February 6, 2017
The critically endangered Nimba
toad is long known for its exceptional reproductive biology. The females of
this unique species give live birth to fully developed juveniles, having for
nine months continuously provided nutrition to the foetuses in the womb
(matrotrophy). While live birth (viviparity) among frogs and toads is rather an
exception than a common characteristic, matrotrophy, in place of alternatives
such as the foetus being fed with yolk, unfertilized eggs, or smaller siblings,
is what makes the Nimba toad one of a kind.
However, more than 40 years of
research had not been comprehensively, accessibly and completely summarised.
The gap has recently been filled with a new paper, published in the open access
journal Zoosystematics and Evolution by German scientists Drs. Laura
Sandberger-Loua and Mark-Oliver Rödel, both affiliated with Museum für
Naturkunde, Berlin, and Dr. Hendrik Müller, Friedrich-Schiller-Universität
Jena.
Studying the phenomenon, the
scientists went through the literature published over four decades to gather
the scattered details. They have also discussed the relationship between the
toad's reproductive biology and its specific habitat of merely 4 km² of high
altitude grasslands located at a minimum of 1,200 m in the Nimba mountains,
West Africa.
The climate of the area is characterised
by a rainy season lasting from April to October and a dry season from November
to February/March. These seasons are found to determine the activity of the
Nimba toads. The amphibians are only active during the rainy season, when
they give birth to their young, mate, and then find shelter underground, where
they stay dormant during the dry season.
Visibly females can be
distinguished from male Nimba toads by their differing cloaca and often larger
size, compared to the males. Also, males show darker backs and, during most of
their adult life, nuptial pads on their thumbs, which look like spiky
swellings. This secondary sex characteristic, in its seasonal change linked to
spermatogenesis, is used by the males to grasp tightly the female while mating.
In this species mating occurs
without a copulatory organ. Instead, the sperm is transferred through
connection of the cloacae, where the male's swells and encloses the female's
cloaca. Furthermore, Nimba toads have a unique behavioural repertoire. Males
crouch on their front legs and as soon as the female moves, follow her and grab
her tightly in the groin. Due to the spiky nuptial pads, the males often injure
their partner.
Giving birth in Nimba toads may
take over two days, depending on the number of offspring, which can be up to 12
in older females - far fewer than the hundreds of eggs in most toad species.
While giving birth, a female assumes a unique "birthing posture" to
compensate for the lack of enough muscle power to expel juveniles. By the time
the juveniles are ready to be born, they have already taken up nearly all the
space in their mother's body. The scientists conclude that the offspring play
an active role in the process, as a juvenile toad's death midway in the oviduct
leads to the mother dying of sepsis.
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