5. Reproduction
Alii amphibians undergo a period of larval development which usually takes place
in pools of water. Some amphibians, especially certain urodeles, exhibit varying
degrees of viviparity, but this is usually in species occupying cold alpine areas and
seems to be an adaptation to low prevailing temperatures, rather than to a lack of
water. Embryonic, and even larval, development can occur on the land, as seen
in some Australian leptodactylid frogs. Such frogs, nevertheless, require a damp
situation in which to lay their eggs and so do not inhabit arid areas (MAIN et al.,
1959).
Frogs living in regions where water is only sporadically available , can breed
only when suitable rain falls. In most desert areas, like those in the centre of Australia, rain is not only sparse but also unpredictable. Leptodacrylid frogs in the
genus Heleioporus, which live in more temperate areas where rain falls more consistently, have a precisely timed breeding season that coincides with the rain . However, other members of this family , like some species of Neobatrachus, which live
in less favourable areas, only come into a reproductive condition when the rainfall
and temperature are suitable. These species are called 'opportunistic breeders'
(MAIN et al., 1959). This pattern of reproductive behaviour is the same as that observed in the budgerygahs and zebra finches which also live in central Australia.
Reproduction is a special endocrine problem and it would be interesting to know
something about the mechanisms which can so promptly initiate breeding.
When the eggs of amphibians are deposited and fertilized, they undergo a period
of embryonic development and then on hatching, the larvae grow and eventually
differentiate and metamorphose into the adult form. In most amphibians this process may take many months, such as in the bullfrog in which it takes more than
a year. In desert areas in which such a prolonged embryonic and larval life is usually
not possible it is far shorter, often only a few weeks. Young, but adult, Scaphiopus
couchi have been observed in the field only four weeks after rain has fallen , while
tadpoles, at an advanced stage of development (with fore and hind limbs), have
been seen after only 10 days (MAYHEW, 1965). Just what are the factors, other than
genetic, that influence the rate of development in such amphibians is not clear. The
propensity of the thyroid to accelerate amphibian metamorphosis suggests that it
may be involved. Environmental conditions such as temperature, food supplies and
possibly the chemical (osmotic?) constitution of the pond undoubtedly influence
the rate of larval growth.
When certain newts prepare to breed, they may undergo a morphological transition and return to an aquatic life. This has been likened to a second metamorphosis. The timing of these changes and the return of such newts to water is influenced by a hormone from the adenohypophysis. The life cycle of the eastern
spotted newt, Diemictylus (also called Triturus) viridescens, takes place in three
stages; an aquatic larval phase, followed by a terrestrial life which lasts from 2 to
4 years (the 'red eft' stage) and finally a return to water as sexual maturity approaches. REINKE and CHADWICK in 1939 showed that when pituitaries from the
sexually mature newts were transplanted into the immature red efts they were induced to seek water ('water drive') and to assume their reproductive form. This
effect also occurred after the injection of mammalian prolactin, even after removal
186
Alii amphibians undergo a period of larval development which usually takes place
in pools of water. Some amphibians, especially certain urodeles, exhibit varying
degrees of viviparity, but this is usually in species occupying cold alpine areas and
seems to be an adaptation to low prevailing temperatures, rather than to a lack of
water. Embryonic, and even larval, development can occur on the land, as seen
in some Australian leptodactylid frogs. Such frogs, nevertheless, require a damp
situation in which to lay their eggs and so do not inhabit arid areas (MAIN et al.,
1959).
Frogs living in regions where water is only sporadically available , can breed
only when suitable rain falls. In most desert areas, like those in the centre of Australia, rain is not only sparse but also unpredictable. Leptodacrylid frogs in the
genus Heleioporus, which live in more temperate areas where rain falls more consistently, have a precisely timed breeding season that coincides with the rain . However, other members of this family , like some species of Neobatrachus, which live
in less favourable areas, only come into a reproductive condition when the rainfall
and temperature are suitable. These species are called 'opportunistic breeders'
(MAIN et al., 1959). This pattern of reproductive behaviour is the same as that observed in the budgerygahs and zebra finches which also live in central Australia.
Reproduction is a special endocrine problem and it would be interesting to know
something about the mechanisms which can so promptly initiate breeding.
When the eggs of amphibians are deposited and fertilized, they undergo a period
of embryonic development and then on hatching, the larvae grow and eventually
differentiate and metamorphose into the adult form. In most amphibians this process may take many months, such as in the bullfrog in which it takes more than
a year. In desert areas in which such a prolonged embryonic and larval life is usually
not possible it is far shorter, often only a few weeks. Young, but adult, Scaphiopus
couchi have been observed in the field only four weeks after rain has fallen , while
tadpoles, at an advanced stage of development (with fore and hind limbs), have
been seen after only 10 days (MAYHEW, 1965). Just what are the factors, other than
genetic, that influence the rate of development in such amphibians is not clear. The
propensity of the thyroid to accelerate amphibian metamorphosis suggests that it
may be involved. Environmental conditions such as temperature, food supplies and
possibly the chemical (osmotic?) constitution of the pond undoubtedly influence
the rate of larval growth.
When certain newts prepare to breed, they may undergo a morphological transition and return to an aquatic life. This has been likened to a second metamorphosis. The timing of these changes and the return of such newts to water is influenced by a hormone from the adenohypophysis. The life cycle of the eastern
spotted newt, Diemictylus (also called Triturus) viridescens, takes place in three
stages; an aquatic larval phase, followed by a terrestrial life which lasts from 2 to
4 years (the 'red eft' stage) and finally a return to water as sexual maturity approaches. REINKE and CHADWICK in 1939 showed that when pituitaries from the
sexually mature newts were transplanted into the immature red efts they were induced to seek water ('water drive') and to assume their reproductive form. This
effect also occurred after the injection of mammalian prolactin, even after removal
186