Paper Example on Sexual Dimorphism in Songbird`s Vocal Learning and Auditory Areas

Introduction

For decades, animals have displayed different characteristics that distinguish males and female. These have emerged in the process of adaptation to the natural environment. Sexual dimorphism refers to these differences in form of varying sexes that belong to the same species. These may include the size of the body, the color of the feathers for birds, the size of horns for some animals among others. For instance, most of the animal's males are larger than females. In addition, most of the variations in sexual dimorphism came about the due struggle of dominance. The world is a jungle where animals must struggle to survive. Therefore some of these characteristics play a vital role in asserting power among the group. Songs sang by birds also differentiate the males and female of different species. Therefore, this paper discusses the sexual dimorphism in songbird`s vocal learning and auditory areas.

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The concept of sexual dimorphism came to be as a result of natural selection. Many males in an effort to gain dominance developed attributes different from females species. This was necessary to conquer the competition. For instance, the bright color of male birds demonstrates to the female that they are fit and deserving to reproduce. However, it posed dangers to some animals as their evolution brought about additional complications. For instance, the bright colored feathers of most male birds are essential to attracting female organisms. On the other hand, the luminous color makes them more visible to predators. Thus many male birds are at risk of attack from predators than their female counterparts. However, during evolution, the male birds had no consideration of this factor since their sole agenda was to attract females. Hence, the quest for dominance in males is more appealing than their safety.

AS a result of sexual dimorphism songbirds have varying ways of learning songs depending on the species. Also, the onset of singing differs. Some learn during the first years of life while some learning is continuous. The mode of learning for different birds varies. Some learn from same-sex birds and other from both. In some instances, male and female sing in unison but sometimes only the male or the female sing alone. However, in all the species the learning process is just like the other learning process. It involves listening, internalizing, imitating while practicing, and perfecting the songs. However, the age at which birds sing depends on the species. This paper reviews the literature on vocal learning and auditory areas of different species of songbirds.

Development of Songs in Birds

Studies on how bird learn songs started as early as the 1950s with William Thorpe initiating the studies ( Wada, 2010). In a laboratory setting, he discovered that young birds that were not exposed to male developed abnormal singing. After introducing taped songs from adult birds of same species to those in the laboratory, the conditions changed. Now, those in confinement were able to develop normal singing abilities. As adults, they sang the same songs. Thorpe deduced that for effective singing, birds must learn from an early age. The mastering of the songs was possible through listening and imitating the sounds of same species birds. Thorpe experiment gave rise to many other studies that tried to explain the phenomenon of sexual dimorphism and vocal learning in birds. Since his discovery, there have been numerous varying studies on the subject. However, there is a consensus that birds learn songs from tutors. In addition, it is a natural process for male birds to sing to the female birds.

Why Birds Learn to Sing

Singing in birds is a way of adapting to the environment. Dominance is essential for survival and reproduction. Thus, songbirds use singing as a way of defending their territories and in the selection of mates. This process is learned from a young age and is perfected into adulthood. Not all sounds that birds make are songs. However, to a layman, may confuse all the humming of birds to be songs. However, the vocals are categorized into songs and calls. Calls are usually brief and signify danger. Birds look after each other. Hence, when they notice dangers especially from predators, the use their call of anguish to warn others of incoming danger. This is a language that birds understand among each other. According to Wada, (2010) birds call to warn others of impending danger.

Songs, on the other hand, are longer and rhythmic. They are also pleasant to listen. The organization of vocals is possible due to the distinct process of learning the songs. Mostly, birds sing to signify the onset of a season for breeding. This is when they sing to attract potential mates. The singing is essential for males since they get selected by a female. On the other hand, the female evaluates the quality of males through their singing. Only the most appealing male will be selected by the female. Birds songs have a distinct organization. Some species of bird learn one song and others use several songs. The brown thrashers, for example, have many songs.

Juvenile songbirds have a distinct way of learning how to sing. Their learning is similar to that of babies (Ravn, 2013). They start by listening to sounds from adult birds. From the voices, they are able to recognize the sequence of the voices. Birds are able to identify songs produced by members of their species. The birds must also hear and recognize their voices in the process of learning. That is how they learn to vocalize. According to Brainard & Doupe (2002), songbirds learning is similar to learning of speech in humans. Birds use sensory as well as sensorimotor learning. The songbirds listen to songs and memorize. They also copy songs from tutor birds by vocal practice over a period of time.

