identical experimental procedures were adopted to make highly controlled interspecific comparisons. However, reducing methodological variability may prevent reliable comparisons because a certain methodology could be more suitable for some species than others. This study sought to build on previous work that investigated the perception of the Delboeuf illusion in reptiles. Reptiles were presented with trials composed of 2 different-sized food portions on 2 identical plates in which they were expected to maximize their food intake. In contrast to the bearded dragons (Pogona vitticeps), tortoises (Chelonoidis carbonaria) performed poorly in all conditions and therefore no firm conclusion regarding their perception of the illusion could be made. Such results could be due to cognitive challenges or due to the experimental setup, because descending a ramp is demanding for the tortoises. In this study, we adopted the same experimental paradigm but in a flat apparatus. Tortoises significantly discriminated the larger food portions in baseline trials, however, their performance did not differ from chance in illusory trials revealing that, under these conditions, they are not sensitive to the Delboeuf illusion. This nonperception could be ascribed to different factors, such as poorer discrimination ability or a low sensitivity to contrast and assimilation phenomena. Our study highlights the importance of additional investigation to better understand the nature of null results, taking in consideration the ecological needs of the species before drawing any conclusions about its abilities. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Forest before the trees in the aquatic world: global and local processing in teleost fishes

Background

The study of illusory phenomena is important to understanding the similarities and differences between mammals and birds’ perceptual systems. In recent years, the analysis has been enlarged to include cold-blooded vertebrates, such as fish. However, evidence collected in the literature have drawn a contradictory picture, with some fish species exhibiting a human-like perception of visual illusions and others showing either a reversed perception or no susceptibility to visual illusions. The possibility exists that these mixed results relate to interspecific variability in perceptual grouping mechanisms. Therefore, we studied whether fish of five species exhibit a spontaneous tendency to prioritize a global analysis of the visual scene—also known as global-to-local precedence—instead of focusing on local details.

Methods

Using Navon-like stimuli (i.e., larger recognisable shapes composed of copies of smaller different shapes), we trained redtail splitfin, zebrafish, angelfish, Siamese fighting fish and three spot gourami to discriminate between two figures characterized by congruency between global and local information (a circle made by small circles and a cross made by small crosses). In the test phase, we put global and local cues (e.g., a circle made by small crosses) into contrast to see whether fish spontaneously rely on global or local information.

Results

Like humans, fish seem to have an overall global-to-local precedence, with no significant differences among the species. However, looking at the species-specific level, only four out of five species showed a significant global-to-local precedence, and at different degrees. Because these species are distantly related and occupy a broad spectrum of ecological adaptations, we suggest that the tendency to prioritize a global analysis of visual inputs may be more similar in fish than expected by the mixed results of visual illusion studies.

Two halves are less than the whole: Evidence of a length bisection bias in fish (Poecilia reticulata)

The horizontal-vertical (HV) illusion is characterized by a tendency to overestimate the length of vertically-arranged objects. Comparative research is primarily confined to primates, a range of species that, although arboreal, often explore their environment moving along the horizontal axis. Such behaviour may have led to the development of asymmetrical perceptual mechanisms to make relative size judgments of objects placed vertically and horizontally. We observed the susceptibility to the HV illusion in fish, whose ability to swim along the horizontal and vertical plane permits them to scan objects' size equally on both axes. Guppies (Poecilia reticulata) were trained to select the longer orange line to receive a food reward. In the test phase, two arrays, containing two same-sized lines were presented, one horizontally and the other vertically. Black lines were also included in each pattern to generate the perception of an inverted T-shape (where a horizontal line is bisected by a vertical one) or an L-shape (no bisection). No bias was observed in the L-shape, which supports the idea of differential perceptual mechanisms for primates and fish. In the inverted T-shape, guppies estimated the bisected line as shorter, providing the first evidence of a length bisection bias in a fish species.

The ontogeny of continuous quantity discrimination in zebrafish larvae (Danio rerio)

Several studies have investigated the ontogeny of the capacity to discriminate between discrete numerical information in human and non-human animals. Contrarily, less attention has been devoted to the development of the capacity to discriminate continuous quantities. Recently, we set up a fast procedure for screening continuous quantity abilities in adult individuals of an animal model in neurodevelopmental research, the zebrafish. Two different sized holes are presented in a wall that divides the home tank in two halves and the spontaneous preference of fish for passing through the larger hole is exploited to measure their discrimination ability. We tested zebrafish larvae in the first, second and third week of life varying the relative size of the smaller circle (0.60, 0.75, 0.86, 0.91 area ratio). We found that the number of passages increased across the age. The capacity to discriminate the larger hole decreased as the ratio between the areas increased. No difference in accuracy was found as a function of age. The accuracy of larval zebrafish almost overlaps that found in adults in a previous study, suggesting a limited role of maturation and experience on the ability to estimate areas in this species.

