Biodiversity

Saturday, April 29, 2006

PARENTAL CARING IN FROGS

I always thought that frogs and toads lay their eggs in water, where they spawn as tadpoles and then metamorphose into frogs (or toads). This group of amphibians have however developed many different kinds of reproductive strategies, all following the egg-tadpole-frog route, but not all opting for the “safety in numbers” strategy.

The “safety in numbers” strategy is purely where the female frog lays hundreds to thousands of eggs in water, but then abandons them. These eggs are then very vulnerable to predation, but the success lies in their numbers. And this strategy has been successful, as many frogs and toads living today, still use this strategy.

Some frogs however have gone a step further. In moist areas, like the South American rain forests some plants provide literal little ponds, as water is collected in their cup-like structure and it is here were some frogs have chosen a home. Here they lay their eggs and the little tadpoles have a secure environment to metamorphose into little frogs.

In Southern Africa there are several frog species that have found terrestrial breeding sites that are safe from predators. One such frog is the Shovel-nosed frog that digs a tunnel with a chamber in the muddy banks of pans, where the eggs are laid and fertilized (fertilization takes place externally in frogs). The female then stays with the eggs, which are kept moist by a rubbery mass. Once the tadpoles hatch, she digs a tunnel to the waters edge, the tadpoles crawl onto the females back and she carries them to the water, where they can the metamorphose into frogs. (Carruthers 2001, p.15)

There are several species of Leaf-folding Frogs, which use the perimeter of pans where grass has been inundated by shallow waters. Here the eggs are laid and the male then fertilizes them and also folds and glues the grass blade into a leaf-tube. As the tadpoles hatch, the glues softens, allowing them to drop into the water to complete their development into frogs. (Carruthers 2001, p.17) Other frogs use trees that hang over water bodies as a nesting site. The Foam Nest Frog female secretes a fluid, which she churns up with her hind legs into a foam ball. It is into this foam ball that the eggs are laid and then fertilized. The outer surface then hardens, but inside the eggs are protected and stay moist. Once the tadpoles have hatched, they break through the crust and drop into the water. (Carruthers 2001, p.20)

A Caribbean frog, the whistling frog, lays fairly large eggs that are hidden in moist areas beneath organic material. Here the tadpoles fully metamorphose inside the fluid filled eggs, feeding on the yoke. The little frogs have a tiny spike on their noses with which they pierce the eggs and then hatch.

Some frogs have become purely terrestrial as the several species of rain frogs. They spend most of their time in underground tunnels and only emerge when it rains. It is then in these underground tunnels that they also breed. Because the rain frogs are so short legged and round the male cannot mount (amplexus) the female and so she secretes a glue that glues them together. The glued pair the lays and fertilize the eggs in a chamber underground, where the tadpoles remain inside the eggs and feed off the egg yolk until they have metamorphosed into frogs. (Carruthers 2001, p.23) So, a strategy away from water has also proven to be successful.

Another amazing adaptation can be observed in the very scarce and localized Ghost Frogs. The tadpoles of this species metamorphose very slowly, due to the cold water of mountain streams and take up to a year. The tadpoles however have enormous mouths that span the whole width of their body. In this big mouth they have sixteen rows of teeth, which they use to pull themselves over rocks while eating off the algae. What makes it remarkable is that this is fast running water. They have even been observed climbing waterfalls, clinging to the algae covered rock surface with their teeth. This “fast running water” environment they metamorphose in is unique to ghost frogs. (Carruthers 2001, p.33)

Some frogs have taken it another step further and have become involved parents. The midwife toad, which is a misnomer as it is the male that does the caring, wraps the strings of eggs around his hind legs protecting them from predation, until the tadpoles hatch.

The pipa toad from Brazil has developed a truly amazing strategy of parental care. These toads are aquatic and as the eggs are laid and fertilized in the water, the male carefully manoeuvres each egg with sort of back-flip onto the females back, where they stick. Up to a hundred eggs can be stuck to her back. Her skin then swells, embedding the eggs and eventually covering them completely under her skin. The eggs hatch in about 3 weeks time, but the tadpoles stay under her skin for another 3 weeks before they emerge from under her skin.

Darwin’s frog from Chile has taken parental caring yet another step further. The male guards the eggs that have been laid on the forest floor until he sees movement in them. He then “eats” the eggs, storing up to 12 eggs in his vocal sack. There they develop through the tadpole stage into fully developed little frogs. The height of parental care however goes to a tiny little frog living on a remote mountain in West Africa. The female stores the fertilized eggs inside her extended oviducts. There they hatch and the tadpoles feed on little white flakes the female secretes internally. For 9 month she carries her young until the next rainy season, when she “gives birth” to fully developed little frogs.


References:

Carruthers V. 2001. Frogs and Frogging in Southern Africa. Cape Town: Struik Publishers (Pty) Ltd

The rest of the information was taken from the 6th episode of the BBC Life on Earth series, by David Attenborough.

Karen Marais
BCB Hons NISL student
University of the Western Cape

5 Comments:

  • Hi Richard

    This is the one I did from memory...

    Cheers,
    Karen

    By Blogger Karen Marais, at May 01, 2006 6:13 PM  

  • Okay...not bad for having been done from memory (incidentally, why?). Very light on references, presumably a a result of the above. Nice coverage of the diversity of parental systems. but some exploration of the evolutionary or ecological principles underlying parental strategy would've been nice.

    Nick

    By Blogger NcK, at May 17, 2006 2:19 PM  

  • Hi Nick

    There were problems with the net that week-end and I did not want to get behind schedule... Wasn't able to get to other refernces apart from the books I had at home.

    Thanks for the positive report and suggestions!

    Karen

    By Blogger Karen Marais, at May 17, 2006 4:46 PM  

  • Hi Karen

    Where did you learn about the frogs? writting this assignment using your memory proved that you really understand everything about the frogs. Have you learned about them at home or school or where?

    That was very informative work, keep the good work up, Karen.

    By Blogger Maleka Evelyn, at May 22, 2006 9:07 AM  

  • your work is very good Karen keep it up.

    By Blogger lethabo, at May 23, 2006 2:27 PM  

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