Theo C. M. Bakker

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• PhD
• professor, retired at University of Bonn

We studied phenotypic and genetic adaptation of the visual system of three-spined sticklebacks, Gasterosteus aculeatus, from North Uist, Scotland. We quantified differences in opsin gene expression of the four cone opsin genes among wild-caught fish from three lakes with clear and from three with tea-stained water and their offspring that were rais...

Generative AI may extend the capabilities of traditional search engines in preparing scientific manuscripts but its misuse must be prevented to maintain the honesty and accuracy of science. With proper disclosure and critical analysis, the use of ChatGTP-generated text in scientific writing may be made ethically legitimate (Eke 2023; Jarrah et al....

Different environmental conditions may lead to diverse morphological, behavioral and physiological adaptations of different populations of the same species. Lighting conditions, for example, vary vastly especially between aquatic habitats, and have been shown to elicit adaptations. The availability of shortwave ultraviolet (UV) light is especially...

Different environmental conditions may lead to diverse morphological, behavioral and physiological adaptations of different populations of the same species. Lighting conditions, for example, vary vastly especially between aquatic habitats, and have been shown to elicit adaptations. The availability of short-wave ultraviolet (UV) light is especially...

Fish that perform paternal care may increase their fitness by choosing nest sites that enhance survival and development of embryos. We studied nest-site choice with respect to dissolved oxygen concentration and water temperature in males of the three-spined stickleback (Gasterosteus aculea-tus), a small fish species with exclusive male parental car...

Ultraviolet A (UVA) signals (320-400 nm) are important in mate choice in numerous species. The sensitivity for ultraviolet signals is not only assumed to be costly, but also expected to be a function of the prevailing ecological conditions. Generally, those signals are favored by selection that efficiently reach the receiver. A decisive factor for...

Ultraviolet (UV) A signals (320–400 nm) are important in mate choice in numerous species. The sensitivity for UV signals is not only assumed to be costly, but also expected to be a function of the prevailing ecological conditions. Generally, those signals are favored by selection that efficiently reach the receiver. A decisive factor for color sign...

Vertebrate cellular immunity displays substantial variation among taxa and environments. Hematological parameters such as white blood-cell counts have emerged as a valuable tool to understand this variation by assessing the immunological status of individuals. These tools have long revealed that vertebrate cellular immune systems are highly plastic...

Increasing UVB-radiation (UVB) reaching earth's surface following stratospheric ozone depletion is linked to serious consequences for organisms. While studies have focused on direct cytocidal and immunomodulatory effects of UVB, indirect consequences for fitness-related life-history traits are largely unexplored, although knowledge is needed to und...

Predicting patterns of variation in sexual traits requires understanding how developmental environments influence mate choice. Here, we studied how long-term perceived high predation risk affects mutual mate choice in the cichlid Pelvicachromis taeniatus. From hatching onwards, fish were exposed regularly to either conspecific alarm cues signalling...

High predation risk during development induces phenotypic changes in animals. However, little is known about how these plastic responses affect signalling and competitiveness during contests. Herein, we have studied the consequences of anti-predator plasticity during the intra-sexual competition of Pelvicachromis taeniatus, a cichlid fish with mutu...

Parasites with complex life cycles often alter the phenotypic appearance of their intermediate hosts in order to facilitate ingestion by the final host. However, such manipulation can be costly as it might increase ingestion by less suitable or dead-end hosts as well. Species-specific parasitic manipulation is a way to enhance the transmission to s...

Spatial orientation is an important skill as it improves, for example, foraging, localisation of recourses, predator avoidance or navigation. Habitat complexity positively affects spatial abilities in various fish species with a more complex environment promoting learning ability. However, to what extent a complex social environment affects cogniti...

Organisms inhabiting shallow aquatic habitats currently experience increasing levels of solar ultraviolet B radiation (UVB). UVB causes damage on cellular and molecular levels and can affect associated life-history traits either through direct exposure or indirectly through oxidative stress generation. We examined UVB effects on pre- and post-matin...

