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Nature’s Most Intriguing Phenomenon: The Evolution of Feathers

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Published: 23rd Sep 2019 in Biology

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Nature’s Most Intriguing Phenomenon: The Evolution of Feathers

Introduction

Feathers are one of nature’s most complex and confounding features in the animal kingdom. They are appendages solely exclusive to that of birds. They provide numerous functions to birds unlike any integuments found on other animals. Feathers are the gateway for scientists to comprehend the events that resulted in the inception of flying birds. While there has been much research on the topic of feather evolution, the pressing question that still remains a mystery why feathers evolved. There has been an ongoing debate in the science realm regarding the answer to this question. Scientists have developed many hypotheses over the years in an effort to understand the feather’s primordial function. One of the first claims was that feathers evolved simply for the purpose of flight (Heilmann 1926, Bock 2000). It makes sense that this became an initial belief because most birds fly and their feathers make that action a reality. Other hypotheses have induced that feathers evolved for insulation and sun-shading (Regal 1975), camouflage and display (Cowen and Lipps 2000), and lastly water repellency (Dyck 1985). Through research on multiple articles related to the topic of feather evolution, one reason appears most plausible. Feathers evolved initially for the purpose of insulation and thermoregulation. This fascinating and complex integument arose in dinosaurs as a result of the selective pressures of climate change.

Background

This section will provide an overview of major discoveries in feather evolution and how that has contributed to science’s understanding of the feather’s origin and primordial function.

Scientists have studied and researched the origin of feathers for over a century. However, the year after Charles Darwin published his ground-breaking scientific literary piece, On the Origin of Species, paleontologist found a feather and (later) skeletal remains of a creature, dating back to 150 million years ago. The structure of the animal reflected both bird and dinosaur-like traits. From Archaeopteryx’s skeletal anatomy, it brought up many questions regarding feather and bird evolution (Padian & Chaippe 1998). Initially feathers were only thought to originate in that of birds. Through to the discovery of Archaeopteryx and later multiple feathered dinosaur fossil records from Northeastern China (including Sinosauropteryx and Caudipteryx) most people within the science sphere have now deduced and accepted that feathers evolved first in that of theropod dinosaurs, before birds (Zhou 2014, Prum & Bush 2003).  This clade of bipedal carnivorous saurischian dinosaurs include popular names such as Allosaurus, Tyrannosaurus rex, and Velociraptors. The feathers also found on these dinosaurs conclude that feathers initially arose for other functions preceding flight (Prum & Bush 2003). Despite all these findings that helped understand feather evolution, the question of why feathers evolved is still a “toss up”.

Figure 1: The figure above displays the hypothesized stages it took for the achievement of flight in dinosaurs, from theropod dinosaurs to modern birds (Padian & Chiappe 1998). It also supports the notion that flight in feathers was a secondary adaptation (Padian & Chiappe 1998).

 

Body

This section will defend the theory of insulation and thermal regulation through evidence from various scientific peer-reviewed journals as well as refute each challenging hypothesis listed in the introduction.

Evidence for Primordial Function of Insulation and Thermoregulation in Feathers

The theory of insulation and thermoregulation being feathers’ primordial function appears to be the most reasonable answer for why feathers evolved. Developing an integument for insulation is a trend that is consistently selected by evolution. It is not surprising that theropod dinosaurs developed such an appendage. What makes feathers different than any other coverings found on animals is its branching ability that allows for its structural diversity (Prum 1999). As with the case for all birds, many considered theropod ancestors to have had a similar endothermic metabolism (Fricke & Rogers 2000). Through examining the isotope ratios of oxygen in fossil remains, scientists have been able conclude this (Fricke & Rogers 2000). With that metabolism must arise a mechanism that retains heat. Hence why the need for the feather arose. Feathers provide better heat retention and warmth compared to scales (Stettenheim 2000). The feathers retain heat by trapping the air molecules (Stettenheim 2000). As concluded by Regal (1975), feathers arose from elongated scales in efforts to act as small sun radiation shields. Regal (1975) also was the first to publish that once the scales elongated and subdivided to feathers it would have provided an insulative purpose. While Regal may be the first to bring the theory of insulation and thermoregulation to life, many scientific scholars consider most of his conclusions outdated and false. Regal (1975) believed that endothermy in birds evolved hand in hand with flight. However, through research and evidence from Fricke and Rogers (2000), endothermy was in flightless feathered theropod dinosaurs long before birds and flight. Prum and Bush view Regal’s belief that feathers evolved from elongated scales as false. Feathers are tubular in structure and in development; the flatness of pennaceous feather derives from the unfolding of the cylindrical sheath (Prum & Bush 2003). The structure of the feather provides a perfect segue into noting a rather new field of science that has contributed to a better understanding of feather origin, evolutionary developmental biology. Evolutionary developmental biology or “evo-devo” is a branch of biology that deals with analyzing the way a current structure or organism develops to infer how that structure or organism initially evolved (Prum & Bush 2003). According to Prum and Bush (2003), feathers evolved through a sequence of phases. The unique traits of feathers evolved over time through this sequence and each stage is essential for the next one to occur (Prum & Bush 2003). A depiction of the various developmental stages of a feather appear in the figure on the next page.

