Julia Gröning

Reproductive interference between animal species

Although sexual interactions between species (reproductive interference) have been reported from a wide range of animal taxa, their potential for determining species coexistence is often disregarded. Here, we review evidence from laboratory and field studies illustrating that heterospecific sexual interactions are frequently associated with fitness loss and can have severe ecological and evolutionary consequences. We define reproductive interference as any kind of interspecific interaction during the process of mate acquisition that adversely affects the fitness of at least one of the species involved and that is caused by incomplete species recognition. We distinguish seven types of reproductive interference: signal jamming, heterospecific rivalry, misdirected courtship, heterospecific mating attempts, erroneous female choice, heterospecific mating, and hybridization. We then discuss the sex-specific costs of these types and highlight two typical features of reproductive interference: density-dependence and asymmetry. Similar to competition, reproductive interference can lead to displacement of one species (sexual exclusion), spatial, temporal, or habitat segregation, changes in life history parameters, and reproductive character displacement. In many cases, patterns of coexistence might be shaped by reproductive interference rather than by resource competition, as the presence of a few heterospecifics might substantially decrease reproductive success. Therefore, interspecific sexual interactions should receive more attention in ecological research. Reproductive interference has mainly been discussed in the context of invasive species or hybrid zones, whereas its influence on naturally-occurring sympatric species pairs has rarely been addressed. To improve our knowledge of the ecological significance of reproductive interference, findings from laboratory experiments should be validated in the field. Future studies should also focus on ecological mechanisms, such as temporal, spatial, or habitat partitioning, that might enable sexually interacting species to coexist. Reproductive interference also has implications for the management of endangered species, which can be threatened by sexual interactions with invasive or common species. Studies of reproductive interference might even provide new insights for biological pest control.

Sexual size dimorphism in Orthoptera (sens. str.) — a review

Abstract Sexual size dimorphism (SSD) is a common phenomenon in animal taxa. While males are the larger sex in many birds and mammals, female-biased SSD predominates among insects, including Orthoptera. We analyzed size differences of 1503 Orthoptera species, suggesting that SSD is rather uniform in Ensifera, with the females being on average 9% larger than the males (ranging from −20 to +40%). In contrast, SSD is usually much stronger in Caelifera (37%) and also more variable (ranging from −20 to +140%). Caelifera with larger females exhibit stronger size differences than smaller species, whereas in Ensifera SSD decreases with male body size, but is not related to female size. Sexual size differences in Orthoptera are usually associated with a higher number of nymphal instars in females, leading to an earlier emergence of adult males (protandry). Both growth rates and the number of instars seem to be affected by genetic and environmental cues. Two major hypotheses have been proposed to explain the ultimate causes for SSD: the intersexual competition hypothesis and the differential equilibrium hypothesis. The first suggests that sexual dimorphism is a mechanism to reduce intraspecific competition, enabling the sexes to specialize on different food items. The differential equilibrium hypothesis proposes that the different body sizes represent sex-specific fitness optima, which are caused by their specific life-history strategies. Females may maximize their reproductive success by increasing the number (or size) of eggs (fecundity selection), whereas males may maximize their reproduction by being more mobile and fertilizing many females in a short period of time. These fundamental differences in the life-history strategies of the sexes may also lead to sexual selection, which has sometimes been referred to as an additional hypothesis. There is still a need for more empirical research on the ultimate causes for SSD. At present, there is much more support for the differential equilibrium hypothesis, but the intersexual competition hypothesis has rarely been tested. We propose some experimental approaches to test both hypotheses in micro- and macroevolutionary contexts.

Habitat preferences of an endangered insect species, Cepero’s ground-hopper (Tetrix ceperoi)

Preventing the global decline in biodiversity is a major task for conservation biologists. Although habitat loss has been identified as a key factor driving extinction processes, our knowledge on the habitat requirements of many endangered species, particularly invertebrates, is still sparse. We present a feasible method to study the microhabitat preferences of insect species. In Central Europe, the endangered Cepero’s ground-hopper, Tetrix ceperoi, is believed to have its only remaining natural habitats in dune slacks of the Wadden Sea Islands. Our results suggest that this species performs an active habitat choice of damp, bare patches with high temperatures. While ponds and fens in dune slacks provide large areas of damp bare ground and algal mats, grasslands, degraded dune slacks and the transitional zone between salt marsh and dunes are less suited as habitats. The major threat for T. ceperoi is found in the succession of its pioneer habitats due to the reduced natural dynamics. In industrialized countries, pioneer habitats and species are threatened substantially by coastal protection and floodplain regulation. This is only compensated in part by anthropogenic creation of secondary habitats, such as different kinds of pits or coal heaps. Nevertheless, there is a strong need for restoration of dynamic habitats by floodplain revitalization and dune slack restoration.