Fishes show a great diversity of mating systems and fertilization mechanisms. This diversity creates an enormous potential for sperm competition. Typically, monogamous species face a low risk of sperm competition and invest less into sperm, and thus show smaller relative testis mass compared to polygamous species with high sperm competition. In cic...

In many species, males signal quality with elaborate traits, but females often show inter-individual variation in preference for these traits. Choosing a mate requires multiple cognitive steps; therefore cognitive style (how an individual processes information) likely influences the perception of sexual signals and ability to choose a high-quality...

Group living is widespread across animal taxa, incurring benefits such as increased foraging efficiency or an enhanced chance of surviving a predator's attack. The chances of escaping a predator are often lower for odd‐looking individuals, as these are detected at a higher rate than uniform looking group members. While this “oddity effect” shall op...

Group living reduces individual predation risk most effectively when group members are behaviorally and phenotypically similar. Group preferences are influenced by the individual, the members of the shoal, and the environmental conditions. While shoaling behavior has been studied extensively in the threespine stickleback (Gasterosteus aculeatus), i...

The ability of organisms to plastically respond to changing environments is well studied. However, variation in phenotypic plasticity during ontogeny is less well understood despite its relevance of being an important source of phenotypic variation in nature. Here, we comprehensively study ontogenetic variation in morphological antipredator plastic...

The ability to compete with conspecifics and to adequately respond to visual stimuli of group mates are important prerequisites for profiting from group benefits such as confusion of predators and greater efficiency in acquiring food. By impairing their host’s physical abilities or making the host conspicuous, even non-contagious parasites that do...

Ultraviolet-B radiation (UVB) reaching the earth's surface has increased due to human-caused stratospheric ozone depletion. Whereas the harmful effects of UVB on aquatic organisms are well studied at the molecular and cellular level, recent studies have also begun to address behavioural changes caused by sublethal amounts of UVB. However, the behav...

With ongoing environmental change, ultraviolet-B radiation (UVB) reaching the Earth’s surface has increased over recent decades with consequences for terrestrial and also aquatic ecosystems. Despite evidence for direct physiological and immunological responses of aquatic animals following enhanced UVB exposure, studies investigating indirect impact...

Parasites with complex life cycles often change intermediate host traits in order to enhance their transmission to the next host. Acanthocephalans are excellent examples of such parasitic manipulation. Here, we summarise evidence for adaptive parasitic manipulation in this group, provide a comprehensive overview of intermediate host traits affected...

There is a need for rapid and reliable molecular sexing of three-spined sticklebacks, Gasterosteus aculeatus, the supermodel species for evolutionary biology. A DNA region at the 5' end of the sex-linked microsatellite Gac4202 was sequenced for the X chromosome of six females and the Y chromosome of five males from three populations. The Y chromoso...

Inbreeding and ageing can affect characteristics of reproductive physiology, influencing an individual’s fecundity, fertility, and thus fitness. The effects of inbreeding and age are expected to depend on several factors such as inbreeding history of a population, lifespan, or environmental influences. Here, we investigated the impact of inbreeding...

Inbreeding and ageing have substantial impact on an individual’s fitness. Both can lead to an accumulation of deleterious alleles resulting in an addition of effects when inbreeding and age interact. The aim of this study was to investigate the separate and simultaneous effects of both factors on primary reproductive traits in females of the West A...

Predation is an important but often fluctuating selection factor for prey animals. Accordingly, individuals plastically adopt antipredator strategies in response to current predation risk. Recently, it was proposed that predation risk also plastically induces neophobia (an antipredator response towards novel cues). Previous studies, however, do not...

The genetic diversity and population structure of a parasite with a complex life cycle generally depends on the dispersal by its most motile host. Given that high gene flow is assumed to hinder local adaptation, this can impose significant constraints on a parasite's potential to adapt to local environmental conditions, intermediate host population...

