Desert spadefoot toad

The desert spadefoot toad (Notaden nichollsi) is a species of frog in the family Limnodynastidae. It is endemic to Australia. Its natural habitats are subtropical or tropical dry shrubland, subtropical or tropical dry lowland grassland, intermittent freshwater marshes, hot deserts, and temperate desert.

This species will burrow underground to survive in the hot, dehydrating arid interior of Australia and is considered to be nocturnal.

Distribution and habitat
The distribution of this species varies across a number of regions of Australia. It occurs throughout the southern Kimberley and Pilbara areas of Western Australia, extending to the southern half of the Northern Territory desert and reaching as far as Western Queensland.

The species is known to occupy specific bioregions, including the Humid Arnhem-Kimberly, Sub-Humid Arnhem Kimberly, Eyrean and Mitchell-grass.

In terms of habitat, this frog is typically found in deep sandy areas and soils within its observed range. This species demonstrates a preference for sandy habitats due to their high water potential and low moisture content, which are beneficial for water retention.

In the Gibson Desert of central Australia, this species has been consistently found in excavated sand dunes, further highlighting its preference for sandy habitats.



Taxonomy
Notaden nichollsi was originally described by George Howard Parker in 1940 and is classified under the family Limnodynastidae, commonly known as Australian ground frogs. The genus Notaden, to which this species belongs, was established by Albert Günther in 1873 based on his early observations.

The family Limnodynastidae is a diverse group of frogs found across Australia. They are characterised by their adaptation to various habitats, including terrestrial, burrowing, and aquatic environments. Notaden nichollsi is one of the notable species within this family, known for its unique behaviours and adaptations to arid desert conditions.

Ecology and behaviour
A notable aspect of the behaviour of this species includes the main call sounds produced by males during the breeding season. These calls have been described as a series of "oo-oo" sounds, reminiscent of the cooing of a pigeon. This vocalisation likely serves as a communication mechanism to attract females and establish territory.

To adapt to the extreme arid conditions, Notaden nichollsi has developed specialised adaptations, including the ability to burrow below the soil surface and undergo aestivation during periods of water shortages. Aestivation is a dormant state similar to hibernation, allowing the species to conserve energy and water during prolonged dry spells. This adaptation helps the frogs survive in environments where water availability is limited.

Unlike some other species within the Limnodynastidae family, Notaden nichollsi does not form a cocoon during aestivation. Instead, it relies on other mechanisms to minimise water loss. Research by Cartledge et al. (2006) suggests that the species' water balance is more closely linked to the hygric properties of the surrounding soil rather than the formation of a cocoon. The water potential of the soil plays a crucial role in the frog's ability to absorb water and reduce loss of body water through its skin. This adaptation allows the species to regulate its water balance effectively in response to soil moisture conditions.

Additionally, the ability to burrow below the soil surface provides this species with protection from extreme temperatures and predation. These burrows serve as shelters where the frogs can seek refuge during periods of extreme heat or dryness, as well as provide a safe environment for reproduction and the development of tadpoles. Notaden nichollsi, like other non-burrowing frogs, relies on mechanisms to maintain water balance. One key aspect is the regulation of the hormone arginine vasotocin, which acts as the primary antidiuretic hormone for most frogs. This hormone helps the frogs conserve water and maintain appropriate hydration levels.

Following rain events, the desert spadefoot is expected to emerge above ground. This is when the frogs will feed and engage in breeding activities.

Diet
Gut analysis of N. nichollsi found within proximity of Ethabuka Reserve just north of the Simpson Desert found it to consist entirely of termites and ants. This has been proposed to be correlated with the presence of ants throughout sand dunes and plains areas during the night time.

Reproduction
After periods of heavy rainfall, breeding occurs for this species in flooded areas characterised by sandy soils with clay-pans that fill with water. In these temporary water bodies, the tadpole development of Notaden nichollsi can occur remarkably quickly, taking as little as 30 days.

During this breeding period it has been proposed that male frogs emit their unique calls while floating within the water, attracting females for mating. These females may then lay spawn consisting of jelly-encased chains, containing up to 1000 eggs. This has yet to be verified by scientific literature.

The tadpoles of Notaden nichollsi exhibit a relatively rapid developmental process, completing their metamorphosis in just over two weeks.

While it is understood that reproduction in this species occurs in response to favourable environmental conditions, there are still some uncertainties and gaps in knowledge regarding the precise details of their reproductive biology. Specifically, the presence or absence of a well-defined tadpole stage in Notaden nichollsi is not yet fully documented or understood due to limited available information. Additional research is required to gain a comprehensive understanding of the reproductive mechanisms and developmental stages of this species.

Conservation
Notaden nichollsi is currently assessed as a species of Least Concern on the IUCN Red List of Threatened Species. This classification indicates that the species is not facing significant threats that would warrant a higher conservation status. However, it is important to continue monitoring the population and its habitat to ensure the species' long-term conservation. The main conservation focus for Notaden nichollsi lies in the protection and preservation of its specialised habitat. The species primarily inhabits sandy and clay soils in arid regions, where it relies on burrows for shelter and reproduction. Efforts should be made to conserve and manage these habitats, particularly by addressing factors such as habitat loss, degradation, and fragmentation caused by human activities such as land clearing and urbanisation.