On dispersal, gene flow, population divergence and learning in insects

                                                                            








One consequence of animal movement and dispersal is gene flow between populations. Gene flow is generally thought to limit local adaptation and population divergence, since local selection is opposed by the homogeneising effects of gene flow. In spite of this, we often find that populations differ in morphological, physiological and behavioural traits, and one way why such population divergence can be achieved is through adaptive phenotypic plasticity. In terms of behavioural traits, it is increasingly being recognized that learning can be important, particularly when it comes to population divergence of mate preferences.



We have studied the effects of learning on the development of female mate preferences in a charismatic insect species: the banded demoiselle (Calopteryx splendens). Males of different species in these calopotergid damselflies are well-known for their enigmatic melanized wing patches, which serve multiple ecological functions, including mate recognition and species recognition for females.

Recently, we have showed that female mate preferences are not entirely genetic, but are partly learned, and develops in females as a result of physical pre-mating interactions and/or during mating with males. The result of this learning is that populations that are even close to each other and hence experience a lot of gene flow in between them, can diverge substantially in mate preferences, due to such learning. It is interesting that even these small insects have such advanced cognitive ability so that they can actually learn whom to mate with, and who is the wrong mate! Our paper has recently been published in the journal Evolution, where we also contributed with the cover photo of a male banded demoiselle aggressively defending his valuable territory (a water lilly). Our article was also covered by the popular science site and media outlet Science Daily.

Below is the link to the article and the abstract:


A ROLE FOR LEARNING IN POPULATION DIVERGENCE OF MATE PREFERENCES  


Erik I. Svensson, Fabrice Eroukhmanoff, Kristina Karlsson, Anna Runemark & Anders Brodin

Article first published online: 13 JUL 2010

DOI: 10.1111/j.1558-5646.2010.01085.x

Learning and other forms of phenotypic plasticity have been suggested to enhance population divergence. Mate preferences can develop by learning, and species recognition might not be entirely genetic. We present data on female mate preferences of the banded demoiselle (Calopteryx splendens) that suggest a role for learning in population divergence and species recognition. Populations of this species are either allopatric or sympatric with a phenotypically similar congener (C. virgo). These two species differ mainly in the amount of wing melanization in males, and wing patches thus mediate sexual isolation. In sympatry, sexually experienced females discriminate against large melanin wing patches in heterospecific males. In contrast, in allopatric populations within the same geographic region, females show positive (“open-ended”) preferences for such large wing patches. Virgin C. splendens females do not discriminate against heterospecific males. Moreover, physical exposure experiments of such virgin females to con- or hetero-specific males significantly influences their subsequent mate preferences. Species recognition is thus not entirely genetic and it is partly influenced by interactions with mates. Learning causes pronounced population divergence in mate preferences between these weakly genetically differentiated populations, and results in a highly divergent pattern of species recognition at a small geographic scale.

Tracking small things with smaller things

A major problem when studying behavior and migration of small organisms is that many questions, easily addressed for larger animals such as birds or fish, cannot be asked when it comes to animals of millimeter size, since tracking devices are too heavy to allow for the organism to act naturally. Recent advances in nanotechnology have, however, made it possible to individually track small animals. In a paper recently published in PloS One (Lard et al. 2010 PLoS ONE 5(10): e13516. doi:10.1371/journal.pone.0013516) we report on a novel approach to track movements and migratory behavior of millimeter sized aquatic animals, using nanometer sized fluorescent probes (quantum dots). Compared to previously used methods to label small animals, the nano-labeling method presented here offers considerable improvements. The method, developed in close cooperation between biologists, chemists and physicists, offers new opportunities to routinely study zooplankton responses to e.g. light, food and predation, i.e. opening up for advancements within research areas such as diel vertical/horizontal migration, partial migration and other differences in intra- and interspecific movements and migration. So, although our results are preliminary and the method still rough, CanMove is advancing the knowledge and opportunities also with respect to small organisms!

//Lars-Anders Hansson

CAnMove seminar on how ecological barriers are shaping migratory strategies of raptors

At this week´s CAnMove seminar, Ugo Mellone was giving a talk about “Ecological barriers shaping the migratory strategies of raptors”. Ugo Mellone is currently doing his PhD at the University of Alacante in Spain but will stay in Sweden until January, working with Thomas Alerstam and Raymond Klaassen.


Two examples of the influence of ecological barriers on migratory strategies have been shown: the Eleonora’s falcon breeds in Mediterranean islands and reach the wintering grounds, located in Madagascar, in a 9’000 km journey. These birds migrate also during night, especially when crossing the Sahara desert, in order to overcome this ecological barrier as soon as possible. Besides this, Eleonora’s falcons are able to cross large water bodies like the Indian Ocean, flying non-stop also for 1’500 km and changing route accordng to weather conditions. Conversely, the Short-toed eagle is a broadwing raptor less adapted to flapping flight and therefore unable to cross large stretches of sea. The migratory strategies of the populations breeding in peninsular Italy are deeply influenced by the geography of the Mediterranean basin.