Analysing spatial differences among macroparasite communities is an important tool in the study of host–parasite interactions. Identifying patterns can shed light on the underlying causes of heterogeneity of parasite distribution and help to better understand ecological constraints and the relative importance of host and parasite adaptations. In th...

Body coloration and color patterns are ubiquitous throughout the animal kingdom and vary between and within species. Recent studies have dealt with individual dynamics of various aspects of coloration, as it is in many cases a flexible trait and changes in color expression may be context-dependent. During the reproductive phase, temporal changes of...

Background: Among fishes, sex reversal by abiotic or social factors is well documented even in species with genetic sex determination. All species of the family Gasterosteidae studied thus far show genetic sex determination, and natural sex reversal is most likely rare. In threespine stickleback (Gasterosteus aculeatus L.), exposure to sex hormones...

Social-rearing environment has profound effects on behaviour, physiology and development. A complex social environment enhances an individual's social skills and competence in many animal taxa. Consequently, complete social deprivation seriously impairs various aspects of behaviour, development, brain function and hormone levels. Although sexual se...

Color signals, including ultraviolet (UV) signals, are widespread throughout the animal kingdom and color changes can be influenced by reproductive and motivational state. However, studies on dynamic changes of UV signals are scarce. Three spine sticklebacks (Gasterosteus aculeatus) that show intraspecific UV communication were used to study dynami...

Please note that "three spine" should be read as "threespine" throughout the manuscript. We are very sorry for this mistake caused by the publisher. Color signals, including ultraviolet (UV) signals, are widespread throughout the animal kingdom and color changes can be influenced by reproductive and motivational state. However, studies on dynamic...

In polyandrous mating systems, male reproductive success depends on both mate-acquisition traits (precopulatory) and sperm competitive abilities (postcopulatory). Empirical data on the interaction between these traits are inconsistent; revealing positive, negative or no relationships. It is generally expected that the investment in pre- and postcop...

Group living is widespread in animals. In nature, groups usually not only differ in phenotypic characteristics but also in the social relationships among group members. Theory predicts that individuals adjusting their shoaling decisions—to join certain groups or not—based on social criteria, such as familiarity or genetic relatedness, can increase...

Individuals have to respond simultaneously to different environmental factors often making trade-offs between conflicting demands necessary. Many freshwater ecosystems are resource-limited and both intra- and interspecific competitiveness is a common requirement to gain and defend resources necessary for reproduction. Although predation risk is an...

Animals usually benefit from joining groups, but joining a group can also come at a cost when members expose themselves to competition and the risk of contracting a contagious disease. Therefore, individuals are expected to adjust grouping behaviour to the ecological circumstances, their own competitiveness and the composition of the group. Here, w...

Grouping is a widespread phenomenon in the animal kingdom and the decision to join a group is a function of individual and environmental conditions, meaning that any advantages and disadvantages have to be pondered constantly. Shoaling decisions in fishes are communicated via a variety of factors, such as colour signals, amongst other ultraviolet (...

The ability to discriminate between different quantities is widespread throughout the animal kingdom, and the underlying mechanisms of quantity discrimination are currently intensely discussed. In contrast, questions elucidating the limits of quantity estimation received rather little attention so far. Here, we examined fine-tuned quantity estimati...

Social aggregations occur in many different animal taxa and mainly result from non-random assortment. Investigating factors that shape and maintain the composition of social aggregations are among others a main topic for understanding ecological speciation processes. Aggregation decisions are mediated by olfactory and visual cues, which in many ani...

Background Mating between close relatives often leads to a reduction of an individual’s fitness, due to an increased expression of deleterious alleles. Thus, in many animal taxa pre- as well as postcopulatory inbreeding avoidance mechanisms have evolved. An increased risk of inbreeding and hence a loss of genetic variation may occur during founder...

