Hamilton's frog, Leiopelma hamiltoni

T R O P I C A L C O N S E R V A N C Y30

Bruce Waldman

Department of ZoologyUniversity of CanterburyPrivate Bag 4800Christchurch NEW ZEALANDEmail bw@zool.canterbury.ac.nzWeb www.zool.canterbury.ac.nz/bw.htm

Imagine the frogs that lived during the time that dinosaursroamed the earth, nearly 200 million years ago, and they

would probably closely resemble living frogs today in thefamily Leiopelmatidae. The primitive morphologicalcharacteristics of the four extant species of Leiopelma frogsresemble those of fossils from the Jurassic era. Thephenotypically similar North American Tailed Frog,Ascaphus truei, is the only other living archaic frog.Together, these frogs represent the most basal lineage ofanuran (tailless) amphibians.

Of the four currently recognized species of New Zealandnative frogs, Hamilton’s Frog, Leiopelma hamiltoni, is therarest and most restricted in distribution. Previouslythought to have lived on both the North and South Islandsof New Zealand, the species now is restricted to a 600 m2

rock bank on Stephens Island in Cook Strait. Like all NewZealand native frogs, Hamilton’s Frog is cryptic, quiet, andnocturnal. This makes estimates of population sizedifficult. Although the habitat probably can support 150 -

200 frogs, recent censuses suggest that substantially fewerremain. Hamilton’s Frog is one of the most endangeredfrogs in the world.

Until recently, a second larger population of frogs onnearby Maud Island was considered to be the same species,but the Maud Island frog has been reclassified as a newspecies Leiopelma pakeka . Whether the geneticdifferentiation between the populations is sufficient tojustify their classification as distinct species is currentlybeing debated. Their appearance, life history, and ecologyall are similar.

Hamilton’s Frogs are small (up to 46 mm long), brown,terrestrial frogs of relatively slender build. Their tongues,unlike those of modern frogs, are hinged at the back of themouth so they lunge at prey with open mouths. They canclimb surprisingly well, and their toes are not webbed. Theyhave large eyes, lack the external eardrum that ischaracteristic of most modern frogs, and can respond tochemical cues produced by other individuals.

Hamilton’s Frogs live in damp habitats, usually hidden increvices during the day. They emerge onto the soil and lowvegetation during the evening especially under humidconditions. They are known to live 30 years or longer andprobably reproduce infrequently (their reproduction hasnever been observed in the wild). Females lay between 2and 19 large eggs (5-6 mm diameter) in moist or drydepressions which males sit over until hatching occurs.

Males guard larvae as they develop, andwhen larvae emerge from ruptured eggcapsules, they are already well advancedwith limbs and digits developed. Theythen climb onto the male where theycontinue to develop, firmly entrenchedon his back as he goes about hisactivities, until they become smallfroglets and hop off.

Habitat destruction is probablyresponsible, in large part , for therestricted distribution of Hamilton’sFrog. The arrival of humans in New

Hamilton’s frog, Leiopelma hamiltoniS P E C I E S B Y S P E C I E S

Hamilton’s frog,Leiopelma hamiltoni,

by Stephen Aitken

B I O D I V E R S I T Y 1 ( 3 ) 31

Zealand within the past 2000 years drasticallyaltered the landscape and biota that had evolvedin the absence of terrestrial mammals. Maori, thefirst human settlers, brought with them kiore(Polynesian rats) that may have devastatedpopulations of many native species includingfrogs. This threat continues to the present, greatlyexacerbated by the many exotic predatorsintroduced since European colonization. Somesmall offshore islands, such as Stephens Island, arefree of mammalian predators.

The low fertility and fecundity of Hamilton’s Frogare consistent with the possibility that they havegone through a genetic “bottleneck” and sufferreduced fitness due to a lack of genetic variation.Because of its highly restricted distribution, thespecies is vulnerable to potential catastrophicevents such as flood, fire, or novel pathogens.Plans are currently under consideration to transferpart of the population to another predator-freeisland. Given the small population size, however,the risks associated with translocations may beunacceptable. Methods to increase fecunditythrough captive rearing programmes need to bedeveloped.

FURTHER READINGBarnett, S. 2000. The trouble with frogs. Forest and Bird295:14-17.Bell, B.D. 1985. Development and parental care in theendemic New Zealand frogs in G. Grigg, R. Shine, and H.Ehmann (eds), Biology of Australasian Frogs and Reptiles.Surrey Beatty and Sons, Chipping Norton, New SouthWales. pp. 269-278Bell, B.D., C.H. Daugherty, and J. M. Hay. 1998.Leiopelma pakeka, n. sp. (Anura: Leiopelmatidae), a crypticspecies of frog from Maud Island, New Zealand, and areassessment of the conservation status of L. hamiltoni from

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Stephens Island. Journal of the Royal Society of NewZealand 28:39-54Brown, D. 1994. Transfer of Hamilton’s frog, Leiopelmahamiltoni, to a newly created habitat on Stephens Island,New Zealand. New Zealand Journal of Zoology 21:425-430.Holyoake, A., B. Waldman, and N.J. Gemmell. 1999. Are-examination of the species status of Leiopelma hamiltoniand L. pakeka. New Zealand Journal of Zoology 26:258.Newman, D.G. 1996. Native Frog (Leiopelma spp.)Recovery Plan. Threatened Species Recovery Plan 18.Department of Conservation, Wellington