Rare morph Lake Malawi mbuna cichlids benefit from reduced aggression from con- and hetero-specifics

Balancing selection is important for the maintenance of polymorphism as it can prevent either fixation of one morph through directional selection or genetic drift, or speciation by disruptive selection. Polychromatism can be maintained if the fitness of alternative morphs depends on the relative frequency in a population. In aggressive species, negative frequency-dependent antagonism can prevent an increase in the frequency of rare morphs as they would only benefit from increased fitness while they are rare. Heterospecific aggression is common in nature and has the potential to contribute to rare morph advantage. Here we carry out field observations and laboratory aggression experiments with mbuna cichlids from Lake Malawi, to investigate the role of con- and heterospecific aggression in the maintenance of polychromatism and identify benefits to rare mores which are likely to result from reduced aggression. Within species we found that males and females bias aggression towards their own morph, adding to the evidence that inherent own-morph aggression biases can contribute to balancing selection. Over-representation of rare morph territory owners may be influenced by two factors; higher tolerance of different morph individuals as neighbours, and ability of rare morphs to spend more time feeding. Reduced aggression to rare morph individuals by heterospecifics may also contribute to rare morph advantage.

Spatial Mixing between Calling Males of Two Closely Related, Sympatric Crickets Suggests Beneficial Heterospecific Interactions in a NonAdaptive Radiation

Sympatry among closely related species occurs in both adaptive and nonadaptive radiations. Among closely related, sympatric species of a nonadaptive radiation, the lack of ecological differentiation brings species into continual contact where individuals are exposed to the risk of reproductive interference. Selection thus should cause divergence in multiple components mediating the reproductive boundary. Besides differentiation of reproductive signals per se, spatial segregation is a commonly proposed mechanism that can mitigate reproductive interference. Studying a pair of broadly sympatric, closely related cricket species from a nonadaptive radiation in Hawaii, we 1) quantified acoustic divergence of male songs and 2) tested alternative hypotheses of spatial distribution of calling males of the 2 species. Acoustic analyses of the recorded songs showed that, while the 2 species differed substantially in pulse rate, no spectral or fine temporal segregation of the pulse structure was evident, indicating the potential for acoustic masking. Moreover, we found that calling males of the 2 species are highly mixed both vertically and horizontally and showed the same preference for calling sites. More surprisingly, calling males were found to form mixed-species calling clusters where heterospecific males are closer to each other than conspecific males. Such an individual spacing pattern suggests low heterospecific aggression and/or high conspecific competition. Because females prefer higher sound intensity, heterospecific males may benefit, rather than interfere, with each other in attracting females. These findings offer a potential mechanism enabling species coexistence in sympatry.

Heterospecific aggression differs predominately by species, rather than sex, in Lake Malawi cichlid fish

ABSTRACTBecause of their striking diversity, Lake Malawi cichlid fish have been well studied for male aggression, particularly among dominant males of closely related sister species within the framework of mate-choice and speciation. However, aggression in females has been largely ignored, and variation in aggressive behaviors between more distantly-related taxa is not well understood despite its potential impact in a complex community structure. To better understand variation in patterns of aggression between species, we presented males and females from five species of Lake Malawi cichlid with a non-predator intruder and recorded all movement and aggressive acts. Additionally, we measured excreted cortisol levels the day after the intruder assay to evaluate one physiological aspect of stress response. We identified species-specific patterns in both specific aggressive acts, and overall level of aggression. Additionally, we found that sexual dimorphism in aggressive acts varies by species and act, where the species with the most aggressive males also has aggressive females. Additionally, cortisol levels vary by taxa, and are associated with restless behavior in the intruder assay, but not levels of aggression. These findings have bearing on understanding sex differences in aggression and their impact on community structure in this important model of rapid evolution.

Aggression and discrimination among closely versus distantly related species of Drosophila

Fighting between different species is widespread in the animal kingdom, yet this phenomenon has been relatively understudied in the field of aggression research. Particularly lacking are studies that test the effect of genetic distance, or relatedness, on aggressive behaviour between species. Here we characterized male–male aggression within and between species of fruit flies across the Drosophila phylogeny. We show that male Drosophila discriminate between conspecifics and heterospecifics and show a bias for the target of aggression that depends on the genetic relatedness of opponent males. Specifically, males of closely related species treated conspecifics and heterospecifics equally, whereas males of distantly related species were overwhelmingly aggressive towards conspecifics. To our knowledge, this is the first study to quantify aggression between Drosophila species and to establish a behavioural bias for aggression against conspecifics versus heterospecifics. Our results suggest that future study of heterospecific aggression behaviour in Drosophila is warranted to investigate the degree to which these trends in aggression among species extend to broader behavioural, ecological and evolutionary contexts.

Aggression and discrimination among closely versus distantly related species of Drosophila

Fighting between different species is widespread in the animal kingdom, yet this phenomenon has been relatively understudied in the field of aggression research. Particularly lacking are studies that test the effect of genetic distance, or relatedness, on aggressive behavior between species. Here we characterized male-male aggression within and between species of fruit flies across the Drosophila phylogeny. We show that male Drosophila discriminate between conspecifics and heterospecifics and show a bias for the target of aggression that depends on the genetic relatedness of opponent males. Specifically, males of closely related species treated conspecifics and heterospecifics equally, whereas males of distantly related species were overwhelmingly aggressive toward conspecifics. To our knowledge, this is the first study to quantify aggression between Drosophila species and to establish a behavioral bias for aggression against conspecifics versus heterospecifics. Our results suggest that future study of heterospecific aggression behavior in Drosophila is warranted to investigate the degree to which these trends in aggression among species extend to broader behavioral, ecological, and evolutionary contexts.

Heterospecific aggression bias towards a rarer colour morph

Colour polymorphisms are a striking example of phenotypic diversity, yet the sources of selection that allow different morphs to persist within populations remain poorly understood. In particular, despite the importance of aggression in mediating social dominance, few studies have considered how heterospecific aggression might contribute to the maintenance or divergence of different colour morphs. To redress this gap, we carried out a field-based study in a Nicaraguan crater lake to investigate patterns of heterospecific aggression directed by the cichlid fish, Hypsophrys nicaraguensi s, towards colour polymorphic cichlids in the genus Amphilophus . We found that H. nicaraguensis was the most frequent territorial neighbour of the colour polymorphic A. sagittae . Furthermore, when manipulating territorial intrusions using models, H. nicaraguensis were more aggressive towards the gold than dark colour morph of the sympatric Amphilophus species, including A . sagittae . Such a pattern of heterospecific aggression should be costly to the gold colour morph, potentially accounting for its lower than expected frequency and, more generally, highlighting the importance of considering heterospecific aggression in the context of morph frequencies and coexistence in the wild.