Body coloration and color patterns are ubiquitous throughout the animal kingdom and vary between and within species. Recent studies have dealt with individual dynamics of various aspects of coloration, as it is in many cases a flexible trait and changes in color expression may be context-dependent. During the reproductive phase, temporal changes of...

The ability to recognize conspecifics is indispensible for differential treatment of particular individuals in social contexts like grouping behavior. The advantages of grouping are multifarious, and there exist numerous additional benefits of joining aggregations of conspecifics. Recognition is based on different signals and transmitted via multip...

Acute and long-term temperature changes caused by global warming could lead to severe ecological and physiological consequences for aquatic organisms. This might be reflected in a higher mortality rate or a reduced hatching success but elevated temperatures might also lead to accelerated growth and egg development due to higher metabolic rates. The...

Some theoretical models of sperm competition make the assumption that in fish species with external fertilisation, sperm length relates positively to swimming speed at the expense of sperm longevity. Few studies have tested this assumption. We used the three-spined stickleback, Gasterosteus aculeatus L., to study functional sperm morphology. In thi...

Kin discrimination in nepotistic as well as in sexual contexts is widespread in animals including humans. However, the underlying mechanisms of kin discrimination are assumed to vary between species and-within species-between contexts. During solitary life stages, kin recognition based on social learning is assumed to be less reliable because kin t...

Alternative reproductive tactics are widespread in fishes, increasing the potential for sperm competition. Sperm competition has enormous impact on both variation in sperm numbers and sperm size. In cichlids, the sperm competition risk is very divergent and longer sperm are usually interpreted as adaptation to sperm competition. Here we examined wh...

The process of ageing is associated with negative effects of mutations acting late in life, which range from those affecting cells to those affecting the whole organism. In many animal taxa, the deterioration of the phenotype with age also affects traits such as males' primary and secondary sexual characteristics. In three-spined sticklebacks (Gast...

Recent studies have revealed that sexually selected traits may signal sperm quality and hence male fertilisation ability. There is also evidence that the expression of male sexual ornamentation and associated sperm characteristics depend on an individual's ability to cope with oxidative stress. Carotenoids are known for their antioxidant properties...

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Exposure to enhanced levels of ambient ultraviolet (UV) radiation (UVR) can have adverse effects on aquatic organisms including damage at the cellular and molecular level and impairment of development, fecundity and survival. Much research has been conducted on the role of the harmful UVB radiation. However, due to its greater penetration in water...

Conditional mating strategies enable individuals to modulate their mating behaviour depending on ‘individual status’ to maximise fitness. Theory predicts that variation in individual quality can lead to differences in mating preferences. However, empirical evidence is scarce particular in terms of variation in male and female strategies. Here, we e...

Many animals are sensitive to ultraviolet light and also possess UV-reflective regions on their body surface. Individuals reflecting UV have been shown to be preferred during social interactions such as mate choice or shoaling decisions. However, whether those body UV-reflections enhance also the conspicuousness to UV-sensitive predators and theref...

Group living has evolved in many animal species as an antipredator behavior, an evolutionary effect that might be augmented by grouping with similarly looking individuals. Consequently, groups are often composed according to species, size, or coloration. During egg ripening or embryo growth, the outer appearance of females often changes drastically...

Mating between relatives often results in inbreeding depression, and is assumed to have a strong effect on fitness traits such as fertility and gonad/gamete quality. However, data concerning this topic are contradictory and particularly scarce in fishes. Three-spined sticklebacks (Gasterosteus aculeatus L.) show inbreeding depression in fertilizati...

The ability to differentiate between kin and nonkin is of importance in nepotistic as well as in mate choice contexts. Phenotype matching is a significant kin recognition mechanism, which is widespread in animals. However, the underlying proximate mechanisms are still poorly understood. Phenotype matching can be based on either self-reference or fa...

Mating preferences for genetic compatibility strictly depend on the interplay of the genotypes of potential partners and are therein fundamentally different from directional preferences for ornamental secondary sexual traits. Thus, the most compatible partner is on average not the one with most pronounced ornaments and vice versa. Hence, mating pre...