You can follow the journey of two eagles tagged with satellite transmitter from this website, where the maps are constantly updated.















Some papers concerning these researches can be downloaded from this website.


//Ugo

Migration as an anti-predator strategy in fish

Common bream (Abramis brama)

Animal migration can take many different forms in nature. Partial migration, where not all of the animals in a population migrate, is extremely widespread and occurs in many species of birds, mammals and fish. Yet despite its prevalence in nature, what drives differences in migratory behaviour between individuals is a mystery that continues to puzzle biologists. Why do some individuals stay and others go?

A collaborative team of researchers from the Danish Technical University and CAnMove at Lund University, led by Dr. Christian Skov, recently investigated this question in a species of partially migratory freshwater fish, the common bream (reported in Proc. R. Soc. B.). Common bream stay in lake habitats over the summer and part of the population migration into streams over the winter. In the lakes during the summer the risk of predation is high, but the rewards are also high (high food availablity). During the winter predation risk decreases as predators become less active and eat less, but food availability also decreases, reducing the benefit of staying in the lake. If the risk/reward ratio for residency in the lake differs over the winter between individuals (when the rewards for staying are relatively low), this may lead to partial migration. One way in which individuals differ that would affect this risk/reward ratio is in their vulnerability to predation. Hence, all else being equal, one would predict that fish that are more susceptible to predators would be more likely to migrate, and that differences in vulnerability between individuals might explain why some migrate and some stay resident.

To test this prediction Christian Skov and his team were able to assign >450 individual bream from 2 lakes in Denmark with a 'predation vulnerability' score, which they calculated by sampling large numbers of piscivorous predators (pike) from each lake. The vulnerability score was calculated by estimating what proportion of predatory pike in a lake could eat each specific bream (as pike are gape limited predators). The bream were individually tagged and released and their migratory behaviour monitored over the winter. As predicted, fish with a high vulnerability score showed a much stronger propensity to migrate, migrated earlier and stayed for longer in the streams. This research is important as it suggests a powerful role for predation in shaping patterns of migration.

So it seems for bream the question of 'should I stay or should I go?' depends upon an individual's vulnerability to predation. For smaller, more vulnerable fish, if they go there will be trouble (after all migration is a costly and risky business), but if they stay there may be double...

//Ben

Read the full article here: http://rspb.royalsocietypublishing.org/content/early/2010/10/26/rspb.2010.2035.full

Here is the full reference:

Skov C, Baktoft H, Brodersen J, Bronmark C, Chapman BB, Hanson L-A, Nilsson PA. 2010. Sizing up your enemy: Individual predation vulnerability predicts migratory probability. Proc. R. Soc. B. (in press)

Thanks to Henrik Baktoft for the bream photograph.

Bird populations are doing gradually better the further away they are from the hottest part of their distribution range, or...

... Success in the north, trouble in the south

Climate change is affecting our fauna in various ways, by influencing distribution range, population size, and migration and breeding phenology. For example, recent studies have shown that European bird species living in a colder climate (that is, generally more northern bird species), are doing relatively poorly compared to species used to a warmer climate (more southern species).

As far as distribution goes, shifts at the poleward limit of the distributional range, or at the upper edge of the altitudinal range, have been documented for many taxa. But beyond changes at range limits, more subtle changes within the ranges of species are also likely, and might have important ecological and evolutionary consequences.

In a French–Dutch–Swedish joint effort we studied how the different populations of 62 common European bird species have developed over 20 years (1989–2008), as recorded in national bird monitoring schemes. France and Sweden were each divided into three equally sized latitudinal belts, and the Netherlands was considered a belt of its own. The following independent factors were used when trying to explain recent population size changes within these latitudinal belts: the thermal distance (in °C) to the thermal maximum of their European distribution (how far the birds are from the warmest part of their range), habitat preference, habitat specialization, body mass, latitude and migration distance.

For the 62 species considered, we found a gradual and linear significant increase in long-term population growth rate along the thermal range, when moving towards a species coolest range limit. Accordingly, the bird populations were doing better and better the further away they were from their thermal maximum (put in another way, the closer they were to the coolest part of their range). This effect was detected beyond other effects expected to affect population growth rates, such as the decline of farmland birds and habitat specialists.

We failed to highlight that long-distance migrants were more prone to decline, as has been shown in several other studies, probably because the studied set of species was restricted, excluding some Afro-Palaearctic migrants that do not occur in all the three studied countries. Also, recent declines in long-distance migrants are most severe in seasonal habitats in Western Europe, whereas we studied a broader array of habitats and countries.

Thus, beyond previously known effects on population dynamics near range limits, we revealed that population dynamics were not randomly distributed within species range, suggesting that European breeding birds are influenced by climate warming, and are experiencing demographic disequilibrium, along their whole thermal range.

Jiguet, F., Devictor, V., Ottvall, R., van Turnhout, C., van der Jeugt, H. & Lindström, Å. 2010. Bird population trends are linearly affected by climate change along species thermal ranges. – Proc. R. Soc. Lond. B 277: 3601–3608. Read the full article HERE.

// Åke Lindström