Phenotypic plasticity in insects: the effects of substrate color on the coloration of two ground-hopper species

SUMMARY The question of how phenotypic variation is maintained within populations has long been a central issue in evolutionary biology. Most of these studies focused on the maintenance of genetic variability, but the phenotype of organisms may also be influenced by environmental cues experienced during ontogeny. Color polymorphism has received particular attention in evolutionary studies as it has strong fitness consequences. However, if body coloration is influenced by the environment, any conclusions on evolutionary consequences of fitness trade‐offs can be misleading. Here we present data from a laboratory experiment on the influence of substrate color on three aspects of the coloration of two ground‐hopper species, Tetrix subulata and Tetrix ceperoi. We reared hatchlings either on dark or on light substrates, using a split‐brood design. Although the type of pronotal pattern changed mainly in response to nymphal development, the basic color was strongly influenced by the substrate color. In both species, black and dark olive color morphs were found more frequently on the dark substrate, whereas the gray color morph dominated on the light substrate. These findings have considerable implications for our understanding of color morph evolution as they show that color polymorphism may not only be maintained by natural selection acting on discrete color morphs, but also by phenotypic plasticity, which enables organisms to adjust to the environmental conditions experienced during ontogeny. This facultative morphology is opposing to the prevailing view of color morph adaptation, which assumes a purely genetic determination and co‐evolution of discrete color morphs with life history traits.

Intersexual niche segregation in Cepero's Ground-hopper, Tetrix ceperoi

Sexual differences in habitat preferences have been reported from a variety of animal taxa. However, the ultimate causes for this intersexual niche segregation remain poorly understood. It has been suggested that sexual dimorphism is a consequence of dimorphic niches based upon different reproductive costs and activities of the sexes. Here we provide evidence from field data to examine this hypothesis by studying the behavioral background of niche segregation in Tetrix ceperoi. Our data revealed distinct sexual differences in the substrates on which the insects perched and in the solar radiation of these locations. Males were found at brighter locations and more often on bare ground than females. Incorporation of behavioral data in our analysis showed that patches of bare ground were mainly utilized during mating behavior, in which males invested more time than females. In contrast, females spent more time resting and feeding in the vegetation. Intersexual differences in the proportion of autotomized individuals indicate that males might suffer higher predation risks. These patterns support the dimorphic niches hypothesis, which suggests that differential habitat utilization is caused by differences in the life history strategies of males and females, since males should accept a higher predation risk due to the benefits of multiple matings. Females should invest more time in gaining nutrients and energy for egg production and survival, whereas males should spend more time with searching for mates. We suggest that behavioral covariates should more often be implemented in ecological analyses, since these might have a strong explanatory power.

Visual communication behaviour as a mechanism behind reproductive interference in three pygmy grasshoppers (Genus Tetrix, Tetrigidae, Orthoptera)

Specific mate recognition systems should enable species to recognize conspecific mates correctly. However, heterospecific matings have been observed in a variety of taxa. One of these cases is the pygmy grasshopper genus Tetrix, in which three species show sexual interactions. T. ceperoi males mount preferably females of T. subulata, but they are rejected as mates. T. subulata males prefer T. undulata females over conspecific females and heterospecific matings occur. Here, we study the underlying behavioural mechanisms of this pattern by analysing the visual courtship behaviour of the three species videographically. We test the hypothesis that the displays of T. ceperoi are highly differentiated from the other species, while the courtship of T. subulata and T. undulata is more similar. This is supported by our results: while T. ceperoi males perform a fast movement of high amplitude (“pronotal bobbing”), the other two species show only minor movements (“lateral swinging,” “frontal swinging”). The first function of a discriminant analysis of the temporal dimensions of the displays explained 96% of the variance. 100% of the T. ceperoi displays were classified correctly, whereas only 50% of the T. subulata and 81% of the T. undulata displays were grouped accurately. A two-way ANOVA revealed no significant effects of the direction of the “swinging” movements (laterally or frontally) and no interactions between direction and species either, whereas each temporal parameter differed significantly between the three species. The highest degree of differentiation was found between T. ceperoi and T. undulata, while T. subulata and T. undulata only differed significantly in two of the six temporal parameters. Our results suggest that the mismatings between T. undulata and T. subulata are caused by an insufficient specificity of the courtship behaviour. Apparently, ecological segregation of these two species could impede sexual interactions in the field.

Niche overlap in allotopic and syntopic populations of sexually interacting ground-hopper species

Abstract  There is accumulating evidence that sexual interactions among species (reproductive interference) could have dramatic effects for species’ coexistence. It has been shown that the fitness of individuals can be substantially reduced as a consequence of reproductive interference. This might subsequently lead to displacement of a species (sexual exclusion). On the other hand, some evolutionary and ecological mechanisms might enable species to coexist, such as the divergence of mate recognition systems (reproductive character displacement), habitat partitioning, clumped dispersion patterns or different colonization capabilities. We have previously shown that the two ground‐hopper species Tetrix subulata and Tetrix ceperoi interact sexually in the laboratory as well as in the field. At sites where both species co‐occur niche overlap was high, suggesting that coexistence is maintained by different niche breadths rather than by habitat partitioning. To test the hypothesis that habitat partitioning does not contribute to species’ coexistence, we examined whether allotopic and syntopic populations of these two species differ in niche overlap (competitive release). Our results show that niche overlap is higher in syntopic than in allotopic populations, suggesting that the site‐specific habitat structure (heterogeneity) has a stronger influence on microhabitat utilization than the presence of heterospecifics. Hence, our data do not support the hypothesis that habitat partitioning plays a substantial role for the coexistence of these sexually interacting species.