# Rarest Living Taxa **URL:** \ **Category:** Factual Questions\ **Created:** [September 19, 2004, 12:13am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921 "2004-09-19T00:13:59Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [September 19, 2004, 12:13am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/1 "2004-09-19T00:13:59Z")
Reading some of the recent threads on zoology, the thought came to me: what are the rarest living vertebrate and tetrapod species, genera, families, orders, etc.? E.g., not just the rarest living species of bird or reptile, but the family among the vertebrates that is the closest to extinction, the rarest species of artiodactyl, the rarest family of rodents, etc. I envision this, not as a single question that admits of a clear answer, but as a kind of catch-all for interesting facts related to the basic question which people have run into. I’m particularly interested in which higher-order taxa are “hanging on by a thread,” so to speak. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [September 19, 2004, 12:51am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/2 "2004-09-19T00:51:12Z")
For birds, it’s probably the family Rhynochetidae. The single member, the Kagu of New Caledonia, is considered Endangered, with a world population of under 1000. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [September 19, 2004, 1:08am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/3 "2004-09-19T01:08:45Z")
As far as the rarest _species_ of bird, that’s a bit tougher, since when they get very rare it’s very hard to be sure just how many there are. However, rarest bird for which we may have a reasonable estimate of number is the [Po’ouli](http://www.hawaii.gov/dlnr/dofaw/pubs/endgrspp/) (_Melamprosops phaeosoma_) of Hawaii, which seems to be down to three individuals. This would also be the rarest genus of birds, since it’s monotypic. ---
**Author:** ![Ilsa\_Lund](https://avatars.discourse-cdn.com/v4/letter/i/a6a055/32.png) [@Ilsa\_Lund](https://boards.straightdope.com/u/Ilsa_Lund)\ **Post date:** [September 19, 2004, 1:16am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/4 "2004-09-19T01:16:47Z")
The rarest species of snake is most likely the Antiguan Racer (_Alsophis antigua_), restricted to an island around 60mi[sup]2[/sup]. The rarest lizard species is likely the Giant Gomeran lizard (_Gallotia gomerana_), restricted to one of the small Canary Islands, numbering (accordong to best estimates) around 150 individuals. The entire order Rhynchocephalia, monogeneric and having two (possibly three) species, is restricted to a few islands of the southern coast of New Zealand. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [September 19, 2004, 1:20am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/5 "2004-09-19T01:20:01Z")
For mammals, the rarest family is probably the Lipotidae, a monotypic family containing only the Whitefin Dolphin _Lipotes vexillifer_ of the Yangtze River in China, which may be down to a few hundred individuals. (This is assuming that the Thylacine, family Thylacinidae, is extinct.) ---
**Author:** ![Ilsa\_Lund](https://avatars.discourse-cdn.com/v4/letter/i/a6a055/32.png) [@Ilsa\_Lund](https://boards.straightdope.com/u/Ilsa_Lund)\ **Post date:** [September 19, 2004, 1:22am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/6 "2004-09-19T01:22:39Z")
The St. Lucian Racer (_Liophis ornatus_) may also vie for the title [world’s rarest snake.](http://www.ahs.org.au/news_lnk.php?name=rare.txt) ---
**Author:** ![Ilsa\_Lund](https://avatars.discourse-cdn.com/v4/letter/i/a6a055/32.png) [@Ilsa\_Lund](https://boards.straightdope.com/u/Ilsa_Lund)\ **Post date:** [September 19, 2004, 1:33am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/7 "2004-09-19T01:33:21Z")
The rarest order of amphibians is Gymnophiona, the Caecilians, numbering around 160 species. Many species are in competition for the title “rarest amphibian,” including the Majorcan Midwife toad (_Alytes muletensis_), Hamilton’s frog of New Zealand (_Leiopelma hamiltoni_) and the Axolotl of Central America (_Ambystoma mexicanum_). ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [September 19, 2004, 1:39am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/8 "2004-09-19T01:39:49Z")
The rarest Order of birds I suppose would have to be the Opisthocomiformes, containing only the Hoatzin (unless you include it in the Galliformes or Cuculiformes, as is sometimes done). However, the Hoatzin is widespread and pretty common. Other small bird orders are the Loons (Gaviiformes) with 5 species, Flamingos (Phoenicopteriformes) with 5, and Mousebirds (Coliiformes) with 6. However, these all include at least some common and widespread species. ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [September 19, 2004, 1:43am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/9 "2004-09-19T01:43:55Z")
The order which contains coelacanths (Coelacanthiformes) contains only one genus (_Latimeria_), and it is thought that there are perhaps only about 300 individuals still around (as of 1995, anyway). Although, I’m not sure that “rarest taxon” really has any meaning (above the species level, anyway), since there is no guidline that any given taxon must contain any specific number of lower taxa, and citing a list of species within a taxon doesn’t give any indication of the relative health of various species within that taxon. After all, there’s only one extant species in the genus _Homo_, for example, and it appears to be doing quite well for the moment. You can get a general feel for the numbers of endangered, threatened, or otherwise vulnerable taxa in various groups [here](http://www.redlist.org/info/tables/table4a.html), though. But, again, those numbers tell us nothing of the relative health, or the total populations, of those groups as a whole. The most likely candidates would be any “critical”, mono-specific groups. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [September 19, 2004, 1:47am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/10 "2004-09-19T01:47:21Z")
My guess for the mammals would be the family Daubentoniidae, consisting of one species, the [Aye-aye.](http://en.wikipedia.org/wiki/Daubentoniidae) Population estimates vary wildly, but I’ve seen estimates as low as 50 surviving individuals. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [September 19, 2004, 1:51am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/11 "2004-09-19T01:51:44Z")
Here’s a list of [the world’s rarest mammals](http://www.animalinfo.org/rarest.htm). Looks like the Whitefin Dolphin is a contender for that, too. Other very rare species include the Vancouver Island Marmot (rarest rodent, order Rodentia), Seychelles Sheath-tailed Bat (rarest bat, order Chiroptera), and Javan Rhino (rarest even-toed ungulate, order Perissodactyla). The other species on the list will of course be the rarest species of their respective orders. The most endangered Order of mammals is undoubtedly the elephants, Proboscidea, with three species, all of them declining in the wild. Although the order Tubilidentata, containing only the Aardvark, has only a single species, it is not in danger. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [September 19, 2004, 1:57am UTC](https://boards.straightdope.com/t/rarest-living-taxa/264921/12 "2004-09-19T01:57:17Z")
> [@MikeS](#): > > My guess for the mammals would be the family Daubentoniidae, consisting of one species, the [Aye-aye.](http://en.wikipedia.org/wiki/Daubentoniidae) Population estimates vary wildly, but I’ve seen estimates as low as 50 surviving individuals. That figure is based on older estimates from the 1960s-1970s. It’s been found to be more widespread than previously thought, and [more recent estimates](http://www.animalinfo.org/species/primate/daubmada.htm) give a range of 1,000-10,000.