Figure 2: The figure shows the 5 major subsequent stages a feather goes through that provides insight into how it initially evolved (Prum & Bush 2003). Modern birds show evidence of each stage of feather development (Prum & Bush 2003).

          Large Feathered Dinosaur from Lower Cretaceous Found in China

In relations to the study of “evo-devo” for feathers, paleontologists found the first documented report of a huge feathered tyrannosauroid by the name of Yutyrannus huali gen. et sp. nov. in the Yixian Formation in China (Xu, Wang, Zhang, et.al 2012). Tyrannosaurids are one of the oldest theropod groups dating back to the Middle Jurassic to the end of the Cretaceous; this finding provides great understanding of feather evolution (Xu, Wang, Zhang, et.al 2012). All 3 of the Yutyrannus huali specimens found have preserved filamentous integumentary structures (Xu, Wang, Zhang, et.al 2012). Research from scientists implies that this tyrannosaurid had a lot of these feathered structures in life and it was primarily insulative in function (Xu, Wang, Zhang, et.al 2012). These feathers appeared to arise as a result of the cold climate noted in China during the Cretaceous (Xu, Wang, Zhang, et.al 2012).

          Advanced Feather use of Archaeopteryx

Paleontologists know through fossil finds, that feathers doused the whole bodyof Archaeopteryx (Foth, Tischlinger, Rauhut 2014). Most of the feathers covering this ancient bird were symmetrical, (indicating they weren’t used for flight) and later experienced and exaptation to have flight functions (Foth, Tuschlinger, Rauhut 2014). The tail of the animal contained the only asymmetrical (flight-capable) feathers, once again indicating that this bird did not fly (Foth, Tuschlinger, Rauhut 2014). As proposed by John Ostrom (1974), he thought these contour feathers arose for regulating heat loss and gain. The primordial function of insulation and thermoregulation seem to be the only logical explanation for the presence of these modern structured feathers (Ostrom 1974).

          Feather Stages of “Evo-Devo” Represented in ‘Yutyrannus huali’ and ‘Archaeopteryx’ and How that Supports the Insulation and Thermoregulation Theory.

Based upon the description of these filamentous structures (found on Yutyrannus huali) by Xu, Wang, Zhang et.al, they seem to reflect that of Stage 1 or 2 in the evolutionary development of feathers (from Prum). As also noted by Griffiths (1998), the unbranched single filamentous structures are more likely to have originally evolved for insulation than branched feathers. The great feather preservation of Archaeopteryx shows the plumage to reflect that of Stage 4 and 5, which Prum and Bush consider to be the most advanced and complex feathers in development. Using the study of feather evolutionary developmental biology as a template, despite the difference in feather structures of Yutyrannus huali (Stage 1 and 2)and Archaeopteryx (Stage 4 and 5) both possess the same function for insulation. Stage 1 in evo-devo reflects the earliest evolutionary phase of feathers (Prum & Bush 2003). Regardless of the diversity and exaptation of the feather, each and every feather structure holds the capability to provide insulative function to the dinosaur (avian or non-avian) (Griffiths 1998). Feathers initially evolved for the purpose of insulation and thermal regulation because of the functional evidence in both primitive and modern feathers of dinosaurs. This case can’t apply to the other hypothesized primordial functions of feathers.

