Birds are a fascinating group of animals that have captured the attention and imagination of humans for centuries. With their ability to fly, colorful plumage, and diverse behaviors, birds represent one of the most unique forms of life on our planet. However, despite being so different from other groups of animals, many people still wonder whether or not birds belong in the same category as mammals.
As an ornithologist or avian biologist, it is important to understand the biological classification system and how organisms are grouped based on shared characteristics. While birds share some similarities with mammals — such as having warm-blooded metabolisms and giving birth to offspring — there are also significant differences between these two groups. In this article, we will explore the question of whether all birds can be considered mammals and examine the evidence behind each argument.
Understanding Biological Classification
It is surprising to know that even in this day and age, people confuse birds with mammals. As an ornithologist or avian biologist, it becomes imperative for me to clear the air on this matter. Birds are not mammals; they belong to a separate biological classification called Aves. Mammals, on the other hand, belong to the class Mammalia.
Biological classification is a system of organizing living organisms into groups based on their shared characteristics. Each group represents a level of organization known as taxonomy. The levels of taxonomy from broadest to narrowest are kingdom, phylum, class, order, family, genus and species. Therefore, birds and mammals differ right at their very first level of classification — the kingdom Animalia.
In terms of traits that set them apart biologically too, birds exhibit unique features such as wings and feathers which are absent in mammals who instead give birth to live young ones and nurse them with milk from mammary glands among other distinguishing features. It’s worth noting that while both classes have warm-blooded animals (endothermic), only mammals possess external ears or pinnae.
Understanding these differences requires careful observation skills since some bird-like dinosaurs like Archaeopteryx existed during prehistoric times before evolving into modern-day birds we see today through natural selection processes over millions of years. In contrast, there is no overlap between mammal anatomy and any non-mammalian animal alive today despite evolution being ongoing.
With this knowledge about what makes up each class distinctively different than others within the Animal Kingdom established firmly let us now proceed towards examining more closely those defining characteristics possessed by members classified under Mammalia specifically next.
The Characteristics Of Mammals
Mammals are a unique group of animals that share several defining characteristics. Firstly, mammals possess hair or fur on their bodies; this fur serves as insulation to keep them warm and protect them from the elements. Secondly, all female mammals produce milk for feeding their young offspring. The production of milk is facilitated by mammary glands located in the female’s breasts. Thirdly, mammals have three middle ear bones — the malleus, incus, and stapes — which allow for efficient hearing.
Another key characteristic of mammals is that they are endothermic (warm-blooded) creatures. This means that their internal body temperature remains relatively constant regardless of external conditions. Endothermy allows for increased metabolic rates and greater activity levels compared to ectothermic (cold-blooded) animals such as reptiles or amphibians.
Furthermore, most mammalian species have well-developed brains relative to other vertebrates and display complex social behaviors. These traits likely evolved due to the challenges posed by mammalian life histories including gestation periods and parental care requirements.
Overall, these distinct features set mammals apart from other animal groups like birds or reptiles. Understanding what makes an animal a mammal can help researchers better understand evolutionary relationships between different taxa and how specific adaptations arose over time. In order to fully appreciate these differences though, we must also explore what sets birds apart in terms of their own distinctive characteristics.
The Characteristics Of Birds
Feathers are a key characteristic of birds and are an adaptation that enable flight. Beaks are well-adapted to the purpose of feeding and can vary depending on species. Wings are the main organ of flight and are usually paired with feathers. Migration is a common behavior of many bird species and is an important part of their life cycle. Nesting is an essential part of bird reproduction as it provides protection for eggs and young birds. Eggs are the primary reproductive product of birds and come in a variety of shapes and sizes.
Feathers
Feathers are one of the most defining characteristics of birds. They are unique to this class of animals and no other vertebrate possesses them. Feathers serve several purposes, including insulation, flight, camouflage and display. They consist of a central shaft or rachis that supports numerous vanes made up of barbs which interlock through tiny hooks called barbules.
The structure of feathers is what makes them so efficient for their intended function. For example, the shape and orientation of the wings’ feathers provide lift during flight, while contour feathers on the body streamline it for easier movement in air currents. The presence of downy feathers underneath provides warmth by trapping heat close to the bird’s body, making it possible for some species to survive in extreme environments such as polar regions.
