The Dodo Dilemma

 

The Evolution and Extinction of the Dodo Bird (Raphus cucullatus)

Introduction

The relationship between extinction and evolution

Evolution and extinction have worked together since the beginnings of life on Earth, removing those species incapable of handling an ever-changing environment and making way for those species that can. Mutations seen in nature can aid in the creation of variants, which may prove helpful in the survival of a species. With more stress-tolerant genetic variants, there comes a higher likelihood of survival (Lindsey, Gallie, Taylor, & Kerr, 2013). Without these necessary genes, however, a species is unlikely to survive in the often unpredictable conditions present on our planet (Lindsey et al., 2013). Stress-tolerant genes can only be selected for through the removal of non-stress tolerant genes—thus, extinction comes into play. So commingled are extinction and evolution that extinction events are believed to affect the trajectory of future evolutionary processes (Lehman & Miikkulainen, 2015). This is believed to be done through the increase of evolvability—by killing off many species and individuals, only those best fit will survive (Lehman & Miikkulainen, 2015). The survivors of these previous extinction events have the genes to live through some of the most drastic environments, thus potentially making them more likely to survive future extinction events (Lehman & Miikkulainen, 2015).

Unfortunately, humans are increasing the rate of extinctions (Ceballos et al., 2015). There have been five mass extinction events over the course of life on Earth, with evidence for a sixth currently occurring (Ceballos et al. 2015). The current rates of extinction are similar to those seen in previous extinction events. If this continues, entire groups of organisms may be gone by the end of the century (McCallum, 2015). Changes are occurring too quickly for adaptations to occur, and the resultant biodiversity loss will make it harder for species to survive any further changes in environment (Ceballos et al., 2015). A variety of species have already been lost due to anthropogenic causes, one of the most famous and illustrative being the dodo bird.

The dodo bird (Raphus cucullatus)

The dodo was a large, flightlessground bird, endemic to the island of Mauritius in the Indian Ocean (Cheke, 2006). The dodo was most commonly spotted in an islet in the bay on Mauritius, called Ile aux Benitiers (Hume, 2006). Mauritius is just east of Madagascar, about 1,134 km from the coast (Figure 1). It is part of an archipelago, containing also the islands Rodrigues and Réunion (Cheke & Hume, 2008). Collectively, they are known as the Mascarene Islands, or Mascarenhas Archipelago. They are all volcanic in origin (Cheke & Hume, 2008).

Mauritius

Figure 1. The location of the Mascarene islands in reference to the island of Madagascar, located off the East coast of Africa (Hume, 2006).

 The dodo had blue-gray plumage, with a large, bald head (Cheke & Hume, 2008). They also had a 23 cm bill, ranging in color from black to a dark red at the tip. The dodo walked on short, fat yellow legs, with a tuft of blue-gray feathers in the back. With small wings hanging at their side, they were incapable of flight and thus stayed on the ground throughout their lives (Cheke & Hume, 2008). A variety of drawings done during the time that the dodo was alive are some of the primary sources for current speculation, one popular drawing being that done by Roelandt Savery in 1626 (Figure 2). Additionally, written records have been of assistance. In what is believed to be the first written record of the dodo, the explorer Heyndrick Dircksz Jolinck referred to the wings as being the size of a pigeon’s (Hume, 2006). Other information includes documentation of the dodo’s diet, which included raw fruit (Hume, 2006). Also documented were their ground-nesting habits, where one egg was laid per batch (Livezey, 1993). Ground nests were safe, due to the lack of predators on the island (Livezey, 1993).

dodo

Figure 2. A depiction of the dodo done by Roelandt Savery in 1626. The image was later referred to by Owen (1872) as a basis for the reconstruction of skeletal remains (Hume, 2006).

