I am broadly interested in evolutionary ecology and biological interactions (see publications). In recent years, my team has been especially active in studying aposematism and mimicry.
Aposematism and mimicry in the novel world
Previous experiments that addressed the question how warning signals -and in particular, warning colours- of aposematic prey initially evolved relied on foraging and learning experiments that used young, inexperienced predators. This did not circumvent the problem that predators have attained genetically determined avoidance of certain colours. To study the predator-prey interaction that selects for aposematism from a “clean plate” we developed the novel world method where instead of colours artificial prey signal with black-and-white symbols in a black-and-white world (see Lindström et al. 2001). With this method we have tested hypotheses on the initial selection pressures that the very first aposematic prey animals would most likely have faced, and have subsequently addressed a range of questions about warning signals and mimicry (see a short video clip from novel world).
Initially, aggregation can facilitate the spread of aposematism in a population supporting Fisher’s (1930) view of kin aggregations as the evolutionary starting point. Unpalatability alone can select for gregariousness, since predators leave an aggregation of unpalatable prey after tasting just a few individuals. However, aggregation without a warning signal does not ensure avoidance learning. Instead, once predators have experience on warning signals, learned avoidance favours aposematism even among solitary prey.
All warning signals do not seem very effective. Predators vary in their tendency to attack defended prey, which can select for weak warning signals in the prey population.
Predators select for signal similarity between defended prey but not automatically in every situation. The evolutionary dynamics between mimetic prey species and predators are sensitive even in simplified systems depending for example on the specific signals. The structure of the whole prey community also affects how strongly predators select for mimetic resemblance. Furthermore, alternative, edible prey can change the dynamics between unequally defended mimetic prey from mutualistic to parasitic but a moderately defended co-mimic per se is not harmful to a strongly defended co-mimic: increasing density of mildly unpalatable co-mimics does not induce attacks on the strongly defended one when predators are learning to avoid their warning signal.
Signal polymorphism in the Wood tiger moth
We have also studied why warning signals of the Wood tiger moth are polymorphic despite strong purifying and positively frequency-dependent selection on warning signals (see Lindström et al 2001)
The Wood tiger moth shows dramatic variation in pigmentation and patterning throughout its distribution, and many morphs may co-occur. We have discovered that this colour polymorphism is an outcome of multiple, sometimes conflicting selection pressures. For example, yellow and white morphs are both common in Scandinavia. Our experiments show that the yellow morph is better protected against avian predators but the white morph has better mating success (Nokelainen et al. 2012). There is also a trade-off between warning signal efficacy (hind wing coloration) and melanisation that influence thermoregulation (Hegna et al. 2013). Colouration of the moths also seems to be genetically correlated with immunity: white males have better encapsulation ability whereas yellow males have increased lytic activity of haemolymph (Nokelainen et al. 2013).
Taken together, instead of the classic view of positive frequency-dependent selection favoring local warning signals, our results are indicative of a more complex dynamic between predators and warning-coloured prey. We have also found that the attack risk of different colour morphs depends on predator community composition: yellow males are better protected when Paridae (e.g. great tits) are abundant, whereas white males have the advantage when Prunellidae (dunnocks) are abundant (Nokelainen et al. 2014).
Chemical defense in the Wood tiger moth
The wood tiger moth produces two distinct defence fluids in response to attack, one targeted towards avian and one towards invertebrate predators (see Rojas et al. 2017). We are now working to identify the chemical composition of these fluids, as well as how they vary within and between sexes, colour morphs, and populations. Our findings suggest that both smell and taste play a role in the anti-predator action of the fluids targeted towards birds. Furthermore, we have identified a pyrazine as the likely source of the warning odour, indicating that the moths are able to synthesise pyrazines de novo.
- Address: University of Jyväskylä
Department of Biological and Environmental Science
P.O. Box 35
FI-40014 University of Jyväskylä
- Phone: +358 14 2602317
- Mobile: +358 40 5372263
- Fax: +358 14 2602321
- E-mail: johanna.mappes(at)jyu.fi