Furthermore, feathers play an important role in communication between birds. Males often use elaborate feather displays to attract females during mating season. Some species also have specialized feathers on their heads that can change color depending on mood or social status within the flock.
In conclusion, feathers are a crucial characteristic that sets birds apart from all other animals. Their intricate structure serves multiple functions essential for survival and adaptation in various environments. As avian biologists continue to study these fascinating creatures more closely, we may discover even more about how their feathers contribute to their success as a class.
Beaks
Beaks are another defining characteristic of birds that sets them apart from other animals. They come in a variety of shapes and sizes, each adapted to the bird’s specific needs for survival and adaptation in different environments. The beak is made up of two parts, the upper and lower mandibles, which are covered by a layer of keratin similar to human fingernails.
The shape and size of a bird’s beak can determine its feeding habits. For example, seed-eating birds have short, thick beaks designed for cracking open seeds, while insect-eating birds have pointed, sharp beaks for catching and piercing their prey. Some species even have specialized hooks or serrated edges on their beaks for tearing flesh or extracting nectar from flowers.
In addition to feeding adaptations, some birds also use their beaks as tools for constructing nests or defending against predators. For instance, woodpeckers use their long chisel-like bills to excavate nest cavities in trees while herons use their spear-like bills to catch fish.
Overall, the diversity and adaptability of bird beaks make them an essential component to avian success. It is through careful study and observation that ornithologists continue to uncover new insights into how these unique characteristics contribute to the remarkable abilities of our feathered friends.
Wings
Birds are known for their unique characteristics that set them apart from other animals. One of the most defining traits of birds is their wings, which allow them to fly and soar through the skies. The anatomy of a bird’s wing consists mainly of feathers, bones, muscles, and tendons that work together in an intricate system to create lift and maneuverability.
The shape and size of a bird’s wing can vary greatly depending on its species and lifestyle. For example, some birds have long, narrow wings ideal for soaring over large distances while others have shorter, broader wings well-suited for quick bursts of speed or agility. Additionally, flightless birds such as ostriches and penguins have evolved modified wings used primarily for balance or swimming instead of flying.
In addition to aiding in flight, wings also play a crucial role in courtship displays and territorial behavior among many bird species. Male birds may use their vibrant wing colors or patterns to attract mates while others will spread their wings wide as a warning signal to intruders encroaching on their territory.
Overall, the evolution of wings has allowed birds to conquer diverse habitats ranging from open oceans to dense forests. Through continued research and observation by ornithologists and avian biologists alike, we can continue to uncover new insights into how these magnificent creatures utilize this incredible adaptation for survival in ever-changing environments.
Differences In Reproduction
Birds are a fascinating species that have evolved to adapt to various environments, from the Arctic tundra to the tropical rainforest. One of the distinguishing features of birds is their unique reproductive system. Unlike mammals, which give birth to live young, birds lay eggs that hatch into chicks. This difference in reproduction has important implications for bird behavior and ecology.
The process of avian reproduction begins with courtship displays that vary between species but often involve elaborate dances or songs. Once a pair has formed, they will build a nest together using materials such as twigs, grasses, and feathers. The female then lays one or more eggs depending on the species, which she incubates by sitting on them until they hatch.
In addition to laying eggs rather than giving birth, there are several other differences in avian reproductive biology compared to mammals:
- Birds have only one functional ovary, while most mammals have two ovaries.
- Avian sperm lack motility and must be transferred directly into the female’s cloaca during copulation.
- Many bird species engage in cooperative breeding where multiple individuals help care for offspring.
By understanding these unique aspects of avian reproduction, we can gain insight into how birds have evolved over time and adapted to different environments. These adaptations may include changes in nesting behavior or egg-laying patterns depending on factors such as temperature or predation risk.
As ornithologists continue to study bird behavior and biology, it becomes increasingly apparent that these creatures are much more complex than many people realize. In the next section, we will explore another aspect of avian anatomy: differences in skeletal structure.
Differences In Skeletal Structure
Having explored the differences in reproduction among various animal groups, it is important to note that birds are not mammals. While both classes of animals reproduce sexually, there are several key distinctions between their reproductive systems. For example, birds have a unique reproductive organ known as a cloaca which serves multiple functions including egg laying and waste elimination.