While all this information can be understood from remains, drawings, and written documents, there has been a great deal of controversy over its correct size and weight (Angst, Buffetaut, & Abourachid, 2011; Brassey, O’Mahoney, Kitchener, Manning, & Sellers, 2016, Louchart & Mourer-Chauviré, 2011). Many drawings done at the time of the dodo’s existence were of a large, fat bird. Calculations for dodo mass estimates have been based on the hind limb dimensions of extant relatives, leading to further support for these observations (Brassey et al., 2016, Louchart & Mourer-Chauviré, 2011). However, this method may lead to biased support of the large, fat model, due to the unique differences seen in the hind limb dimensions of the dodo (Brassey et al., 2016). Other methods, such as measurements of bones and analysis, along with whole-body computed-tomography (CT) mass estimation techniques, have yielded evidence for a much slimmer bird (10.5 kg, in comparison with the original 23 kg) (Angst et al., 2011, Brassey et al., 2016). The many images of it being fat could be due to overweight birds in captivity or the habit of puffing up feathers when feeling aggressive or threatened (Angst et al., 2011). Other possibilities for misconceptions could stem from fat and thin seasonal cycles or wrong reconstruction of skeletal remains (Hume, 2006). Owen (1872) put together two possible examples for reconstruction of skeletal remains, using the drawing done by Roelandt Savery (Figure 2) as a model. He supplied two possible constructions, one allowing for a fatter version of the dodo (Figure 3a) and the other more upright, representing a much skinnier version of the dodo (Figure 3b).

Figure 3a. The skeletal reconstruction of the dodo bird by Owen (1872), based on Roelandt Savery’s depiction done in 1626. This reconstruction allows for a fatter version of the dodo bird (Hume, 2006).

Figure 3b. The skeletal reconstruction of the dodo bird by Owen (1872), based on Roelandt Savery’s depiction done in 1626. This reconstruction allows for a slimmer version of the dodo bird (Hume, 2006).

The dodo is arguably one of the most well-known birds in human history. It is popular in today’s culture as a fat, dumb, flightless bird, recognized both for its extinction and the awareness it brought to the conservation field. There is more to this bird, though, and much to be learned from its untimely end. Its existence as an endemic, flightless species on an island brings up many questions that are still being researched. Learning more about this species and the unique evolutionary path it followed opens the door to new knowledge on evolution, extinction, endemic species, and what we can do to protect those we have left.

The dodo’s extinction

The dodo is believed to have gone extinct on the island of Mauritius by the 1640s, with any in captivity or brought to other islands extinct by 1662 (Cheke, 2006, 2014). There is, however, a great deal of controversy about these dates (Jackson, 2014). Some papers support a much later date for the dodo’s extinction, including as late as the 1690s (Hume, Martill, & Dewdney 2004, Roberts & Solow, 2003). As Cheke (2014) states, the difference of these few decades is of little importance, though can be interesting in understanding the ultimate cause of extinction.

Despite its extinction, the dodo was believed to be well-adapted to its life on Mauritius. With no predators present and food available on the ground, flight was unnecessary for these creatures. No predators also led to a trusting disposition (Cheke and Hume, 2008). However, as well adapted as the dodo was to life on Mauritius, with the sudden appearance of Dutch settlers and the predators they brought with them, their flightlessness, ground nests, and trustworthiness quickly became a liability. A variety of anthropogenic factors played a role in the dodo’s end. The Dutch settlers arrived on the island in 1598 after weeks of sea travel, hungry and surprised by the presence of these birds that did not run away from them (Cheke and Hume, 2008). They began to hunt them for food, reducing their population quickly (Hume, 2006). While this is often believed to be the primary reason for the extinction of the dodo, further research has proved otherwise. The Dutch settlers introduced a variety of predators, which the dodos were not adapted to dealing with. This included rats and pigs, who ate the eggs and young from their unprotected ground nests (Hume, 2006). Rats would have also eaten the same food as the dodos, providing competitors that the dodo was not adapted to dealing with (Hume, 2006). Additionally, the Dutch settlements and expeditions would have pushed the dodo out of its original territory, into the furthest recesses of the island (Figure 4) (Cheke, 2006).

island

Figure 4. When settlers arrived on Mauritius, they quickly altered the landscape to their needs. Many endemic species were affected by this. The dodo and a tortoise species can be seen in the top left (Hume, 2006).

