The International Code of Zoological Nomenclature (ICZN) is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals. It is also informally known as the ICZN Code, for its publisher, the International Commission on Zoological Nomenclature (which shares the acronym "ICZN"). The rules principally regulate:
- How names are correctly established in the frame of binominal nomenclature
- Which name must be used in case of name conflicts
- How scientific literature must cite names
Zoological nomenclature is independent of other systems of nomenclature, for example botanical nomenclature. This implies that animals can have the same generic names as plants.
The rules and recommendations have one fundamental aim: to provide the maximum universality and continuity in the naming of all animals, except where taxonomic judgment dictates otherwise. The code is meant to guide only the nomenclature of animals, while leaving zoologists freedom in classifying new taxa.
In other words, whether a species itself is or is not a recognized entity is a subjective decision, but what name should be applied to it is not. The code applies only to the latter. A new animal name published without adherence to the code may be deemed simply "unavailable" if it fails to meet certain criteria, or fall entirely out of the province of science (e.g., the "scientific name" for the Loch Ness Monster).
The rules in the code determine what names are valid for any taxon in the family group, genus group, and species group. It has additional (but more limited) provisions on names in higher ranks. The code recognizes no case law. Any dispute is decided first by applying the code directly, and not by reference to precedent.
The code is also retroactive or retrospective, which means that previous editions of the code, or previous other rules and conventions have no force any more today, and the nomenclatural acts published 'back in the old times' must be evaluated only under the present edition of the code. In cases of disputes concerning the interpretation, the usual procedure is to consult the French Code, lastly a case can be brought to the commission who has the right to publish a final decision.
In regulating the names of animals it holds by six central principles, which were first set out (as principles) in the third edition of the code (1985):
This is the principle that the scientific name of a species, and not of a taxon at any other rank, is a combination of two names; the use of a trinomen for the name of a subspecies and of uninominal names for taxa above the species group is in accord with this principle.
This means that in the system of nomenclature for animals, the name of a species is composed of a combination of a generic name and a specific name; together they make a "binomen". No other rank can have a name composed of two names. Examples:
- Subspecies have a name composed of three names, a "trinomen": generic name, specific name, subspecific name:
- Taxa at a rank above species have a name composed of one name, a "uninominal name".
In botanical nomenclature, the equivalent for "binominal nomenclature" is "binary nomenclature" (or sometimes "binomial nomenclature").
This is the principle that the correct formal scientific name for an animal taxon, the valid name, correct to use, is the oldest available name that applies to it. It is the most important principle—the fundamental guiding precept that preserves zoological nomenclature stability. It was first formulated in 1842 by a committee appointed by the British Association to consider the rules of zoological nomenclature. Hugh Edwin Strickland wrote the committee's report.
There are approximately 2-3 million cases of this kind for which this principle is applied in zoology.
The principle of coordination is that within the family group, genus group and species group, a name established for a taxon at any rank in the group is simultaneously established with the same author and date for taxa based on the same name-bearing type at other ranks in the corresponding group. In other words, publishing a new zoological name automatically and simultaneously establishes all corresponding names in the relevant other ranks with the same type.
In the species-group, publishing a species name (the binomen) Giraffa camelopardalis Linnaeus, 1758 also establishes the subspecies name (the trinomen) Giraffa camelopardalis camelopardalis Linnaeus, 1758. The same applies to the name of a subspecies; this establishes the corresponding species name.
In the genus-group, similarly, publishing the name of a genus also establishes the corresponding name of a subgenus (or vice versa): genus Giraffa Linnaeus, 1758 and subgenus Giraffa (Giraffa) Linnaeus, 1758.
In the family-group, publication of the name of a family, subfamily, superfamily (or any other such rank) also establishes the names in all the other ranks in the family group (family Giraffidae, superfamily Giraffoidea, subfamily Giraffinae).
Author citations for such names (for example a subgenus) are the same as for the name actually published (for example a genus). It is immaterial if there is an actual taxon to which the automatically established name applies; if ever such a taxon is recognised, there is a name available for it.
This is the principle that in cases of conflicts between simultaneously published divergent acts, the first subsequent author can decide which has precedence. It supplements the principle of priority, which states that the first published name takes precedence. The principle of the first reviser deals with situations that cannot be resolved by priority. These items may be two or more different names for the same taxon, two or more names with the same spelling used for different taxa, two or more different spellings of a particular name, etc. In such cases, the first subsequent author who deals with the matter and chooses and publishes the decision in the required manner is the first reviser, and is to be followed.
Linnæus 1758 established Strix scandiaca and Strix noctua (Aves), for which he gave different descriptions and referred to different types, but both taxa later turned out to refer to the same species, the snowy owl. The two names are subjective synonyms. Lönnberg 1931 acted as first reviser, cited both names and selected Strix scandiaca to have precedence.