In the laboratory, wild-caught male Rhinoderma darwinii frogs, but not females, changed body colour from brown to green starting with the appearance of green dorsal spots or a greenish dorsal tone on a brown body colour. After a year in captivity, most males exhibited a greenish or complete green dorsal colour, probably induced by the terrarium's s...

Visual signalling can be affected by both the intensity and spectral distribution of environmental light. In shallow aquatic habitats, the spectral range available for visually mediated behaviour, such as foraging, can reach from ultraviolet (UV) to long wavelengths in the human visible range. However, the relative importance of different wavebands...

We demonstrate that Darwin's frogs (Rhinoderma darwinii) show sex differences in dorsal pattern and body coloration. Males possessed higher variability than females, which were mainly brown; two dorsal patterns and a green body colour were found exclusively in males. Males at different reproductive stages differed significantly in body colour and d...

Microsatellites used in the analysis of the three wild Pelvicachromis populations. P. taeniatus Moliwe (N = 200), P. taeniatus Nigeria (N = 9), P. pulcher Nigeria (N = 31). Shown are the number of alleles per locus (A), expected (He) and observed heterozygosity (Ho), results of Hardy-Weinberg probability test for deviation from expected Hardy-Weinb...

Microsatellite diversity indices at the six sampling sites of the Moliwe population. Sample size (N), number of loci typed in the sample (Loci), mean number of alleles per locus (A), median of number of alleles per locus (Amedian), mean expected (He) and observed heterozygosity (Ho), results of Hardy-Weinberg probability test for deviation from exp...

In wild animal populations, the degree of inbreeding differs between species and within species between populations. Because mating with kin often results in inbreeding depression, observed inbreeding is usually regarded to be caused by limited outbreeding opportunities due to demographic factors like small population size or population substructur...

Sexual selection theory proposes correlated evolutionary changes in mating preferences and secondary sexual characters based on a positive genetic correlation between preference and the preferred trait. Empirical work has provided support for a genetic covariation between female preference and male attractiveness in several taxa. Here, we study par...

Fast acclimatisation of experimental animals to laboratory test conditions is crucial for effective research. During highly standardized experiments, confounding parameters must be minimized which might lead to animals behaving unnaturally. The objective of the present study was to develop a simple method to enhance activity of fishes under laborat...

Predation risk is one of the major forces affecting phenotypic variation among and within animal populations. While fixed anti-predator morphologies are favoured when predation level is consistently high, plastic morphological responses are advantageous when predation risk is changing temporarily, spatially, or qualitatively. Three-spined stickleba...

Adaptive female coloration is likely to occur when males largely invest into reproduction or variance of quality between potential mating partners is high. Although recent studies have shown male choosiness of female traits, little is known about the extent to which female ornamentation signals benefits to males. Female ornamentation might signal i...

Sexual selection is an important force in the evolution of body size. Both intersexual selection, that is, preference for large individuals, and intrasexual selection, that is, increased competitiveness of large individuals, are involved in this process. Furthermore, preferences based on body size of the choosing individual might also influence bod...

Parasites with a complex life cycle are supposed to influence the behaviour of their intermediate host in such a way that the transmission to the final host is enhanced, but reduced to non-hosts. Here, we examined whether the trophically transmitted bird parasite Polymorphus minutus increases the antipredator response of its intermediate host, the...

Studies addressing the adaptive significance of female ornamentation have gained ground recently. However, the expression of female ornaments in relation to body size, known as trait allometry, still remains unexplored. Here, we investigated the allometry of a conspicuous female ornament in Pelvicachromis taeniatus, a biparental cichlid that shows...

Theory predicts that parents should adjust their parental investment to the reproductive value of the brood. Previous studies have mainly investigated the relationship between brood size, brood age and the intensity of care. However, the impact of brood quality traits such as the offspring’s average body size on parental investment has received rel...