Phylogeny

The origin of the dodo has been a conundrum for years. Many scientists speculated its relatives, naming it a rail, small ostrich, vulture, or albatross (Hume, Cheke, and McOran-Campbell, 2009). In the 1800s, the dodo was placed into the Columbiformes, an order which includes our many pigeons and doves (Figure 5) (Shapiro et al., 2002, Soares et al., 2016). This was done through DNA analysis of remains of the dodo (Figure 6). The Columbiformes contain over 300 species, almost ubiquitous in their presence on our planet (Pereira, Johnson, Clayton, & Baker, 2007). They are absent only from the Arctic, Antarctic, and some oceanic islands. The dodo is part of the family Raphidae, located within the Columbiformes (Shapiro et al., 2002). The Rodrigues solitaire is the only other species in the Columbiformes to join the dodo in this family (Soares et al., 2016). It is believed to be a sister species to the dodo—it is similarly endemic to the nearby island of Rodrigues, flightless, and now extinct (Shapiro et al., 2002).

Figure 5. The phylogenetic tree for the Columbiformes, containing the dodo and its sister species, the Rodrigues solitaire (Soares et al., 2016).

Figure 6. The fossilized remains of the Oxford dodo, located in the Oxford University Museum of Natural History. It contains the only known DNA samples for experimentation (Nowak-Kemp & Hume, 2017).

There has been a significant amount of controversy over the inclusion of a second solitaire in the family Raphidae. It was believed up through the 1980s that a white dodo had been seen on the island of Réunion (Hume & Cheke, 2004). This was based on a sighting by Ysbrantz Bontekoe in 1619. With no further sightings of a dodo, the belief in a white dodo on Réunion nevertheless stuck. Drawings of a white dodo further encouraged this controversy (Figure 7). By 1646, mentions of a bird dubbed the Réunion solitaire started to appear as well. Speculation took hold, and a variety of hypotheses about the existence of a white dodo and a Réunion solitaire were not distinguished until just recently, when a fossilized specimen was found and studied. It has been concluded that this bird was actually an ibis (Threskiornis solitarius), relegating it into a different family than that of the dodo and Rodrigues solitaire (Hume & Cheke, 2004).

white dodo

Figure 7. Many drawings of a white dodo surfaced, including this one, that encouraged the controversy of a white dodo being present on the island of Réunion (Hume & Cheke, 2004).

When the Columbiformes began to diverge from their common ancestor has been a topic of debate. Pereira et al. (2007) concluded that the Columbiformes radiated during the early Eocene to the mid-Miocene. However, Soares et al. (2016) concluded that the Columbiformes radiated later, during the late Oligocene, with continued radiation seen into the Miocene. The difference is a much slower and drawn-out radiation, as dictated by Pereira et al. (2007), than the relatively quicker radiation seen in the conclusions of Soares et al. (2016). As a result, the dodo and solitaire may have radiated from their common ancestor at the end of the Oligocene (Pereira et al., 2007, Shapiro et al., 2002) or sometime in the early to mid-Miocene (Soares et al., 2016).

nicobar

Figure 8. The Nicobar pigeon (Caloenas nicobarica), the most closely related species in the Columbiform order alive today (Encyclopedia of Life). 

The common ancestor of the dodo and solitaire is believed to be Caloenas nicobarica (Figure 8) (Heupink, Grouw, & Lambert, 2014; Pereira et al., 2007; Shapiro et al., 2002; Soares et al., 2016). The Nicobar pigeon has its roots in India, and today is often found on remote islands (Heupink et al., 2014). The radiation time of the dodo and solitaire from the Nicobar pigeon, be it late Oligocene or mid-Miocene, is significant in that it was before the islands of Mauritius, Rodrigues and Réunion were created (Pereira et al., 2007; Shapiro et al., 2002; Soares et al., 2016). As previously mentioned, this archipelago was created by volcanic action. Mauritius did not appear until the late Miocene, with Rodrigues not appearing until the Pleistocene (Shapiro et al., 2002). Flightlessness is believed to have been lost in the dodo’s ancestors around ten million years before they reached the island, making flight from Africa to Mauritius not possible (Shapiro et al., 2002; Soares et al., 2016). It has been suggested that island ‘stepping stones’ may have finally brought the dodo’s ancestors to Mauritius, with these islands later sinking back into the ocean (Heupink et al., 2014; Pereira et al., 2007; Shapiro et al., 2002). There are ridges present along the Mascarene Plateau that may represent these early stepping stones (Shapiro et al., 2002).

