Monarch Butterflies Wing Morphology and Migratory Behaviour
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SUMMARY: This study aims to evaluate the correlation between wing morphology of monarch butterflies and their migratory behaviours which are classified by Altizer et al. (2010) in three categories according to the distance travelled by monarchs during their annual migration (long-distance migration, short-distance migration, and nonmigratory monarchs). By collecting data on these adaptations, Altizer et al. (2010) aimed to show that there is an evolutionary response to each kind of migratory strategy, and aimed to better understand the migratory behaviours of monarchs so that they can be more efficiently protected (Lyons et al., 2012).
Related article in Evolution: Altizer S., Davis A.K. (2010). Populations of Monarch butterflies with different migratory behaviors show divergence in wing morphology. Evolution. 64(4):1018-28.
Migratory behaviour has led to some morphological adaptations in animals, in order to increase flight performances. Altizer et al. (2010) focused their research on the morphological adaptations of several populations of Monarch, according to their different migration strategies. Altizer et al. (2010) identified three different migration strategies. The first one is the long-distance migration. The second one is the short-distance migration, and the last one is the nonmigratory status.
Studies focusing on insects and birds have previously shown that the size and the shape of the wings as well as the ratio between body mass and wing area (Dingle et al., 1980; Hedenstrom et al., 1992) are the main features which permit the animals to adapt their bodies according to the distance they have to cover while they migrate (Calmaestra et al., 2001). Because migratory pathways are not the same between each species of butterflies, Altizer et al. (2010) have hypothesized that a specialisation may have occurred. Besides, according to Lyons et al. (2012), the fact that monarchs have different migratory pathways and destinations leads to less genetic mixing, which has for consequence to increase genetic divergence which can cause speciation. The wide distribution of the Danaus plexippus L. (monarch butterflies) and their important differences in their migratory behaviour make them a good system to analyse the variation in flight morphology in comparison to the geographic location to which they belong. Altizer et al. (2010) assumed that long-distance migratory populations of monarchs had larger and longer forewings because previous study conducted by Dudley (2000) has shown that wing size is positively correlated to more efficient flight by reducing wingtip-induced drag, which is a downforce causing by air resistance.
Study system: wild monarchs and captive-reared monarchs
Altizer et al. (2010) conducted their study in order to examine variation in flight morphology between different populations of migratory monarchs and nonmigratory monarchs. The butterflies have been collected from six different populations: the migratory monarchs came from eastern and western North America, and the nonmigratory monarchs came from South Florida, Puerto Rico, Costa Rica, and Hawaii. Altizer et al. (2010) have chosen these six different populations because they show important differences in their migratory behaviour. Indeed, populations from eastern North America have a long-distance migration strategy while populations from western North America have a short-distance migration strategy. Populations from South Florida, Puerto Rico, Costa Rica and Hawaii have been chosen because they are nonmigratory populations, so their wing morphology can be compared to the wing morphology of monarchs which have a different strategy.
Previous studies (Beall et al., 1945) have been conducted on the migration of butterflies, but only a few have focused on the morphological differences between monarchs. Besides, these studies have been done before the migratory pattern of these butterflies was well known. North America butterflies have been found to have only little variations between each other (Beall et al., 1945), but some differences have been found in the wing length between North America butterflies and South America butterflies.
A total of 822 wild adult monarchs have been studied by Altizer et al. (2010). The butterflies have been collected from six different locations between 1996 and 2009. In addition to wild monarchs, 1019 captive-reared butterflies have also been collected in four different locations in North America in 1996, 1997, and 2003. Altizer et al. (2010) chose to analyse captive-reared butterflies by making the experiment with full sibling progeny reared under four different conditions in order to show the influence of genetic factors, which can lead to increase the knowledge on the evolutionary change as well. The conditions under which butterflies have been reared differed by the kind of food given. Altizer et al. (2010) wanted to see if the change of food impacted the development of the wings. This theory has then be tested by Johnson et al. (2014). They conducted an experiment to test the effects of food deprivation in the larval stage. Then, they measured multiple traits of adult wing morphology of monarch butterflies. Their conclusion was that the kind of food given to butterflies impacts their wing morphology.
Altizer et al. (2010) have differentiated males and females for their analyses because it is known that males butterflies are slightly larger than females (Braby, 2000).
