Amazon Milk Frog

An Amazon Milk Frog. Image taken at the Oakland Zoo.

This little froggy is an Amazon Milk Frog. No wait, it’s a Mission Golden-eyed Tree Frog. No wait, it’s a Blue Milk Frog. Oh no, I was wrong on all accounts. What this frog is, is a perfect example for why scientists use scientific names rather than common names. (Except often with birds, since the common names are relatively standardized.) So, this frog is Trachycephalus resinifictrix. But for now, we’ll call him an Amazon Milk Frog. Why milk frog? Well, it’s a fitting name for an animal that oozes out poisonous white milky stuff through its skin when it’s threatened.

There is dramatic variation in the life cycles of frogs and  often it is how the amphibians bring about the next generation that mesmerizes herpetologists. This particular frog is no exception; he has an interesting story to tell.

It all starts about an hour after sunset. Mr. male Amazon Milk Frog, who lives high up in the trees, begins to call. He is seeking a female and until one appears, he will keep calling and calling. He might end up making 4,000 calling notes in one night. But, once he attracts a female they will mate and she will deposit up to 4,000 eggs (average is less than 3,000 ) into his water-filled tree hole. She leaves in the morning, but dad is not done. The eggs will hatch in less than 24 hours (!) and there are a lot of hungry tadpoles to feed. At first, they’ll eat whatever detritus they can find, but this won’t last.

So, Mr. dad Amazon Milk Frog will begin to call for a female again. Why is he calling again? He already has more mouths than he can feed! But it turns out, there is method to this madness. When the next female comes, he will not fertilize her eggs when she deposits them. Instead, they become food for his already hatched tadpoles to eat and grow on.

Amazon milk frog

An Amazon Milk Frog, front view, eyes partially covered. Image taken at the Oakland Zoo.

And so you might wonder, why would this deception evolve? What problems are being solved by this method of reproduction and parental care? To try to figure this out, we have to look at what makes this frog different from other frogs. And the answer is simple: his home. Other frogs more typically use ponds and streams as their breeding sites. But living in the rain forest means that there are many more temporary bodies of water that can be used for breeding. These sites offer a few advantages- the most important being way less competition and fewer predators. They also don’t have to worry about currents. However, in order to exploit these potential breeding sites, they have to overcome some obstacles. First, the water in these plant structures can dry out quickly. So, for a frog to use this area to reproduce,  the young have to hatch and develop quickly, to ensure they can leave the water before the water leaves them. The 24 hour hatch time is an important adaptation that allows this frog to remain in the trees. The second problem is a lack of food. This frog species solves that problem by having females lay nutritious eggs for the tadpoles to eat.

Of course, other frog species solve this same problem without the deception. A close relative of this frog, T. hadroceps (no English common name), solves this problem with parental care from the genetic mom to her own offspring. She returns to deposit unfertilized eggs for her tadpoles every 2-3 days.

Either way, these behaviors evolved to solve a basic problem and to exploit a relatively predator and competitor free breeding site, that allows these frogs to spend their entire life up in the trees.

Amazon milk frog

An Amazon Milk Frog resting on a tree branch. Image taken at the Oakland Zoo.

Once again, I can’t talk about frogs without mentioning the world wide amphibian decline and to recommend visiting Save the Frogs for more information on the extinction threat many frog species are facing and to find out what you can do to help.