This is the principle that the name of each taxon must be unique. Consequently, a name that is a junior homonym of another name must not be used as a valid name.
It means that any one animal name, in one particular spelling, may be used only once (within its group). This is usually the first-published name; any later name with the same spelling (a homonym) is barred from being used. The principles of priority and first reviser apply here. For family-group names the termination (which is rank-bound) is not taken into account.
Genera are homonyms only if exactly the same — a one-letter difference is enough to distinguish them.
In species, there is a difference between primary and secondary homonyms. There can also be double homonyms (same genus and species). A slight difference in spelling is tolerated if Article 58 applies.
Primary homonyms are those with the same genus and same species in their original combination. The difference between a primary junior homonym and a subsequent use of a name is undefined, but it is commonly accepted that if the name referred to another species or form, and if there is in addition no evidence the author knew that the name was previously used, it is considered as a junior homonym.
Secondary homonyms can be produced if taxa with the same specific name but different original genus are later classified in the same genus (Art. 57.3, 59). A secondary synonym is only a temporary state, it is only effective in this classification. If another classification is applied, the secondary homonymy may not be produced, and the involved name can be used again (Art. 59.1). A name does not become unavailable or unusable if it was once in the course of history placed in such a genus where it produced a secondary homonymy with another name. This is one of the rare cases where a zoological species does not have a stable specific name and a unique species-author-year combination, it can have two names at the same time.
Article 59.3 states that in exceptional cases, junior secondary homonyms replaced before 1961 by substitute names can become invalid, "...unless the substitute name is not in use," an exception of the exception. However, the ICZN Code does not give an example for such a case. It seems that this passage in the ICZN Code is widely ignored. It also does not define what the expression "is not in use" should mean.
Double homonymy (genus and species) is no homonymy: if the genera are homonyms and belong to different animal groups, the same specific names can be used in both groups.
For disambiguating one genus-group name from its homonym, it is important to cite author and year. Citing the author alone is often not sufficient.
The name Ansa can only be used for a lepidopteran taxon. If that name cannot be used (for example because an older name established prior to 1858 takes precedence), this does not mean that the 1868 name can be used for a hemipteran genus. The only option to use the 1868 name for the hemipteran taxon is to get the 1858 name officially suppressed by the commission.
In some cases, the same genus-group or species-group name was published in the same year by the same author. In these cases it is useful to cite the page where the name was established.
There are cases where two homonyms were established by the same author in the same year on the same page:
Animal, plant, and fungi nomenclature are entirely independent from each other. The most evident shortcoming of this situation (for their use in biodiversity informatics) is that the same generic name can be used simultaneously for animals and plants. For this kind of homonym the expression "hemihomonym" is sometimes used. Far more than 1000 such names are known.
For names above the family level, the principle of homonymy does not apply.
Homonyms occur relatively rarely in families (only if generic names are identical or very similar and adding an ending "-idae" produces identical results). Discovering such a homonymy usually produces the same problems as if there were no rules: conflicts between entirely independent and unconnected groups of taxonomists working in different animal groups. Very often the Commission must be asked to take a decision.
This is the principle that each nominal taxon in the family group, genus group, or species group has—actually or potentially—a name-bearing type fixed that provides the objective standard of reference that determines what the name applies to.
This means that any named taxon has a name-bearing type, which allows the objective application of that name. Any family-group name must have a type genus, any genus-group name must have a type species, and any species-group name can (not must) have one or more type specimens (holotype, lectotype, neotype, syntypes, or others), usually deposited in a museum collection. The type genus for a family-group name is simply the genus that provided the stem to which was added the ending "-idae" (for families). Example:
The type species for a genus-group name is more complicated and follows exactly defined provisions in articles 67-69. Type species are very important, and no general zoological database has recorded the type species for all genera. Except in fishes and some minor groups, type species are rarely reliably recorded in online animal databases. In 60% of the cases the type species can be determined in the original publication. The type species is always the original name of the taxon (and not the currently used combination).
Designation and fixation have different meanings. A designation is the proposal of the type species. It is not necessary to have spelled the name of the genus or species correctly with correct authors (articles 67.2.1, 67.6, 67.7), type species are always the correctly spelled name. If the designation is valid, the type species is fixed.
A designation can also be invalid and ineffective—for example—if the genus had already a previously fixed type species, or if a type species was proposed that was not originally included, or contradicted the description or figure for a genus for which no species had originally been included.