Exploring the Müller-Lyer illusion in a nonavian reptile (Pogona vitticeps)

Visual illusions have been widely used to compare visual perception among birds and mammals to assess whether animals interpret and alter visual inputs like humans, or if they detect them with little or no variability. Here, we investigated whether a nonavian reptile (Pogona vitticeps) perceives the Müller-Lyer illusion, an illusion that causes a misperception of the relative length of 2 line segments. We observed the animals' spontaneous tendency to choose the larger food quantity (the longer line). In test trials, animals received the same food quantity presented in a spatial arrangement eliciting the size illusion in humans; control trials presented them with 2 different-sized food portions. Bearded dragons significantly selected the larger food quantity in control trials, confirming that they maximized food intake. Group analysis revealed that in the illusory test trials, they preferentially selected the line length estimated as longer by human observers. Further control trials excluded the possibility that their choice was based on potential spatial bias related to the illusory pattern. Our study suggests that a nonavian reptile species has the capability to be sensitive to the Müller-Lyer illusion, raising the intriguing possibility that the perceptual mechanisms underlying size estimation might be similar across amniotes. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Anisotropy of perceived space in non-primates? The horizontal-vertical illusion in bearded dragons (Pogona vitticeps) and red-footed tortoises (Chelonoidis carbonaria)

The horizontal-vertical illusion is a size illusion in which two same-sized objects appear to be different if presented on a horizontal or vertical plane, with the vertical one appearing longer. This illusion represents one of the main evidences of the anisotropy of the perceived space of humans, an asymmetrical perception of the object size presented in the vertical and horizontal space. Although this illusion has been widely investigated in humans, there is an almost complete lack of studies in non-human animals. Here we investigated whether reptiles perceive the horizontal-vertical illusion. We tested two reptile species: bearded dragons (Pogona vitticeps) and red-footed tortoises (Chelonoidis carbonaria). In control trials, two different-sized food strips were presented and animals were expected to choose the longer one. In test trials, animals received two same-sized strips, presented in a spatial arrangement eliciting the illusion. Only bearded dragons significantly preferred the longer strip in control trials; in test trials, bearded dragons selected the strip arranged vertically, suggesting a human-like perception of this pattern, while no clear choice for either array was observed in tortoises. Our results raise the interesting possibility that the anisotropy of perceived space can exists also in a reptile brain.

Perception of the Müller-Lyer illusion in guppies

The Müller–Lyer illusion is a well-known distortion illusion that occurs when the spatial arrangement of inducers (i.e., inwards- or outwards-pointing arrowheads) influences a line’s perceived relative length. To date, this illusion has been reported in several animal species but only in 1 teleost fish (i.e., redtail splitfins Xenotoca eiseni), although teleost fish represent approximately 50% of vertebrate diversity. We investigated the perception of this illusion in another teleost fish: guppies Poecilia reticulata, a species that diverged from the redtail splitfin 65 million years ago. The guppies were trained to select the longer between 2 lines; after meeting the learning criterion, illusory trials were presented. Control trials were also arranged to exclude the possibility that their choices were based on potential spatial biases that relate to the illusory pattern. The guppies’ overall performance indicated that they were susceptible to the Müller–Lyer illusion, perceiving the line with the inwards-pointing arrowheads as longer. The performance in the control trials excluded the possibility that the subjects used the physical differences between the 2 figures as the discriminative cue in the illusory trials. Our study suggests that sensibility to the Müller–Lyer illusion could be widespread across teleost fish and reinforces the idea that the perceptual mechanisms underlying size estimation might be similar across vertebrates.

Can reptiles perceive visual illusions? Delboeuf illusion in red-footed tortoise (Chelonoidis carbonaria) and bearded dragon (Pogona vitticeps)

Optical illusions have been widely used to compare visual perception among vertebrates because they can reveal how the system is able to adapt to visual input. Sensitivity to visual illusions has never been studied in reptiles. Here, we investigated whether red-footed tortoises, Chelonoidis carbonaria, and bearded dragons, Pogona vitticeps, perceive the Delboeuf illusion. This illusion involves the misperception of the size of a target circle depending upon the context in which it is presented. We adopted the same size discrimination for both species to compare their performance. Animals were presented with two different types of trials. In control trials, they received two different-sized food portions on two plates of the same size. In test trials, they received two same-sized food portions but presented on two different-sized plates. If they perceived the illusion in the same way as humans, we expected them to select the food portion presented on the smaller plate. The tortoises exhibited poor performance in the control trials, which prevented us from drawing any conclusions about their perception of the Delboeuf illusion. In contrast, the bearded dragons selected the larger amount of food in control trials. In test trials, they selected the portion presented on the smaller plate significantly more often than chance, suggesting a human-like sensitivity to the Delboeuf illusion. Our study provides the first evidence of the perception of a visual illusion in a reptile species, suggesting that rather than simply detecting visual input, they interpret sensory information captured by photoreceptors. (PsycINFO Database Record (c) 2019 APA, all rights reserved).