At the right you see two mating treefrogs. They are Squirrel Treefrogs, Hyla squirella. The smaller male is on top. As you can see in the photograph, when frogs mate, the male grasps the female's trunk with his forelimbs. The technical name for this special kind of embrace is amplexus. A more general term for fertilization accomplished outside the female's body, and therefore including amplexus, is external fertilization. Frogs and toads don't have penises. During amplexus the female discharges eggs -- usually into water -- while the male sheds sperms over the eggs.
The picture below shows some Wood Frogs, Lithobates sylvatica, during amplexus. It was sent to us by Greg Scott in Wisconsin, who took the picture. Greg writes about it:
I photographed one unfortunate female ... who had been "attacked" by several males. Each had tried grasping her and in the process of each holding on to a different part of her body they had held her under water long enough that she had drowned.... but they were still holding on to various parts of her body.
In other words, at the left you see a dead female frog with several males trying to mate with various part of her body. How could such a crazy thing happen in nature?
Maybe the answer lies with a study on toads, not frogs, conducted by Dr. Susumu Ishii in Japan. He found that when adult male toads find any pliable object as large as an adult toad during the breeding season, they mount it and try to clasp it. If the object they have mounted doesn't respond they keep holding on for hours. If the object is a female ready to mate, then amplexus proceeds normally. However, if the object mounted is a female not ready to produce eggs, she will vibrate her body. When the clasping male feels the vibration, he suddenly stops clasping, jumps from the back, and leaves quickly. The situation may be similar with Wood Frogs, but only further studies will make that clear.
One thing is made clear by the following photo, however: That shows a male toad clasping a completely different species of frog -- a Green Frog. Since the Green Frog probably won't vibrate its body the way a female toad would to get the male toad to go away, the poor Green Frog may be stuck with the toad for some time!
At the right you see a female Squirrel Treefrog with her belly filled with eggs prior to mating. Here you see eggs through her tight belly skin. She had been in a parked car and apparently couldn't get to water where she could mate. One day the car was taken to the carwash and the frog hopped out, surely eager to encounter a male of her species, for she was obviously ready.
Some frogs don't need much water when they mate. Sometimes a little rain pooled in a curled leaf is all that's needed for a female to deposit her eggs there. When a frog's eggs first make contact with water their protective layers of jelly soak up the water and swell, forming a jelly-like blob.
At the right you see some frog eggs discovered in a drainage ditch near my home. The black specks are frog embryos suspended in the eggs' gelatinous mass, which is attached to grassblades. If you discover an egg mass, let it stay where it is but return to it regularly and watch what happens. The black specks will grow larger and larger and finally the eggs will "hatch" into tiny, legless tadpoles, sometimes called pollywogs, consisting of little more than heads and tails. Then they will wriggle out of the gelatinous mass and begin eating algae and other things in the water around them. As these tadpoles grow they will sprout legs and absorb their tail, and their diet will change gradually from a vegetarian one to a carnivorous one. Finally, if their puddle of water doesn't dry out so that all the tadpoles die, the newly formed frogs who have managed to escape hungry birds, raccoons and the like will hop away to begin their land lives.
Instead of laying masses of eggs as in the above picture, toads usually lay their eggs in strings such as is shown at the left. Typically such strings are attached to vegetation, but I found this one lying on moist sand. Toad eggs hatch into tiny black tadpoles, which weeks later metamorphose.
The picture below shows a tadpole about an inch long I captured and put into a jar, brought home, placed on my scanner, and scanned! The neat thing about this picture is that you can see not only the spaghetti-like intestines coiling around in the body part, but you can also see tiny back legs beginning to sprout at the base of the tail. Because the tadpole was out of water during the scanning, gravity twisted its body so that you see a side view of the tadpole's back half, but a view from directly below of the front half. Since this tadpole was already becoming a frog I knew that getting extra air as it lay for about 30 seconds on the scanner wouldn't hurt it, and indeed after the scanning I plopped him back into the jar and he was very lively as I carried him to a nearby pool where, if a raccoon doesn't get him, he may well have become a frog -- a Southern Leopard Frog, Lithobates pipiens, I think.
You can review books about frogs and toads available at Amazon.com by clicking here.