There are various possible modes of type species designation. This is their order of legal importance, with approximate proportions of occurrence and examples:
- Superior type fixation:
- Type fixation in the original work:
- Subsequent methods of type fixation:
A species-group name can have a name-bearing type specimen, but this is not a requirement. In many cases species-group names have no type specimens, or they are lost. In those cases the application of the species-group name is usually based on common acceptance. If there is no common acceptance, there are provisions in the Code to fix a name-bearing type specimen that is binding for users of that name. Fixing such a name-bearing type should only be done if this is taxonomically necessary (articles 74.7.3, 75.2, 75.3).
Links to the separate articles:
- Principle of coordination
- Principle of typification
The code divides names in the following manner:
- Names above the family group
- Family-group names
- Genus-group names
- Species-group names
The names above the family group are regulated only as to the requirements for publication; there is no restriction to the number of ranks and the use of names is not restricted by priority.
The names in the family, genus, and species groups are fully regulated by the provisions in the code. There is no limitation to the number of ranks allowed in the family group. The genus group has only two ranks: genus and subgenus. The species group has only two ranks: species and subspecies.
In the species group gender agreement applies. The name of a species, in two parts, a binomen, say, Loxodonta africana, and of a subspecies, in three parts, a trinomen, say Canis lupus albus, is in the form of a Latin phrase, and must be grammatically correct Latin. If the second part, the specific name (or the third part, the subspecific name) is adjectival in nature, its ending must agree in gender with the name of the genus. If it is a noun, or an arbitrary combination of letters, this does not apply.
- For instance, the generic name Equus is masculine; in the name Equus africanus the specific name africanus is an adjective, and its ending follows the gender of the generic name.
- In Equus zebra the specific name zebra is a noun, it may not be "corrected" to "Equus zebrus".
- In Equus quagga burchellii the subspecific name burchellii is a noun in the genitive case ("of Burchell").
If a species is moved, therefore, the spelling of an ending may need to change. If Gryllus migratorius is moved to the genus Locusta, it becomes Locusta migratoria. Confusion over Latin grammar has led to many incorrectly formed names appearing in print. An automated search may fail to find all the variant spellings of a given name (e.g., the spellings atra and ater may refer to the same species).
Many laymen, and some scientists, object to continued adherence to this rule, especially those who work with butterflies and moths. This is for historical reasons. In 1758, Linnæus placed all butterflies in the genus Papilio, which, after a few decades, contained thousands of species. From the beginning, the gender of Papilio was unclear, undecided, and disputed. Some authors regarded it as masculine, others as feminine. Linnæus knew this problem and avoided any statement. All his 250 specific names in the genus Papilio were either nouns, indeclinable adjectives, or adjectives ending in -is (which can be masculine or feminine but not neuter). He did not use a single adjective ending in -us, -a, -um. P. Brown, Cramer, Fabricius, Fueßlin, Goeze, Poda and Schrank regarded Papilio as masculine, Ménétriés, Pontoppidan and most modern authors as feminine. In ICZN Opinion 278 from 1954, it was regarded as masculine. In many cases lepidopterists would not change the ending of a name as used by the author who established a name. So we find for example Papilio fuscus or Papilio macilentus, but also Papilio osmana and Papilio paradoxa. Only in a few cases are both versions found in the Web (an example is Papilio multicaudatus and Papilio multicaudata). This works also with other butterfly genera of which the gender is undisputed. Graphium appears neuter, but only the inconsistent versions Graphium angolanus and Graphium mandarinus are used, while Graphium sandawanum can only be found with a neuter specific name. Likewise, pairs are more frequently found in genera of which the gender is not obvious: Delias castaneus and Delias gigantea, Belenois albumaculatus and Belenois rubrosignata, Mylothris arabicus and Mylothris ruandana. Even in moths, such pairs occur: Xylophanes obscurus and Xylophanes turbata, Manduca boliviana and Manduca caribbeus, Sphinx caligineus and Sphinx formosana, Macroglossum albolineata and Macroglossum vicinum. It may also occur that a lepidopteran subspecies can have a different gender from the name of the species, as for example in Papilio multicaudata pusillus Austin & Emmel, 1998, and Papilio torquatus flavida Oberthür, 1879.
Written nomenclatural rules in zoology were compiled in various countries since the late 1830s, such as Merton's Rules and Strickland's codes going back to 1843. At the first and second International Zoological Congresses (Paris 1889, Moscow 1892) zoologists saw the need to establish commonly accepted international rules for all disciplines and countries to replace conventions and unwritten rules that varied across disciplines, countries, and languages.
Compiling "International Rules on Zoological Nomenclature" was first proposed in 1895 in Leiden (3rd International Congress for Zoology) and officially published in three languages in 1905 (French, English, German; only French was official). From then on, amendments and modifications were subsequently passed by various zoological congresses (Boston 1907, Graz 1910, Monaco 1913, Budapest 1927, Padua 1930, Paris 1948, Copenhagen 1953, and London 1958). These were only published in English, and can only be found in the reports of these congresses or other official publications.