Moving on from reproduction, another significant point of differentiation between birds and mammals lies in their skeletal structures. Birds possess lightweight bones designed for flight and aerodynamic efficiency. In contrast, mammalian skeletons tend to be denser and stronger in order to support larger bodies and provide protection for vital organs.
Furthermore, avian digestive systems differ significantly from those of mammals. Birds lack teeth; instead they swallow small stones which aid in grinding up food in their gizzard. Additionally, unlike most mammals who rely on stomach acids for digestion, birds use enzymes found in their saliva to break down food before it reaches the stomach.
In conclusion, while some similarities exist between birds and mammals such as sexual reproduction through internal fertilization, crucial differences can still be observed regarding their skeletal structure and digestive system. These variances ultimately reflect how each group has evolved over time to adapt to different environmental pressures and ecological niches. Moving ahead into the next section about differences in digestive systems, we will explore these adaptations more closely.
Differences In Digestive Systems
It is a well-known fact that birds are not mammals. It seems ridiculous to even suggest such a thing, yet it is surprising how often this question arises. While both groups may share certain traits, they differ in many fundamental ways, particularly when it comes to their digestive systems.
Birds have evolved unique adaptations in order to efficiently process food. Their beaks and tongues are specialized for different types of diets; some species have sharp beaks for tearing flesh while others have longer beaks used for probing flowers or the ground for insects. Unlike humans and other mammals, birds do not have teeth in their mouths, instead swallowing their food whole or breaking it down with a muscular organ called the gizzard.
The way in which birds digest their food also sets them apart from mammals. Birds possess what is known as a two-chambered stomach: one chamber stores food before it moves on to the second chamber where enzymes break down the nutrients. This allows birds to consume large quantities of food at once and then store it until needed later on.
Overall, the differences between bird and mammal digestive systems highlight just how diverse life can be on our planet. By studying these contrasting mechanisms we gain insight into how evolution has shaped each group’s biology and behavior. In turn, this knowledge helps us better understand our own place within the natural world.
Moving forward, it is important to note that respiratory systems are another area where birds exhibit remarkable adaptations compared to mammals. These differences allow avian species to fly higher and farther than any mammal could ever dream of achieving.
Differences In Respiratory Systems
The respiratory systems of birds and mammals differ in several ways. Birds possess a unique set of lungs that operate differently than those found in mammals. Unlike mammalian lungs, avian lungs are rigid and contain multiple air sacs, which means they do not expand or contract during breathing like human lungs do. Instead, birds rely on the movement of air through their lungs to facilitate gas exchange.
One key advantage of this system is that it allows birds to extract more oxygen from each breath, making them well-suited for high-energy activities such as flight. Additionally, since bird lungs do not move during breathing, they are less susceptible to damage caused by repetitive motion or injury.
Another notable difference between the respiratory systems of birds and mammals is the presence of unidirectional airflow in birds. This means that instead of inhaling and exhaling through the same set of airways as humans do, birds’ respiratory systems have separate pathways for incoming and outgoing air. As a result, fresh air always moves across the surface where gas exchange takes place, maximizing efficiency.
Overall, the unique respiratory adaptations present in avian species play an essential role in allowing these animals to thrive in diverse environments around the world. By optimizing oxygen uptake while minimizing energy expenditure and risk of injury, bird lungs represent a remarkable example of evolutionary adaptation at work.
Moving forward into a discussion about differences in circulatory systems among various animal groups will shed further light on how evolution has led to these fascinating physiological adaptations over time.
Differences In Circulatory Systems
Avian circulatory systems differ significantly from those of mammals. The former have a four-chambered heart, while the latter typically have three chambers. This allows birds to maintain higher metabolic rates and sustain flight for extended periods without fatigue. Additionally, avian blood cells are nucleated, unlike mammalian red blood cells which lack nuclei.
Blood flow in birds is also unique due to the presence of air sacs connected to their lungs. These sacs act as bellows that facilitate unidirectional airflow through the respiratory system during both inhalation and exhalation. Oxygen-poor blood flows into the right side of the heart where it is pumped to the lungs via pulmonary arteries. After oxygen exchange, oxygen-rich blood returns to the left side of the heart before being distributed throughout the body.
Another notable difference between bird and mammal circulatory systems concerns their temperature regulation mechanisms. Birds possess an organ called a gizzard which grinds food with rocks or hard objects they swallow instead of teeth like most mammals. As this process generates heat, some species use it as a means of thermoregulation by increasing internal temperatures when necessary.
