Have you ever found a frog in your toilet? Or perhaps had one living in your water tank? If you live in Queensland, New South Wales or in the tropics, then you would probably be very familiar with the green tree frog (Litoria caerulea). You may of also heard the strumming of the pobblebonk or banjo frog (Limnodynastes sp.). Frogs often go unnoticed as many are minute, quiet and have secretive behaviours. There are more than 6200 species of frogs recorded and are present on all continents bar Antarctica (Chai 2014). We have 241 species of native frogs in Australia. These species of are divided into six frog families in Australia:
- Hylidae – tree frogs
- Myobatrichidae – Southern frogs
- Limnodynastiae – ground and burrowing frogs
- Ranidae – true frogs
- Microhylidae – narrow-mouthed frogs
- Bufonidae (introduced true frogs
What makes a frog a frog?
A frog is an animal that belongs to the order Anura. Anura in Latin means no tail which is very apt as Anuran adults lack tails. There are adult frogs that do appear to have tails but in fact it is a part of their reproductive system. They also share the following characteristics:
- can blink
- covered ear opening
- small teeth on their upper jaw
- a tongue so long it requires folding to be housed in its mouth
- their body temperature is dependant on their environment (poikiothermic)
The life cycle of a frog depends very much on its species. Does it start its life as a tadpole or a froglet? Are the eggs laid in water or on land? Does the adult frog climb trees or does it burrow?
Poikiotherm means many, varied temperature. A poikiotherm’s environment determines their body temperature and behaviour. Temperature will affect: vocalisation, muscle contraction, movement and digestion. An undesirable temperature leads to: slothfulness, hibernation, torpor, immunosuppression, bacterial infection, weight loss, agitation and death.
Frogs have a relatively simple digestive system. Their excretory system is a little more complex. This is necessary due to their need to conserve water as their skin readily allows water to escape their body. Their skin is ‘semi-permeable’. This allows a frequent excretion of chemicals such as hormones and oxygen (they can breathe through their skin). They even have the unique ability to absorb water through their hip area. They are also capable of decreasing water loss by slowing down the rate of the kidney filtration. This does have the negative effect of ammonia building up in the tissues.
Frogs have a novel method of respiration. Adult frogs are capable of breathing through their skin but they do also have lungs. Whereas tadpoles utilise gills to breathe. While froglets are still encased in their egg, they use their vascular tail for oxygen transfer.
The diet of a frog depends on what stage of the life cycle it is in. Is it an egg, a tadpole, a froglet or a frog? Tadpoles are either herbivores (eat plant matter) or are omnivorous (eat anything in front of them). Adult frogs are exclusively carnivorous. They prey upon: invertebrates, mice, fish and other small vertebrates. Land dwelling frogs eat only prey that moves. Aquatic frogs use scent to determine their choice of food.
Internal Body Guard
The immune system in frogs is very important. In humans, the skin helps keep out foreign materials and organisms. Frogs skin however, is semi-permeable which means that pathogenic organisms can enter their body. To fight this, their skin produces antimicrobial chemicals to provide protection. There are instances of these secretions containing irritants or lethal compounds to discourage predation. Their liver is also critically involved in the immune system. It can: break down materials; synthesis nitrogenous compounds and produces antioxidation reactions.
Frogs reproduce through amplexus. Amplexus is, “the pairing of adult frogs prior to and during egg-laying” (Anstis 2013, p17). Unlike most mammal reproduction, amplexus is undertaken external to the body (with exception to Ascaphus sp.). Amplexus is initiated by: rainfall; temperature; food availability and/or a change in daylight hours. It occurs near to where the eggs will be laid which is usually in water or at least a moist environment.
When it is time to breed, male frogs are sent into a frenzy. They desperately want to mate with a female. Some individuals have been observed attempting to mate with a rock or branch during this period.
Before amplexus can begin, the male must court the female frog. He does this by repeatedly calling and sending hormonal signals to the female. Once a female has been attracted he will often climb onto her back. The female then lays the eggs and he fertilises them after they have been laid.
Eggs and Embryos
Anuran eggs have a high degree of variation and there are twenty-three designations of eggs in Australia. The categories are: aquatic, non-foamy; aquatic, foamy or with bubbles that reduce; semi-terrestrial, foamy; terrestrial, non-foamy and para-viviparous. Non-foamy eggs lack bubbles in the jelly. Foamy eggs have bubbles in the jelly which occur by the female beating the egg jelly repeatedly. Egg are laid in clutches, tiers, layers, strings, rosary, strands or membranous cord.
There are 46 stages of tadpole development! This means that for an egg to turn into a frog there are 46 steps it needs to take. There are exceptions to this. For example, Myobatrachus gouldii (turtle frog) does not live as a tadpole but instead emerges from its egg as a fully formed frog. To cope with these differences, tadpoles have been classified into 23 separate groups according to their method of development.
Tadpoles can also be classified according their mouth type. The type of mouth determines how the tadpole can feed and where it lives. Tadpoles that live in rapidly flowing water need a mouth that can firmly grasp and suction onto rocks so they do not get washed downstream whilst feeding. There are in fact 20 separate types of tadpole mouths.
The ARAZPA Amphibian Action Plan (Gillespie et al. 2007) states that of the 219 anuran species native to Australia 27% face the threat of extinction (Gillespie et al. 2007) There are 33 species categorised as either endangered or critically endangered (Gillespie et al. 2007).
The threat of habitat loss or change of habitat is an issue for many organisms including anurans. Changes of habitat include: urban development; deforestation; climate change; environmental pollution (i.e. herbicides, pesticides and fungicides); change in the water table and drainage of coastal swamps and wetlands. The long term effect of environmental pollutants on anurans is at yet unknown.