The 1905 rules became increasingly outdated. They soon sold out, and it became increasingly difficult to obtain to a complete set of the Rules with all amendments. In Copenhagen 1953 the French and English texts of the rules were declared of equivalent official force, and a declaration was approved to prepare a new compilation of the rules. In 1958, an Editorial Committee in London elaborated a completely new version of the nomenclatural rules, which were finally published as the first edition of the ICZN Code on 9 November 1961.
The second edition of the code (only weakly modified) came in 1963. The last zoological congress to deal with nomenclatural problems took place in Monte Carlo 1972, since by then the official zoological organs no longer derived power from zoological congresses. The third edition of the code came out in 1985. The present edition is the 4th edition, effective since 2000. These code editions were elaborated on by editorial committees appointed by the International Commission on Zoological Nomenclature. The ICZN Commission takes its power from a general biological congress (IUBS, International Union of Biological Sciences). The editorial committee for the fourth edition was composed of seven persons. Such new editions of the ICZN Code are not democratically approved by those taxonomists who are forced to follow the code's provisions, neither do taxonomists have the right to vote for the members of the commission or the editorial committee.
As the commission may alter the code (by declarations and amendments) without issuing a new edition of the book, the current edition does not necessarily contain the actual provision that applies in a particular case. The Code consists of the original text of the fourth edition and Declaration 44. The code is published in an English and a French version; both versions are official and equivalent in force, meaning, and authority. This means that if something in the English code is unclear or its interpretation ambiguous, the French version is decisive, and if there is something unclear in the French code, the English version is decisive.
The rules in the code apply to all users of zoological names. However, its provisions can be interpreted, waived, or modified in their application to a particular case when strict adherence would cause confusion. Such exceptions are not made by an individual scientist, no matter how well-respected within the field, but only by the International Commission on Zoological Nomenclature, acting on behalf of all zoologists. The commission takes such action in response to proposals submitted to it.
- Carl Linnaeus named the domestic cat Felis catus in 1758; Johann Christian Daniel von Schreber named the wildcat Felis silvestris in 1775. For taxonomists who consider these two kinds of cat a single species the principle of priority means that the species ought to be named F. catus, but in practice almost all biologists have used F. silvestris. In 2003, the commission issued a ruling (Opinion 2027) that "conserved the usage of 17 specific names based on wild species, which are pre-dated, by or contemporary with those based on domestic forms", confirming F. silvestris for the wild cat. Taxonomists who consider the domesticated cat the same species as the wild cat should use F. silvestris; taxonomists who consider the domesticated cat a subspecies of the wild cat should use F. silvestris catus; taxonomists who consider the domesticated cat a separate species should use F. catus.
The latest amendments enacted by the commission concern electronic publishing, which is now permitted for works published under an ISBN or ISSN after 2011 in a way that ensures registration with ZooBank as well as archival of multiple copies.
Local usage and name changes
The ICZN is used by the scientific community worldwide. Changes are governed by guidelines in the code. Local changes, such as the changes proposed by the Turkish government, are not recognised by ICZN.
The current (fourth edition) code is cited in scientific papers as ICZN (1999) and in reference lists as:-
ICZN 1999. International Code of Zoological Nomenclature. Fourth Edition. The International Trust for Zoological Nomenclature, London, UK. 306 pp.
- Strickland, H.E. [et al.] 1843. Report of a committee appointed "to consider of the rules by which the Nomenclature of Zoology may be established on a Uniform and Permanent Basis." ["The Strickland Code".] In: Report of 12th Meeting of the British Association for the Advancement of Science, June 1842, p. 105-121. BHL  . [Also published in the Philosophical Magazine and the Annals of Natural History.]
- Strickland, H.E. 1878. Rules for Zoological Nomenclature. John Murray, London. Internet Archive  .
- Blanchard, R., Maehrenthal, F. von & Stiles, C. W. 1905. Règles internationales de la nomenclature zoologique adoptées par les Congrès Internationaux de Zoologie. International Rules of Zoological Nomenclature. Internationale Regeln der Zoologischen Nomenklatur. Rudeval, Paris. Google Books  .
- ICZN. 1961. International Code of Zoological Nomenclature: adopted by the XV International Congress of Zoology. The International Trust for Zoological Nomenclature, London, UK. BHL  .
- ICZN. 1964. International Code of Zoological Nomenclature. Second edition. The International Trust for Zoological Nomenclature, London, UK. BHL  .
- ICZN. 1985. International Code of Zoological Nomenclature. Third edition. The International Trust for Zoological Nomenclature, London, UK. BHL  .
- ICZN. 1999. International Code of Zoological Nomenclature. Fourth edition. The International Trust for Zoological Nomenclature, London, UK. BHL  . The Code Online (ICZN)  .