Interestingly, a connection has been found between the dodo and the spotted green pigeon (Caloenas maculata) (Heupink et al., 2014). Evidence has been provided that the spotted green pigeon is not just a variety of the nicobar pigeon, but its own species. In this way, its closest relative is the nicobar pigeon, then the dodo and solitaire. It shows the ability to fly, semi-terrestrial habitats, and a tendency to live on islands. These habits provide evidence for the stepping-stone hypothesis mentioned previously (Heupink et al., 2014).

The unique evolution of the dodo

There is little doubt that the dodo followed a very unique evolutionary trajectory. The Columbiformes are well known for their unspecialized bodies, allowing them to live in both arboreal and terrestrial conditions (Pereira et al., 2007). While their sizes and colors can vary a great deal, they are all widely recognizable (Pereira et al. 2007). The dodo is anything but unspecialized, however—their flightless, terrestrial lifestyles and large bodies speak to this. The speculation required to place the dodo into the Columbiformes further speaks to its specialization, due to the fact that most Columbiformes are easily identified. The traits that make the dodo so unusual, however, are what made it well-adapted to its life on Mauritius.

The dodo’s large body size and reduced pectoral limbs are traits seen in other bird species, as well, such as moas (Dinornithidae and Emeidae), elephant birds (Aepyornithidae), and of course the Rodrigues solitaire (Noble, 1991). This large body size and small limb condition are characteristics of gigantism (Livezey, 1993). It is often associated with increased longevity, thermoregulation, physiological changes, and longer fasting times (Livezey, 1993). Being bigger also allows for the ingestion of more food, along with being able to eat bigger food items (Noble, 1991). Fruits and stones too large for other animals become ingestible by these larger birds (Noble, 1991). These may have been evolutionary advantages selecting for the gigantism seen in the dodo bird on the island Mauritius.

Charles Darwin mentions specifically the loss of flight in island birds and insects (Darwin, 1859/2004). Throughout much of his life, Darwin followed one of Jean-Baptiste Lamarck’s theories, known as use and disuse (Burkhardt, 2013). In fact, he built upon it much more than Lamarck ever did (Burkhardt, 2013). In island communities, Darwin states that birds or insects may have lost their ability to fly because it was no longer needed (Darwin, 1859/2004). With no predators to require a quick escape, this flight mechanism may not have been important. Furthermore, in insects, flight may have just become unhelpful or even detrimental. Those who could fly and did so may have been blown to sea. This would have resulted in death, and therefore natural selection would have selected against it (Darwin, 1859/2004). With each successive generation flying less, wings may have disappeared or become useless, as seen in the dodo bird.

However, Lamarck’s theory of use and disuse no longer holds much credence in today’s scientific culture. While the idea that those individuals who could fly had a higher possibility of being blown to sea, thus allowing natural selection to act against flight, holds true to current evolutionary beliefs, there may be further explanations for the flightlessness seen in the dodo. For example, flight requires a lot of energy, especially as the size of the bird gets larger (Noble, 1991). With gigantism traits becoming common in the dodo, it may have become more practical to lose the ability to fly. McNab (1994) states that flightless rails were found to have lower basal rates than their flying relatives. This may have been due to the reduction of pectoral muscles that were previously needed for flight. McNab (1994) also states that the loss of flight in insects would lead to less expenditure of energy. Having wings requires the energy to make them and care for them (Phillips, Gibb, Crimp, & Penny, 2009). Finally, by not flying, the birds would have been consistently in the environment where they could find their food. This is in contrast with being in the air, away from food sources (Noble, 1991).

Besides its obvious size and flightlessness, the dodo bird had other characteristics that aided in its survival on Mauritius. It was loosely plumaged, a characteristic that is seen in some species of birds today, such as Columba livia (Livezey, 1993). It is believed that loose plumage helps with heat loss. In the warm environment of Mauritius, this would have been especially important (Livezey, 1993). The body proportions of the dodo, associated with gigantism and derived further in the dodo, may have also led to heat loss. It is documented that the legs are much better at thermal radiating than the wings, a possible additional explanation for the dodo’s evolutionary advantage of small wings (Livezey, 1993).

