What are the 3 domains and how are they different

In 1994, Professor David Mellor and Dr Cam Reid proposed a new model as a means of systematically identifying and grading the severity of different forms of welfare compromise by reformulating the Five Freedoms as ‘Five Domains’ of nutrition, environment, health, behaviour and mental state [1] (Table 1).

Table 1 Five Freedoms and Five Domains – simplistic form

Five FreedomsFive Domains 1. From hunger and thirst1. Nutrition 2. From discomfort2. Environment 3. From pain, injury and disease3. Health 4. To express normal behaviour4. Behaviour 5. From fear and distress5. Mental state

This approach allowed a distinction to be made between the physical and functional factors that affect an animal’s welfare and the overall mental state of the animal arising from these factors. Over the past 20 years this paradigm has been widely adopted as a tool for assessing the welfare impacts of research procedures, pest animal control methods and other interventions in animals’ lives.

The Five Freedoms and Five Domains frameworks contain essentially the same five elements. However, the Five Domains explore the mental state of an animal in more detail and acknowledge that for every physical aspect that is affected, there may be an accompanying emotion or subjective experience that may also affect welfare. This is useful in terms of reinforcing the message that emotional needs are equally important as physical needs for animals.

One of the most important strengths of the Five Domains is the clarity it provides that merely minimising or resolving negative physical or mental states does not necessarily result in positive welfare, but may only provide, at best, a neutral state. To have good welfare, animals need more than this.

To help ensure animals have a ‘life worth living’ they must have the opportunity to have positive experiences, such as anticipation, satisfaction and satiation. To enable this, those responsible for the care of animals need to provide them with environments that not only allow, but encourage animals to express behaviours that are rewarding. This shift in understanding is the basis for the Five Domains model incorporating positive welfare states [2, 3]. You can read about this in detail here.

Thus, the Five Domains provide a means of evaluating the welfare of an individual or group of animals in a particular situation, with a strong focus on mental well-being and positive experiences. The Five Domains also allow us to extend our thinking beyond the Five Freedoms to place even greater emphasis on providing opportunities for animals to be exposed to or engage in activities which provide positive experiences.


[1] Mellor DJ & Reid CSW (1994) Concepts of animal well-being and predicting the impact of procedures on experimental animals. In Improving the Well-Being of Animals in the Research Environment; Australian and New Zealand Council for the Care of Animals in Research and Teaching (ANZCCART): Glen Osmond, SA, Australia, pp. 3–18.

[2] Mellor DJ (2017) Operational details of the Five Domains Models and its key applications to the assessment and management of animal welfare. Animals 7(8):60. doi:10.3390/ani7080060

[3] Mellor DJ & Beausoleil NJ (2015) Extending the ‘Five Domains’ model for animal welfare assessment to incorporate positive welfare states. Animal Welfare 24:241–253. doi: 10.7120/09627286.24.3.241.

Video Transcript


In science, we are always looking for ways to better categorize what we are researching and studying. After all, it makes it easier to look up information and draw comparisons and relationships between things if they are categorized and organized properly. One such place where we take great pride and detail in this is in biological taxonomy.

Biological taxonomy is the hierarchical breakdown of the different ways to categorize living things. We break everything in the world into living vs. nonliving things. The system for categorizing living things was revised around 1990 by Carl Woese, a microbiologist. He suggested adding a more general term above the category kingdom, and he added domain.

Our current taxonomic system looks like this:

  • Domain
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

You can remember this by the mnemonic: Did King Phillip Come Over For Great Spaghetti.


Domains are our way of breaking down living things more generally than before when we just went into kingdoms. We have found through research that many of the kingdoms were not exactly aligned as best as they could be. By adding domains, we can now show how some kingdoms are actually closely related under a specific domain.

There are three distinct domains in biology. They do an excellent job of making it easy to understand what goes under that domain. There is the Archaea, the Bacteria, and the Eukarya. We will look at each one individually.


The first and oldest known domain is the Archaea. These are ancient forms of bacteria that were originally grouped under the kingdom Monera (now defunct) as Archaeabacteria.

We know them to be prokaryotic (lacking membrane-bound nuclei and organelles) that are found in all habitats on Earth. They are single celled microbes that find their origins as the first organisms of life here on Earth. Hence, we give them the prefix archaea, which in Greek means ‘ancient things.’

Archaea organisms are also different from the other domains in that many are extremophiles, meaning they can live in intense environments with high temperature, high acid, and high salt levels. One type of extremophile is the methanogens, or those organisms that produce methane as a product of their metabolism.


The Bacteria domain includes all other bacteria that are not included in the Archaea domain. They are prokaryotic and again found in all of the habitats on Earth. They are very similar to the Archaea domain, except that bacteria gain energy by being phototrophs (getting energy from light), lithotrophs (getting energy from inorganic non-carbon compounds), or finally organotrophs (getting energy from organic carbon-containing compounds).


The final domain is the Eukarya. This domain contains all the organisms that are eukaryotic, or contain membrane-bound organelles and nuclei. These would be considered fairly ‘modern’ since the other domains existed first. Scientists believe that Eukarya evolved from the symbiotic relationship exhibited by a prokaryotic bacteria that ate other prokaryotic bacteria but did not digest them. We also see here the more complex and larger organisms.

Within the Eukarya domain are four kingdoms: Animalia, Plantae, Fungi, and Protista.