Understanding these differences in avian circulatory systems has significant implications for our knowledge about how animals adapt to different environments and challenges related to evolution and conservation biology. In order to fully appreciate how birds survive in their respective habitats, one must consider all aspects of their physiology rather than just individual traits such as feather structure or beak shape.
Transitioning into discussing nervous systems: While much research has been conducted on bird circulatory systems, comparatively less attention has been paid to their nervous systems despite its vital role in controlling behavior and survival strategies.
Differences In Nervous Systems
Having discussed the differences in circulatory systems among various bird species, it is now imperative to delve into their nervous systems. As with any animal, the nervous system of birds plays a crucial role in controlling and coordinating their behavior as well as regulating other physiological processes. While there are similarities between avian and mammalian nervous systems, several notable distinctions exist.
One key difference is the structure of the brain. The cerebrum, which is responsible for conscious thought and complex behaviors, makes up a larger proportion of the bird brain than that of mammals. Additionally, birds possess a unique neural structure known as the Wulst, which may play a critical role in visual processing and navigation.
Another significant variation lies in how information is processed within neurons. Unlike mammalian neurons, avian neurons have a much lower density of ion channels on their cell membranes. This leads to slower conduction speeds but also allows for greater control over neuronal activity patterns. Birds also exhibit more pronounced lateralization of brain function than most mammals do; different tasks are often delegated to specific hemispheres rather than being distributed across both sides equally.
In summary, while certain basic principles hold true for all animals’ nervous systems, including those of birds and mammals alike, there are clear differences between these two groups as well. Understanding these disparities may shed light not only on evolutionary history but also on how we can better protect vulnerable bird populations from environmental threats such as habitat loss or climate change.
Moving forward into our next section about differences in sensory systems, we’ll explore further variations between avian biology and that of other animals through examining how they sense and respond to stimuli like sound or touch.
Differences In Sensory Systems
- Birds have a well-developed auditory system, which allows them to pick up a wide range of frequencies, including higher-pitched sounds than mammals can hear.
- In addition to having excellent hearing, birds also have excellent vision, with many species having superior binocular vision compared to mammals.
- Birds also have a well-developed sense of smell, although it is not as acute as the sense of smell found in some mammals.
- Birds have small olfactory lobes in their brains, which are used to process odors, and some species even have specialized olfactory organs which can detect odors more accurately.
- The olfactory system of birds is mainly used for finding food and detecting predators, but some species also use smell for navigation and locating mates.
- The sense of smell in birds is also used to identify potential nesting sites and detect changes in the environment that may signal danger.
Hearing
Birds are fascinating creatures that have evolved unique sensory systems to meet the demands of their environment. One of the most important senses for birds is hearing, which they rely on for communication, navigation, and survival. Compared to mammals, birds’ auditory systems are highly specialized and adapted to detect a wide range of sounds.
In terms of structure, bird ears differ from those of mammals in several ways. Most notably, birds lack external ear structures such as pinnae or auricles. Instead, sound waves enter through the openings at the sides of their heads and travel directly to the eardrum. Birds also have a single middle ear bone called the columella instead of three ossicles found in mammalian ears.
A key feature of bird hearing is its sensitivity and ability to locate sounds accurately. Many species can hear frequencies beyond the human range and distinguish between subtle variations in pitch and intensity. This allows them to identify different calls from other birds and localize prey or predators with great precision even when they cannot see them.
Overall, while there are some similarities between avian and mammalian auditory systems, differences exist due to each group’s unique evolutionary history. The study of bird hearing not only provides insight into how these animals function but also has practical applications in fields such as conservation biology and bioacoustics research.
Vision
Birds have evolved unique sensory systems to meet the demands of their environment. In addition to hearing, vision is another essential sense for birds. Compared to mammals, birds’ visual systems are highly specialized and adapted to detect a wide range of colors, shapes, and movements.
One notable difference between avian and mammalian eyes is that birds possess four types of color receptors in their cones compared to three in humans. This allows them to see ultraviolet light and distinguish between subtle variations in color that are invisible to us. Additionally, many bird species have a much wider field of view due to their eyes being positioned on either side of their head instead of facing forward like ours.