The dodo and the tambalacoque tree

There is some interesting controversy surrounding the dodo and its relationship with the tambalacoque tree (Sideroxylon grandiflorum). It was hypothesized by Temple (1977) that the dodo and the formerly named Calvaria major tree, the tambalacoque, had an obligatory mutualistic relationship. As Temple (1977) states, the tambalacoque was previously widely present on the island, even used for lumber by the settlers. However, by 1973, the population had declined to thirteen mature trees, all over 300 years old. With no young trees present on the island, and with no luck germinating the seeds in a nursery, Temple (1977) tried passing the seeds through the gastrointestinal tract of turkeys. Once passed or regurgitated, three of the seventeen seeds were able to germinate.

Temple (1977) concludes that the decline of the tambalacoque tree is directly related to the extinction of the dodo. The dodo went extinct by the mid- to late- 1600s. With the oldest tambalacoque trees being older than that, the extinction of the dodo may have had an effect on the tambalacoque’s germination rates. He believed this had to do with the thick endocarp present on the tambalacoque, making it difficult for it to germinate on its own (Figure 9). In his paper, he hypothesized that the dodo would eat the seeds from the tambalacoque tree, breaking down the tough endocarp coating in its strong gizzard. It would then pass through the dodo’s system. With the endocarp gone, the seed could then freely germinate. Without the tough endocarp, the seed would have been destroyed in the dodo’s gizzard. His research, he believed, provided evidence for a mutualistic relationship (Temple, 1977).

tambalacoque

Figure 9. The seeds of the tambalacoque tree (Sideroxylon grandiflorum). Notice the thick endocarp (Atlas Obscura).

However, Temple received a lot of backlash for his hypothesis. Witmer & Cheke (1991) came out with documentation of hundreds of trees on the island, of many age classes, after Temple’s paper was written. This provided evidence for the germination of the seeds without the need for abrasion. They attacked Temple’s scientific design specifically, saying that he provided no control in his research, such as planting the seeds without any abrasion to see if they would germinate. Additionally, they stated that Temple’s results provided evidence for the abrasion within the gizzards being too much for the endocarp to handle—seven of seventeen seeds were completely destroyed by the turkey’s gizzard.

Despite all of this, Temple’s hypothesis may have some credence. The dodo may have been helpful in dispersing the seed, even if the abrasion from the gizzard wasn’t necessary (Witmer & Cheke, 1991). Additionally, there were many other species on the island that may have contributed to the abrasion that Temple thought necessary. Iverson (1987) suggests that the testudinid tortoise, a member of the genus Cylindraspi, that inhabited Mauritius may have played a part in this abrasion. This is specifically seen in the Galapagos tortoises, where their ingestion of the Galapagos tomato and prickly pear allows for better germination (Iverson, 1987). Whatever the case, due to the amount of human effect on the island, it will be hard to ever find any true evidence of mutualistic relationships between the tambalacoque tree and other species on the island (Iverson, 1987).

Extinction as a driver for evolution

Darwin’s theories

Darwin (1859/2004) speaks about extinction regularly. He speaks of it as both a part of natural selection and an outcome of it (Raup, 1994). As variations occur and new generations show better adaptations, so they will outcompete earlier generations and be the cause of their decline. The subsequent loss in these comparatively ill-adapted features from the population will only make the species, and future variants, better adapted to their surroundings (Darwin, 1859/2004). Darwin also mentions the incredible amount of extinctions that have occurred on our planet, describing these species as fallen branches underneath the tree of life. The current fossil record has been documented to contain about 35,000 genera and 4,000 families (Raup, 1994). There is estimated to be one-quarter of these families still living today (Raup, 1994). Darwin’s fallen branches are the other two-thirds, representing those species that could not keep up with an ever-changing environment. There are many ways to study his theories surrounding the importance of extinction, one such way being on islands.

Island biogeography

Islands have long been revered by scientists as a sort of controlled field experiment. With little intrusion from outside factors, evolution can be observed at its finest in these mostly isolated conditions. Extreme evolutionary trends, such as that seen in the dodo, become the ideal candidates for study for a better understanding of evolution’s workings. A great deal of our current knowledge on evolutionary trends comes from previous work on islands (Pâslaru, 2014). One of the most prominent theories related to islands seen in ecology and evolution today is that of island biogeography. This theory, developed by E. O. Wilson and Robert MacArthur, refers to how species get to islands and how they evolve once there (Pâslaru, 2014).

