Saturday, April 29, 2006


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.


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


All animals with a backbone or a stiffened rod (notochord) are part of the phylum Chordata. Cartilaginous fish or Chondrichthytes (sharks, rays and skates) do not have a bony skeleton, but rather as their name suggests a lighter cartilaginous one. Osteichthytes are the bony fish. From fossil records it seems as if the link between these two classes are the placoderms or plate-skinned archaic jawed fishes, which had a combination of cartilage and bone. They evolved in the Silurian period. But it was in the Devonian period (about 400-320 million years ago), when the big split in the fish dynasty must have taken place.

It is however not only cartilage that makes the Chondrichthytes different from their bony cousins. Unlike other fishes that fertilize their eggs outside the body, these fishes copulate and fertilization takes place internally. (Males can be easily distinguished from females by their claspers, the Chondrichthytes version of a penis). (1) They then either give birth to live pups or they may lay leathery-shelled eggs out of which the pups then hatch. (2, 3, 4) Another interesting difference is that they have relatively large brains relative to their bodies and relative to bony fishes. (2) One of the explanations of why their brains are so large is that the nerve cell density is much lower than in bony fish, thus using less energy and allowing the brain to be bigger. (2)

Cartilageous fish need to produce their red blood cells elsewhere as they do not have bone marrow. The spleen, special tissue around the gonads (organs that produce reproductive cells) as well as a special organ called Leydig's Organ is responsible for red cell production. (2) Another organ unique to these fish is the epigonal organ . Its role is not clear but it might have a role in the immune system. (2)

They also have a spiracle behind each eye. It is used to pump water through the gills when these fish are at rest. (5) This brings me to another major difference. Because the cartilaginous fishes do not have a swim bladder, they do not have much buoyancy and need to keep swimming to prevent them from sinking. Sharks, rays and skates store large quantities of oil in their livers to improve their buoyancy (1), but it is not enough to let them hover in one place. They thus keep on the move and their large pectoral fins provide them with lift, much in the same way as the wings of an airplane, provided a minimum speed is maintained. (1) Sharks cannot suddenly stop due to their rather rigid anatomy. They use their tail for propulsion, whereas rays and skates use their modified pectoral fins, which are attached to their heads and are quite flexible. Their reduced tail is mainly used as a rudder. (3, 4) The lack of buoyancy has basically limited the cartilaginous fishes to a limited number of body shapes and to basically two lifestyles. The ever-moving hunters (although some are filter feeders) and the seabed prowlers. Their skin is also quite different to most bony fishes, by not having true scales, but rather dermal teeth that make their skin feel like sandpaper. (2) Lastly their eyes cannot distinguish colour (that is why fishes in this group are mainly drab in colour), although they can see well and are especially well adapted to seeing in darker waters.

The diversity of bony fishes is extraordinary compared to the cartilaginous fishes. This is mainly because of their maintenance of buoyancy through their swim bladder. The earliest bony fishes had both lungs and gills that came in handy during periods of drought, but today only two species of fish still have true lungs. (6) Most bony fishes also do not need to gulp air anymore to fill their swim bladders as they have become closed off and are filled with gas which is secreted by special glands. Other glands again can absorb gas, thereby controlling the amount of gas in the swim bladder. They could now regulate at what height they wanted to stay afloat. (6) This has allowed the bony fishes to start using their fins and tail for other maneuvers. They now had the freedom to adapt to many new environments, taking on a great variety of body shapes and fin shapes, allowing them to brake, swim backwards and generally becoming more maneuverable and agile. They have managed to fill every niche in the ocean. (6) Their eyes can also distinguish colour and many bony fishes have therefore become very colourful. Bony fishes also have an operculum covering the gills on either side. This allows them to breathe even when they are stationary. (7) The bony fishes can also be divided into two groups, the ray-finned fish and the lobe-finned fish, with the ray-finned being by far the majority of all fishes found in the ocean and in freshwater. The lobe-finned fish only have 7 living species of which the coelacanths are one. (7)

1. Wikipedia contributors. Shark [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 27, 17:58 UTC [cited 2006 Apr 28]. Available from:
2. Wikipedia contributors. Chondrichthyes [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 26, 21:29 UTC [cited 2006 Apr 28]. Available from:
3. Wikipedia contributors. Batoidea [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 29, 23:26 UTC [cited 2006 Apr 28]. Available from:
4. Wikipedia contributors. Rajiformes [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 4, 13:27 UTC [cited 2006 Apr 28]. Available from:
5. Wikipedia contributors. Spiracle [Internet]. Wikipedia, The Free Encyclopedia; 2006 Mar 11, 00:14 UTC [cited 2006 Apr 28]. Available from:
6. Reader’s Digest Editors. 1974. Animal Families. Marvels and mysteries of animal behaviour. Pages 200-211 in Animals in Action. Hong Kong: Reader’s Digest Association Far East Limited.
7. Wikipedia contributors. Osteichthyes [Internet]. Wikipedia, The Free Encyclopedia; 2006 Apr 24, 09:37 UTC [cited 2006 Apr 29]. Available from:

Karen Marais
BCB Hons NISL student
University of the Western Cape
Private Bag X17