Avoiding errors by identify heavily infected butterflies
Because wild populations are often infected by a protozoan (Ophryocytis elektroscirrha), a selection has had to be done. Indeed, it has been demonstrated that the infection by this protozoan can affect the body mass of the butterflies as well as the wing area (Altizer et al. 1999, de Roode et al. 2000). A previous study conducted by Altizer et al. (2000) has described a method to examine butterflies for parasite loads. Small pieces of ScotchTM tape have been put on the ventral abdomen of the butterflies. Thus, scales have been collected from the monarchs and analysed with a high power microscope to identify spores, which appear in dark brown colour. Altizer et al. (2010) used this technique in their experiment. They only kept the butterflies which were not heavily infected, using a score described by Altizer et al. (2000).
Altizer et al. (2010) have shown that the wing area is a result of two components, the forewing and the hindwing. When butterflies are flying, most of the hindwing area is covered by the forewing. For this reason, the characteristics measured on the butterflies have been focused on the forewing. The characteristics measured include the size (length of the bigger axis, width of the longest part), the shape, and the area (by measuring the perimeter) of the forewing. Then, these measures have led to two ratios: the aspect ratio which is calculated by dividing the length by the width, and the roundness, which takes into account the perimeter of the forewing and its area. To complete the analysis, right forewings have been cut off and the dorsal sides have been scanned.
Using principal component analysis to reduce the observed variables into a smaller number of artificial variables
Because a lot of characteristics have been examined, Altizer et al. (2010) used a mathematical method called “principal component analysis” (PCA), described first by Pearson (1901). This statistical procedure is useful to transform correlated observations into a set of uncorrelated linearly variables by using an orthogonal matrix (Shlens, 2003). This technique has the advantage to reduce dimensionality of the data, while the loss of informations is very low. This procedure had led to two values: the PC-size which gives an indication about the forewing area, the length and the width, and the PC-shape which gives indication about the aspect ratio and the roundness. Consequently, the analysis is simplified: high values of PC-size indicate that butterflies have large forewings, and high values of PC-shape indicates that butterflies have elongated wings.
Connections to Physics Concepts
This article relies on the fact that morphology adaptations are a response to Physics issues. It assumed that the bigger the body, the less important the boundary layer interactions. As described by Dudley (2000), the term “boundary layer” described the interactions between the body and the surrounding fluid.
Another physical principle that is discussed in this article is the thrust, produced by wing flapping. Thrust helps to reduce wingtip-induced drag and relies on muscular force. Then, Altizer et al. (2010) expected bigger wings in butterflies which use powered-flight, and smaller body size in butterflies which use gliding flight (flight without using thrust). Altizer et al. (2010) wanted to show that the shape of the wings is an evolutionary response to the constraint of the boundary layer.
Their study has led to some other studies about the evolutionary adaptations of the wings, like the structure of the scales which is assumed to decrease drag and increase thrust and lift during flapping and glided flight (Demko et al., 2012).
Experimental results and further studies
The hypothesis assumed by Altizer et al. (2010) that long-distance migratory populations of monarchs had larger and longer forewings than short-distance monarchs and nonmigratory monarchs had been proven by this study. In addition, Altizer et al. (2010) shown that the roundness of the wings were more important in nonmigratory populations than the wings of migratory populations. Besides, their results have allow them to make the assumption that the big size of the migratory butterflies allow them to stock energy (lipids) which can increase their survival.
Altizer et al. (2010) said that further studies could be conducted in order to see if other selective forces than distance travelled could influence variation in wing morphology. For instance, Davis (2009) hypothesised that forewing colors may allow for greater ability to fly. Butterfly forewing coloration could provide an adaptive advantage for flight, because more heavily pigmented wings absorb more solar radiation and this allows ectothermic butterflies to fly at low temperatures (Watt, 1969; Guppy, 1986).
Criticisms of the study
Hanley et al. (2013) reported a possible bias in the study conducted by Altizer et al. (2010) about the measure of the roundness of the wings. Altizer et al. (2010) used circularity to describe the roundness of the wings, but according to Hanley et al. (2013), they should have use in addition to the circularity descriptor the roundness descriptor . Indeed, the formula used by Altizer et al. (2010) can be greatly impacted by wing wear (Foster et al., 2011). Since wild monarchs vary in wing wear, circularity and roundness have to be related to wing wear and wing perimeter.
Questions for Further Exploration:
- To analyse the contribution of genetic factors in the differentiation of wing morphology, the authors used full-sibling family from multiple mated adult females. What could be the drawback of this method?
- For the four experiments on the captive-reared monarchs, the authors fed the larvae with different food. Why did they use this method and what did they expect?
- The authors state that one goal of this study is to increase the knowledge on the evolutionary change in wing morphology. Have they collected enough data for that? Why can it be useful to have informations about the genetic variability of monarchs for the future?
- This study investigates the relation between the distance of the flight and the morphology of the wings. What other characteristics which may have influenced the wing morphology could have been studied?
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