Sensory Phase and Sensorimotor Phase

Most birds learn songs during the first year of life. They have the ability to distinguish songs from their species and those from other species. Brainard & Doupe (2002) states that birds have an inherent ability to distinguish songs from their species. That is why even in the jungle, young ones will only learn to sing the songs from their species. In the initial stages of sensorimotor, the songs of juvenile birds are usually not properly developed. They are sometimes low pitched and not identifiable to a particular species. Later, the songs become recognizable to the species and are louder. After perfecting the art of singing, duplicate songs are produced similarly to those of adults.

The same ideas have been expressed by Pytte (2016), who states that songbirds learning in sensorimotor stages is characterized by trial and error until the birds can sing perfectly. He adds by saying that the songs learned may vary in duration, the degree by which the song is imitated, the seasons in which the songs are learned, and the number of songs learned. These conditions influence the learning processes of songbirds. Therefore, much as the learning of the songs is through imitation and there is always a likelihood in variation in any two learners.

For both sensory and sensorimotor phase, the motor controller comprises of structures that facilitate breathing as well as vocal muscle control. "Song system" as they are collectively known as a "motor pathway, cortical-basal ganglia circuit, and forebrain pathway." ("Anatomy of a songbird basal ganglia circuit essential for vocal learning and plasticity"). The three are responsible for developing the sequence of singing as well as timings for songs. Therefore, the three elements are vital in singing since the process requires coordination of breathing and muscle control. With inefficiencies in this process, the birds may not develop normal singing capabilities even when they have memorized the songs

The duration by which birds learn the songs is relative depending on the species. Some take a shorter time while others take a longer. For instance, it only takes white-crowned sparrows a few months after hatching to memorize a song. What they learn is used throughout their life without change. On the other hand, some species learn new songs from time to time. In a lifetime, they will have added many songs to what they learned as young birds. The two forms of learning are known as close-ended and open-ended (Wada, 2010). Examples of birds with open-ended learning include canaries and European starlings.

According to Eriksen, (2011), for a bird to qualify as an open-ended learner, there should be evidence that what has been seemingly learned in adults may not have been heard in the juvenile stage of initial learning. Those that learn multiple songs do so in seasons. Usually, the males sing to the female in an effort to dominate other males. Therefore, birds struggle to outshine others just like any other animal. The ones that are stronger, control the environment and in birds, are selected by females as the preferred mate for breeding.

A study from Nowicki, Peters & Podos (1998) gave a new insight into the reasons for birds learning how to sing. According to the findings, the way the birds learn enables female birds to evaluate their potential partners. For effective reproduction, birds need quality mates. The male songs enable a female to identify the developmental process of learning the songs. The female bird can also identify the time that the songs were learned, which is also important is shaping a potential mate. Also, singing reflects the period of brain development. More mature birds have louder and more rhythmic vocals than the young ones. Therefore, a combination of these factors is identifiable through male`s` songs hence making it possible for a female to make the right choice of a quality mate. Again, these go back to the issues of adaptation, where the males depend on the female to select them depending on their singing. And since the male wants to reproduce, they strive to learn and sing attractively.

Birds have exceptional brain plasticity that has several functions. Brain regeneration occurs through the growth of new neurons that replace the old ones. (Brenowitz & Larson, 2015). Developmental of learning of a new song depends on the addition of new neurons in adults birds brain. The process of song control is essential for evaluating the addition of neurons. However, the for the new neurons to survive they must be supported by testosterone produced by gonads. A male with the ability to absorb new neurons will have better song control. Consequently, it increases their appeal to the female birds. There is also a correlation between the death of existing neurons the regeneration of new ones (Brenowitz & Larson, 2015). The new neurons in some cases replace the mature neurons. The process is necessary for continued development of song control. Throughout the lifespan of a male bird, their ability to sing means higher chances of reproducing.

Barnea (2009), states that the new neurons sometimes die without reaching to various brain regions. Those that manage to get to the brain, replace the mature neurons. The replacement comes with the ability to acquire new information. Therefore, this process is vital in adapting to environmental changes. For instance, birds change the volumes of their voices depending on the time of the year. They may increase or reduce the volume of the brain parts that are not necessary during some times of the year. This leads to reduced weight. Consequently, the energy that a bird requires for the brain is significantly reduced.

Usually, in spring, birds tend to sing loudly than other seasons of the year. This is the breeding season for the birds (Kroodsma & Haver 2009). The high pitched singing often comes from the male songbirds....