Another fascinating aspect of bird vision is their ability to perceive polarized light. Some bird species can use this information for navigation by detecting the polarization pattern created by sunlight reflecting off water or other surfaces. Certain seabirds also use polarized light as a wayfinding tool during long-distance migration.
Overall, while there may be some similarities between avian and mammalian visual systems, differences exist due to each group’s unique evolutionary history. Understanding these differences not only provides insight into how birds function but also has practical applications in fields such as conservation biology and ornithology research.
Smell
Birds’ sensory systems have evolved to meet their specific environmental demands, and this includes the olfactory system. The sense of smell has long been considered less important in birds compared to other senses such as vision and hearing. However, recent research suggests that many bird species possess highly developed olfactory abilities that allow them to navigate, forage, and communicate with one another.
One significant difference between avian and mammalian olfaction is the number of functional odor receptor genes. While humans have approximately 400 functioning receptor genes, some birds can have over a thousand. This suggests that birds may be capable of detecting an incredibly wide range of scents. Additionally, unlike mammals who breathe through both their mouth and nose simultaneously, birds have separate air passages for breathing and smelling which may enhance their ability to detect odors while flying.
The importance of smell varies across different bird species depending on their ecological niche. For example, scavenger birds like vultures rely heavily on their sense of smell to locate carrion from great distances away. In contrast, seabirds use scent primarily for locating breeding colonies or finding prey at sea when visual cues are limited.
Overall, although the study of avian olfaction is relatively new compared to vision and hearing research; it highlights the incredible adaptability and complexity found within bird sensory systems. Understanding these differences not only provides insight into how birds function but also has practical applications in fields such as conservation biology where identifying critical habitats based on scent could play a vital role in protecting endangered species.
The Role Of Dna In Classification
The classification of birds as a distinct group from mammals is based on several factors, one of which is their genetic makeup. DNA plays an important role in the determination of species relationships and phylogenetic history. The use of molecular genetics has revolutionized our understanding of evolutionary relationships among organisms, including birds and mammals.
One key difference between birds and mammals lies in their respective types of DNA. Birds have a unique genome structure that sets them apart from other vertebrates, including humans. The bird genome contains fewer genes than mammalian genomes but has undergone significant changes over time due to genomic rearrangements. These genomic differences are reflected in both the physical characteristics and behavior of birds compared to mammals.
Another aspect where DNA plays a pivotal role in distinguishing birds from mammals is through comparative genomics analysis. By comparing the complete genomes or specific gene sequences between different taxa across diverse groups, we can infer patterns of common ancestry or divergent evolution. The comparison reveals that while there may be some similarities between certain bird and mammal lineages at the molecular level, overall they remain distinctly separate groups.
Overall, the role of DNA in classification provides compelling evidence for why all birds cannot be classified as mammals. Their genetic makeup differs significantly from that of mammals, resulting in distinct morphological features and behaviors. Understanding these differences helps us better understand how avian biology evolved over time independently from other animal groups such as mammals, making it imperative to classify them accurately according to their genomic characteristics.
Conclusion: Birds Are Not Mammals
Based on the classification system used in biology, birds are not mammals. This is evident from the characteristics that separate these two classes of animals. For instance, all birds have feathers while no mammal has them. Feathers provide insulation and allow for flight which is a unique feature among avian species. On the other hand, most mammals have fur or hair as their epidermal covering.
Another distinguishing factor between birds and mammals lies in their reproductive systems. Mammals give birth to live young ones while birds lay eggs. In addition, the skeletal structure of both groups varies significantly with mammals having flexible spines compared to rigid spines found in birds. Furthermore, all birds possess beaks or bills whereas only some mammals such as platypus and echidnas have specialized mouth parts.
Apart from physical attributes, genetic studies also point out clear differences between these animal groups at molecular levels. For example, while every bird possesses an enzyme called uricase which helps break down nitrogenous waste into uric acid before excretion, it is absent in almost all mammals except primates and few others like dalmatians.
In conclusion, although some similarities may exist between birds and mammals such as being warm-blooded vertebrates that breathe air; they differ mainly due to diverse adaptations brought about by evolutionary processes over time. The distinctions cited above clearly indicate that despite sharing some traits, birds cannot be classified under the same category as mammals using standard biological criteria.
Frequently Asked Questions
What Is The Purpose Of Biological Classification?