Comparing the Domains

The Archaea domain is the oldest, followed by Bacteria, and finally Eukarya. In both Archaea and Bacteria, the chromosomes (genetic material/blue prints for life) are circular, whereas in Eukarya, we have a double helix DNA structure (looks like a twisted ladder, again, blue prints for life). In Eukarya, there are membrane-bound organelles as well as nuclei, whereas the other two lack these. Archaea and Eukarya both will continue to grow in the presence of antibiotics, whereas organisms in the Bacteria domain do not.

Under the domains Archaea and Bacteria, we see many different organisms, but they are all lumped together under their respective domains. Most of these are single-celled organisms. It is in the Eukarya domain that we see multicellular organisms falling in complex categories.

Lesson Summary

Biological taxonomy is the hierarchical breakdown of the different ways to categorize living things. The taxonomic break down of biology starts with our most general category, domain, which is our way of breaking down living things more generally than before when we just went into kingdoms. There are three domains in biology, and each one has distinct characteristics and organisms under it. The three domains include:

  • Archaea – oldest known domain, ancient forms of bacteria
  • Bacteria – all other bacteria that are not included in the Archaea domain
  • Eukarya – all the organisms that are eukaryotic or contain membrane-bound organelles and nuclei

All living organisms consist of elementary units calledcells. Cells are membrane-enclosed compartments that contain genomic DNA(chromosomes), molecular machinery for genome replication and expression, atranslation system that makes proteins, metabolic and transport systems thatsupply monomers for these processes, and various regulatory systems. Scientistshave performed careful microscopic observations and other experiments to showthat all cells reproduce by different forms of division. Cell division is anelaborate process that ensures faithful segregation of copies of the replicatedgenome into daughter cells. The best-characterized cells are the relativelylarge cells of animals, plants, fungi, and diverse unicellular organisms knownas protists, such as amoebae or paramecia. These cells possess an internalcytoskeleton and a complex system of intracellular membrane partitions,including the nucleus, a compartment that encloses the chromosomes. Theseorganisms are known as eukaryotes because they possess a true nucleus (karyon in Greek). In contrast, the muchsmaller cells of bacteria have no nucleus and are named prokaryotes.

In the twentieth century, scientists devised new imaging methodslike electron microscopy, which can be used to view tiny particles that aremuch smaller than cells, to detect a second fundamental form of biological organization:the viruses. Viruses are obligate intracellular parasites. These selfishgenetic elements typically encode some proteins essential for viral replication,but they never contain the full complement of genes for the proteins and RNAsrequired for translation, membrane function, or metabolism. Therefore, virusesexploit cells to produce their components.

Classifying organisms (known as systematics or taxonomy) isone of the oldest occupations of biologists. Carolus Linnaeus constructed hisnow famous taxonomic system — certainly one of the foundations of scientificbiology — in the middle of the eighteenth century. How did he classifyorganisms? Since Linnaeus was not an evolutionist, his classifications strivedto reflect only similarities between species that were considered immutable.The goals of systematics changed after Charles Darwin introduced the concept ofthe Tree of Life (hereafter, TOL). At least in principle, the TOL was perceivedas an accurate depiction of the evolutionary relationships between alllife-forms. After Darwin,evolutionary biologists attempted to delineate monophyletic taxa, which aregroups of organisms that share a common ancestry and thus form a distinctbranch in the TOL. Until the last quarter of the twentieth century, however, taxonomistsworked with phenotypic similarities between organisms, so monophyly remained ahypothesis based on the hierarchy of similar features. Accordingly, biologistscould boast substantial advances in the classification of animals and plants,and to a lesser extent, simpler multicellular life-forms, such as fungi andalgae. However, taxonomy was nearly helpless when it came to unicellularorganisms, particularly bacteria, which have few easily observed features tocompare. As a result, microbiologists were skeptical about whether it waspossible to establish the evolutionary relationships between microbes. Howcould they compare these tiny organisms?

A revolution occurred in 1977 when Carl Woese and his co-workersperformed pioneering studies to compare the nucleotide sequences of a moleculethat is conserved in all cellular life-forms: the small subunit of ribosomalRNA (known as 16S rRNA). By comparing the nucleotide sequences of the 16S rRNA,they were able to derive a global phylogeny of cellular organisms for the firsttime. This phylogeny overturned the eukaryote-prokaryote dichotomy by showingthat the 16S rRNA tree neatly divided into three major branches, which becameknown as the three domains of (cellular) life: Bacteria, Archaea and Eukarya (Woese et al.1990). This discovery was enormously surprising, given thatsuperficially the members of the new Archaea domain did not appear particularlydifferent from bacteria. Since archaea and bacteria looked alike, how differentcould they be?

Learning is not an event. It is a process. It is the continual growth and change in the brain’s architecture that results from the many ways we take in information, process it, connect it, catalogue it, and use it (and sometimes get rid of it).

Learning can generally be categorized into three domains: cognitive, affective, and psychomotor. Within each domain are multiple levels of learning that progress from more basic, surface-level learning to more complex, deeper-level learning. The level of learning we strive to impact will vary across learning experiences depending on 1) the nature of the experience, 2) the developmental levels of the participating students, and 3) the duration and intensity of the experience.

When writing learning objectives, it is important to think about which domain(s) is relevant to the learning experience you are designing. The tables below provide further information about each domain.

Written by Jane