Refutation of Alternative Hypotheses

          Debunking Flight Theory

Considered to be one of the earliest hypotheses for why feathers evolved, flight theory has always remained a concludable belief by many naïve and uniformed onlookers and scientists. It is understandable why early 20th century scientists sided with this claim. When people think of feathers they automatically think of flight. Feathers are the only integuments to belong to one group of animals, birds. Most modern birds have the ability to fly. Through this subsequent thought process and some research, this theory most likely arose. The belief is that birds evolved from non-flying bipedal arboreal animals that jumped from tree to tree and developed contour feathers that over time changed to flight feathers capable of flight (Bock 2000). The claim, initially championed by Heilmann (1926) also states that these contour and flight feathers evolved from elongated scales along the forelimbs and hindlimbs of the arboreal animals (Bock 2000). Despite these assertions they are many fallacies. As a result of numerous fossil findings and more advanced research from the later 20th century, paleontologists have governed that birds descended from a group of ground-based dinosaurs known as theropods (Padian & Chiappe 1998). Stated earlier in this research paper, a diversity of early feathers appear in theropod fossils, concluding that feathers originated in this dinosaur group before the origin of birds or flight (Prum & Bush 2003). In addition, the belief by many flight theorists that feathers evolved from an elongation of scales is biologically incorrect as noted by Prum and Bush (2003). Feathers are tubular in structure, unraveling and branching from filamentous cylindrical composition (Prum & Bush 2003). Scales on the other hand are flat in anatomy and elongate in the same way (Prum & Bush 2003). Lastly, the belief of a bipedal non-flying arboreal ancestor is also incorrect to assume by flight theorist. Declared by Cowen and Lipps (1982), in modern times, there a no non-flying bipedal tree living creatures causing the assumption that the exaptation of bipedality in trees was either unsuccessful or never happened.  

           Debunking Display/Camouflage Theory

 Another hypothesis that seems to be the closest competitor to the insulation and thermal regulation theory is the proposition of display and camouflage being the primordial function for feathers. The theory declares that feathers evolved for some form of display whether that be for sexual selection, intraspecific competition (for food or hierarchy), or for protection through camouflage (Cowen & Lipps 2000). All the examples of display presented by Cowen and Lipps, are very credible and evident in modern birds today. As a result of the forms of display seen in birds today (including peacocks, owls, etc) , it is reasonable to deduce that feathers had developed the same functions during the time of the theropod dinosaurs. The issue at hand, however, is the failure of display theorist to include all the possible evolutionary stages and structures of feathers. The evolutionary development of feathers, highlighted by Prum and Bush (2003), note that there are 5 stages a feather develops and evolves through. Every stage has a different structure and capability (Prum & Bush 2003). Display theorists seem to only focus on the advanced and more modern feathers in relations to sexual selection and competition instead of the primitive feathers. Cowen and Lipps (2000) attest that down feathers only purpose is for insulation and thermoregulation. While that statement may be true, when analyzing the subsequent phases of feather evolution and development, down feathers fall with the initial stages. Stage 1 begins with a filamentous cylindrical fiber reflecting that of hair (Prum & Bush 2003). Stage 2 reflects that of a down feather without barbules (Prum & Bush 2003). Stage 3 consists of two structures: a modern down feather with barbules and the initial pennaceous feather (Prum & Bush 2003). Stages 4 and 5, observed in Archaeopteryx and more modern birds (Prum & Bush 2003), seem to be the only ones addressed by Cowen and Lipps. They completely ignore the evolutionary process of the feather, in which down feathers proceeds any pennaceous feather, implying that the display function is not the feather’s initial cause of origin. Display theorists support of camouflage as a primordial function is also problematic. Understanding how dinosaurs experienced a structural change in integument from reptilian scales to the first feathers is a process that still requires more research (Bock 2000). In modern reptiles it is evident how many have the ability to alter their skin color to blend in with their environment. Reptiles such as geckos and chameleons use camouflage. Since many reptiles have the ability to alter their skin color, the assumption made is that dinosaurs were able to do the same thing before developing feathers. Hence, it brings up the question of the feather’s primordial function since the use of camouflage seemed to already appear selected in dinosaur scales . 

