Up in the treetops hides an entire unexplored verdant palace.
It’s easy to forget that there are spheres of existence that occur beyond what is directly observable–we could get poetic here and talk about the feeling of looking out into the stars or into the ocean’s abyss, but from a more ordinary standpoint, it’s easy to forget about life directly under our feet or above our line of sight.
In the Amazon with trees reaching several stories tall, your mind might wander to the birds and monkeys that so noisily call from above– but there are other animals that make use of the vertical gradient from the forest floor to the treetops that aren’t winged or . . . thumbed(?) and make for much less annoying roommates. We, of course, are led back to our fabulous amphibian friends. In this study I am here to take you on three different narratives between the lives and interactions of three different species of Neotropical frogs and provide you a brief introduction to a tadpole’s guide to the galaxy.
Illustrations by Andrius Pašukonis
In this study, we went through the jungle looking for babies.
Which is a great sentence on its own, but I’ll elaborate. Following babies is good for several reasons: first, it saves us the trouble from having to follow the parents which would (1) be exhausting and (2) a waste of time. By identifying tadpoles, we can say with certainty that at least one adult of that species has been at the location in a not-so-distant past. Secondly, these microhabitats (also called phytotelmata, also called pools) are . . . small. . . and thus, it’s quite easy to measure a ton of chemical aspects of the pool, identify the kind/density of predators, and cons/heterospecifics.
This gives us a really beautiful picture of the ecology of these habitats and also shines a light into the decisions parents make when deciding where to deposit their tadpoles. What is shaping deposition decisions in frogs, and does it change dramatically based on the natural history of the species?
You might be tempted to think that water accumulating in plants and trees and dead palms is created equal. But not to the frogs, friends. Not to the frogs.
A brief backgound: Allobates femoralis is a small poison frog that can’t climb for shit. They are great parents, where fathers will dutifully carry their babies to small pools of water throughout the jungle. Osteocephalus oophagus is an arboreal froggo, which means that they chill in the trees pretty much their whole lives. These guys are a bit different from the poison frogs: they deposit their eggs in small pools (into which the tadpoles then hatch)– then mamas will come and feed their babies unfertilized eggs as a nutritious snack throughout tadpole development!
Dendrobates tinctorius is a little amphibious spider money with amazing parenting skills and is just overall just the best creature out there. Their tadpoles are aggressive cannibals and survive an amazing range of chemical properties.
So, what did we find, climbing ’round the forest for two years?
Unsurprisingly, we see that A. femoralis dads deposit their babies on the ground, which is not all that surprising as adults can’t climb. O. oophagus, the arboreal frog, actually climbs down these humungous trees to deposit their egg clutches around 1.5-2m in height (6ish feet). We explore why parents might do this below. Finally, D. tinctorius can’t be bothered to care: from the forest floor to the tops of trees, dads will sometimes make huge energetic decisions regarding where to put their babies.
So, into what kinds of pools are these babies being deposited within their respective vertical columns?
Interestingly, for A. femoralis (grey points below) and O. oophagus (yellow points below), deposition decisions are pretty clearly be delineated by pool size and height– these decisions fit nicely within the context of the parental care strategies of both species.
We find that O. oophagus tadpoles almost exclusively occur in small phytotelmata with close to zero leaf litter (a variable that can help inform water turbidity); in other words: small, clear tiny pools. And this makes sense, right? Remember that mamas feed their babies trophic eggs, so clear water is probably important for the maintenance of potential feeding cues; also, feeding their babies means consistent, nutritious meals, which may be what allows them to choose these extremely small pools (higher risk of drying out counteracted by short development times from yummy eggs). Small pools also exclude other aquatic predators (such as dragonfly larvae), which is another advantage of choosing these dangerously small pools. A. femoralis tadpoles on the other hand occur in larger pools terrestrially– these pools can be big and murky– this means more predator risk, but also more food opportunity (i.e. detritus) resulting from the decomposing leaves, which is what tadpoles of these species primarily eat. Leaves and sticks in these pools also provide some much needed hiding places for these tadpoles to hang out and hide.
Again, here we are stumped by the amazing flexibility of D. tinctorius. Because these tadpoles appear to tolerate such vast conditions, we spend the rest of our time looking at the pool choices of this species to see if there was any particular condition they preferred.
Well, the story is not so simple– but in broad strokes here’s what we found: D. tinctorius tadpoles occur in higher densities in pools that occur higher in the treetops, these pools are also chemically distinct– they have higher alkalinity (KH), hardness, and salinity– which may all be important variables for tadpole growth and development (which is the next step to experimentally explore).
Overall, the beauty of this study are its contributions to natural history and behaviour of poorly studied species. I think one of the coolest parts of being a biologist is having the opportunity to work with the natural world, ask questions, and slowly untwine the nature of why things are and how they came to be. If you enjoyed this brief overview, feel free to take a look at our preprint which you can access here (now accepted at Ecology and Evolution, I’ll update this page once it’s out for all you die-hard fans 😉 ).