The purpose of biological classification is to organize and categorize living organisms based on their shared characteristics, ancestry, and evolutionary relationships. This system allows scientists to study and compare species in a more efficient way. For ornithologists or avian biologists specifically, the classification of birds helps to identify similarities and differences between different bird species and understand their unique adaptations for survival. Birds are classified as members of the class Aves, which distinguishes them from other classes of animals such as mammals, reptiles, amphibians, fish, and insects. While birds share some traits with mammals (such as being endothermic or warm-blooded), they differ significantly in terms of reproductive strategies, skeletal structure, respiratory systems, and other key characteristics that define their place within the animal kingdom.
How Are Mammals Different From Birds In Terms Of Their Reproductive Systems?
As avian biologists, we understand the intricate differences between birds and mammals. In terms of reproductive systems, these two groups diverge significantly. While birds have a unique system that involves laying eggs externally and incubating them through brooding, mammals carry their offspring internally before giving birth to live young ones. This distinction is primarily due to the fact that birds are oviparous while mammals are viviparous; an adage often used in this context is "birds lay eggs, but they don’t give birth." Additionally, mammalian reproduction usually involves internal fertilization with sperm introduced into the female’s body via copulation or other means. On the other hand, bird reproduction may entail external fertilization where male gametes combine with those released by females outside their bodies. Therefore, when it comes to reproductive systems specifically, there are fundamental variations between birds and mammals that highlight each group’s distinct evolutionary history and adaptations for survival.
What Are The Differences Between The Skeletal Structures Of Birds And Mammals?
The skeletal structures of birds and mammals differ significantly. Mammals possess a ribcage that is attached to the spine, which provides support for their internal organs. Birds, on the other hand, have a keeled sternum that serves as an attachment point for their powerful flight muscles. Additionally, bird bones are hollow and lightweight, allowing them to fly efficiently through the air. In contrast, mammalian bones tend to be denser and heavier in order to support their larger bodies. Overall, these differences in skeletal structure reflect the unique adaptations that each group has evolved in order to thrive within its respective environment.
How Do Birds And Mammals Differ In Terms Of Their Respiratory Systems?
Birds and mammals differ significantly in terms of their respiratory systems. Mammals have a diaphragm, which separates the thoracic cavity from the abdominal cavity and aids in breathing by expanding and contracting the lungs. In contrast, birds lack a diaphragm and rely on air sacs located throughout their body to help regulate airflow through their lungs. These air sacs act as bellows, pushing fresh air into the lungs during both inhalation and exhalation. Additionally, birds possess a unique one-way flow of air through their lungs that enables more efficient oxygen exchange compared to mammals’ two-way flow system. Overall, these differences in respiratory anatomy reflect adaptations for flight and high metabolic demands in birds versus terrestrial living in mammals.
Why Is Dna Important In The Classification Of Animals?
It is a well-known fact among those in the field of ornithology that DNA plays an essential role in determining the classification of animals. In this day and age, it would be utterly preposterous to ignore the intricacies of genetic code when attempting to categorize our feathered friends. The use of molecular techniques has revolutionized our understanding of avian taxonomy, allowing us to discern previously unseen relationships between species. Through analyzing differences in DNA sequences, we can identify similarities and divergences across various bird lineages, providing invaluable insights into their evolutionary history. Without considering such fundamental aspects as genetics, one could hardly claim to have a comprehensive grasp on the diversity and complexity found within the avian world.
Conclusion
Biological classification is a way of organizing living things based on their characteristics. Birds and mammals are two distinct groups within the animal kingdom, with several key differences between them. Mammals give birth to live young, while birds lay eggs. Additionally, mammalian bones have marrow, but bird bones do not.
The respiratory systems of birds and mammals also differ significantly; birds have air sacs that allow for more efficient oxygen exchange during flight, while mammals rely on lungs alone. Furthermore, DNA analysis has become an important tool in classifying animals due to its ability to reveal genetic relationships between species.
Interestingly, there are over 10,000 known species of birds worldwide compared to around 5,500 species of mammals. This makes birds one of the largest and most diverse groups within the animal kingdom. As ornithologists and avian biologists continue to study these fascinating creatures, our understanding of their unique traits and behaviors will undoubtedly expand. The classification of birds as separate from mammals highlights just how diverse life on Earth truly is.