There are three main characteristics to this theory. That relating to extinction is the third characteristic, referring to species turnover. It states that immigrations and extinctions are common phenomena in island communities (Pâslaru, 2014). In species turnover, immigrations and extinction work together to create a sort of equilibrium in species diversity (Pâslaru, 2014). With more species immigrating to an island, more species will go extinct. With less immigration to the island, less will go extinct (Figure 10). As such, extinction plays a large role in shaping the species present on an island, as well as the genes that will be present in future populations. This process has also been known to create a great deal of endemic species, such as the dodo, as well as very fragile ecosystems (Simberloff, 1974). This theory is apparent in the story of the dodo, where island biogeography would have aided in creating the conditions and biodiversity on Mauritius and the unique evolution of the dodo.

island biogeography

Figure 10. As more species immigrate to an island, more species will go extinct. With fewer species immigrating to an island, fewer will go extinct. In such a way, an equilibrium can be reached (Pâslar, 2014).

Extreme evolution

As previously mentioned, the dodo is an example of extreme evolution. Its inability to fly, ground nesting and feeding habits, and trustworthiness would have been ill suited to most other locations. However, with Mauritius being devoid of predators and sufficient in food on the ground, there was little need to waste energy or stress on such things as flying or getting away from predators. In fact, their innate trustworthiness may have led to a curiosity that, in many ways, was beneficial. Studying these unique forms of evolution can give us insight into the more common forms of everyday evolution. One such example is seen in Darwin’s finches, located on the Galapagos Islands (Grant & Grant, 2002). Through studying two species of these finches, the medium ground finch (Geospiza fortis) and cactus finch (Geospiza scandens), over the course of 29 years the researchers were able to identify obvious patterns of evolution. Through interpretation of their data, they got insight into hybridization and natural selection, as well as identified the effects of rare events on species (Grant & Grant, 2002). The evolutionary track of Darwin’s finches are well known. Their common ancestor arrived on the Galapagos Islands, only to radiate into the variety of species that are seen today. The island conditions are believed to be one of the biggest reasons for the evolutionary track they followed. This is similar to the dodo, where living on an island played a large role in its evolution.

As with the Galapagos finches, there is much to be learned from the dodo’s evolutionary history. Besides gaining knowledge having its own merits, studying the extreme evolution seen in the dodo can give us better knowledge of gigantism, island hopping, and the Columbiformes. Additionally, studying the causes for the dodo’s end can give us a better understanding of extinction and its consequences, something that has been touched upon in the tambalacoque mutualism hypothesis. It can also raise awareness of conservation in these areas, and what we can do to prevent further anthropogenic-caused extinctions. Finally, as with all island systems, studying its inhabitants can give us a better understanding of island biogeography theory.

Conclusion

Over years of study, a great deal has been discovered about the dodo bird and its home island of Mauritius. However, these discoveries have not been possible without some amount of speculation. This is understandable, given the often mutually exclusive information that has surfaced from the time period. Lack of significant evidence and a subsequent reliance on hearsay and anecdotal accounts provide further frustration for researchers. Despite all of this, research on the dodo bird is very important. The unique evolutionary processes that led to the dodo as we know it are important in understanding many other aspects of evolution. These include the aforementioned, such as extinction’s effects on evolution, island biogeography, gigantism, and how island inhabitants originally reached their new home. Additionally, by understanding the dodo, we can further fill in the Columbiform phylogenetic tree and have a better understanding of those extant species today.

Finally, learning more about the dodo also gives us a better idea about the effects that we are having on our planet and its flora and fauna. By understanding our effects, and how to limit them, there will be fewer future instances similar to that of the dodo. On the island of Mauritius, the loss of the dodo and other endemic species has raised conservation awareness. Indeed, hands-on approaches have become popular on the island as it became more apparent to conservationists that simply leaving the forests alone would not suffice to protect it (Cheke & Hume, 2008). Projects have been worked on by international conservation organizations, with focuses on reversing habitat destruction, invasive species removal, and species management. Conservation management areas on the island have seen success, though keeping the invasive species out is an ongoing battle (Cheke & Hume, 2008). However, the research and successful conservation strategies learned on the island can be helpful in restoring other islands and management areas around the world. By learning from our past mistakes, we can prevent the situations see on Mauritius. By implementing conservation strategies, we can ensure the continued presence of evolutionarily unique creatures, such as the dodo, for years to come.

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