     -      Debunking Water Repellency Theory

 The last hypothesis relates to the idea that the porous structure of a pennaceous feather indicates that the feather initially evolved for water repellency (Dyck 1985).  The surface of a porous integument resist water better than an impermeable surface (Dyck 1985). He also suggests that feathers may have evolved from reptiles settling off the coast of a sea or ocean (Dyck 1985).  While there are some cases of modern birds with water proof feathers, that evidence alone appears to be the only potent support for this theory. Dyck fails to include the primitive stages of feather evolution in his hypothesis like the proponents of the display theory. He only incorporates pennaceous feathers into his observation which questions whether water-repellency was the initial cause of feather development. In addition, not all feathers in modern birds are water repellent (Bock 2000). Some birds (like some Pelecaniformes) that are even around water all the time are not water proof. This brings up the question of whether the feather itself is even responsible for water repellency. As described by Eric Fabricius (1956), the water repellency of plumage arises from a secretion deriving in the oil glands of a bird. The secretion covers all over the feathers (Fabricius 1956). The feather structure itself does play some part in the repellence of water but secretion seems to have a more prominent role.

Conclusion

 The feather is easily the most fascinating, complex, and successful integument nature has to offer. Feathers are the perfect example of an exaptation because of its structural diversity that subsequently arises from its development. Initially feathers evolved for some specific purpose but over time developed the many suitable functions shown in birds today. It is clear that feathers arose primarily to provide insulation and thermoregulation to theropod dinosaurs. Since theropod dinosaurs were endothermic, it was essential that they retain their body heat. As a result of the selective pressures of unpredictable weather, the need for an additional appendage to provide insulation and heat-retention arose in these dinosaurs. Through the 5 major stages of evolutionary developmental biology (evo-devo), proposed by Prum and Bush, the role of insulation and thermoregulation applies in all. The discovery of thin filamentous cylindrical on Yutyrannus huali show parallels to the most primitive stages of feather evolution in evo-devo (Stages 1 and 2). The extensive plumage covering Archaeopteryx reflect that of advanced feathers depicted in Stages 4 and 5. Despite the difference in structure, the feathers on both creatures serve the primordial role of insulation and thermoregulation. Competing hypotheses for feather evolution also are easily disproven. Flight theory’s primary fallacy is that there is substantial fossil evidence of feathered theropod dinosaurs that were not geared toward flight. Display theory and Water repellency theory both do not account for the primitive feathers within evo-devo’s stages. Both theories only elaborate on advanced modern pennaceous feathers, indicating that both functions served as secondary adaptations. As a result of valuable evidence and thorough refutations, the theory for insulation and thermoregulation appears to be the most reasonable explanation for why feathers evolved.

References

         Foth, C., Helmut, T., Rauhut. O.W.M . (2014, July 3). New specimen of Archaeopteryx provides insights into the evolution of pennaceous feathers. Retrieved from https://www.nature.com/articles/nature13467#abstract

         Fricke, H.C., Rogers, R.R. (2000). Multiple taxon–multiple locality approach to providing oxygen isotope evidence for warm-blooded theropod dinosaurs. Retrieved from https://pubs.geoscienceworld.org/sgf/geology/article/28/9/799/188886

         Ostrom, J.H. (1974, March). Archaeopteryx and the Origin of Flight. Retrieved from https://www.journals.uchicago.edu/doi/abs/10.1086/407902

         Regal, P.J. (1975, March). The Evolutionary Origin of Feathers. Retrieved from https://www.journals.uchicago.edu/doi/abs/10.1086/408299

         Stettenheim, P.R. (2000, August 1). The Integumentary Morphology of Modern Birds—An Overview. Retrieved from https://academic.oup.com/icb/article/40/4/461/101379

         Xu, X., Wang, K., Zhang, K., Ma, Q., Xing, L., Sullivan, C.,…Wang, S. (2012, April 17). A gigantic feathered dinosaur from the Lower Cretaceous of China. Retrieved from https://web.archive.org/web/20120417134949/http:/www.xinglida.net/pdf/Xu_et_al_20                             12_Yutyrannus.pdf

         Zhou, Z. (2014). Dinosaur Evolution: Feathers Up for Selection. Retrieved from https://www.sciencedirect.com/science/article/pii/S0960982214008434

 

 

 

 

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