SFB_TRR 212

Niche choice and niche conformance in colour polymorphic grasshoppers

We know that grasshoppers from the subfamily Gomphocerinae, i.e. the wider sibship of the steppe grasshopper, have genetic colour morph determination, while other species show phenotypic plasticity in colour. Since colour morphs are relevant to performance in local habitats, this gives a brilliant opportunity to compare colour changers with non-changers. Both types have the opportunity for niche choice, but only colour changers have the option to conform phenotypically to local environments.

The body colour of grasshoppers has consequences for their daily activities. We know so far that body colour affects body temperatures: Brown individuals have 1-2 °C higher body temperatures in their natural habitat than green individuals and at least some species show thermal melanism with the proportion of brown morphs increasing with altitude in the mountains. Such differences in body temperature are likely to alter the time window for their daily activities, in particular under borderline conditions such as mornings, evenings or overcast conditions. This might be particularly important to males that invest substantially in advertisement songs and in search for fertile females.

Besides influencing thermoregulation in grasshoppers, body colour influences crypsis and morph ratios are thus likely shaped by predation pressure. Interestingly, grasshoppers are notable also for pronounced pattern polymorphisms that effect both the distribution of green areas (mostly a lateral-dorsal divide), but also in the presence of shape-dissolving black patches. The trade-off between crypsis and thermoregulation will favour alternative survival strategies and niche specialisation.

We will study the behavioural consequences of individualised body colour in grasshoppers, contrasting multiple species with genetic colour morph determination and a species with the ability to plastically change body colour. We will test four hypotheses in separate work packages. The hypotheses will focus on the discrete difference between green and brown morphs, but also on inter-individual variation in body darkness within colour morphs.

1. Colour morphs have different thermal preferences. We predict that brown morphs (and darker individuals) have higher capacity to heat up under radiant heat and therefore lower preference for warm microhabitats.
2. Colour morphs have different time windows for their daily activities. We predict that brown morphs (and darker individuals), due to their thermoregulatory advantage, are more active under colder conditions compared to green morphs (and lighter individuals), and that this benefits in particular males in courtship display.
3. Individuals choose matching substrate colours, but niche chooser (= non-plastic species) invest more in habitat sampling when mismatched than niche conformers.
4. Colour morph identity is reflected in cuticular hydrocarbon (CHC) profiles and song traits and influences mating decisions. Both assortative and disassortative are possible and both would be relevant for understanding the maintenance of niche specialisation.

This project offers a remarkable opportunity to study the interaction between different niche-alternating (NC³) mechanisms in closely related species with similar ecology.