Frogs are Amphibians
**This versions of blog post were originally posted on birdhard and reserva las tangaras in collaboration with Juliana Smith.
Juliana and I have concluded our second month here in the cloud forests of Mindo, Ecuador. The month of February rained more than we could have expected. The rainfall totaled at 591.20 mm, higher than the last three Februarys here at Las Tangaras. It is no surprise that this soaking creates an amphibian paradise. Mindo, and its plentiful rain seasons, harbor one of the most unique and biodiverse collection of frogs in the world. These frogs have taught me so much I did not know and captivated me throughout our stay. Here is one way analyze the anuran diversity in the Cloud Forests of Ecuador:
Wet conditions are often associated with amphibians because water is needed for reproduction. Most amphibians start their lives in the water and bear little (if any) resemblance to their adult form. They then undergo a drastic transformation¹, called metamorphosis, providing them with characteristics and tools to help them survive on land. From here on out, there is a varying amount of dependency each species has on water, but most are restricted to reproduction in an aquatic habitat². Many frogs exemplify this process by laying their eggs in the water, which then hatch into a strictly aquatic form. This form is commonly known as a tadpole— essentially a large round head with a tail used for aquatic propulsion. When they reach a certain point in their juvenile lives, they will begin to change. They begin to trade their tails for legs and hop into the frog form we are most familiar with.
There is a ton involved in this process and I am over simplifying and generalizing for brevity. But what is important to note is this is the primitive reproduction method for amphibians. It has existed in frogs for approximately 220 million years and is still the predominate form of reproduction today. Around 50% of all anurans (frogs) use this method of water eggs > tadpole > metamorphosis > terrestrial (or semi-terrestrial) adult³. Clearly this is a successful strategy and has some huge benefits. For one, problems of water loss during embryonic development are nonexistent because the eggs are laid in the water. Also, restricting juveniles and adults to their own individual environments prevents competition for food amongst the same species. Juveniles can eat for growth in the water, adults can eat to nourish reproduction on land, and neither has to do so at the expense of the other.
This development strategy is used by most of the frogs I work with in South Carolina and it is also the strategy used by several species here in the Cloud Forest of Ecuador:
The Babbling Torenteer (Hyloscritus alytolylax). A Hylid (treefrog) whose name indicates the rage of the waters it breeds near. This frog will mate near rivers or waterfalls and then lay eggs in the small pools that form on the edges of these water systems. The photo above pictures a metamorph— This froglet recently underwent metamorphosis, came on land, but still possesses some of its tail.
The Executioner Clown Frog (Dendropsophus carnifex)4 congregates in low lying areas and reproduces in the standing or slowly flowing water that collects from heavy rainfall. At Las Tangaras, these frogs are most commonly spotted at the start of the Motmot Trail. We frequently find them in a flattened area that serves as a water catchment from the hillsides surrounding it.
Though the above frogs are incredibly fascinating, I am certainly not constructing this article to discuss the common reproductive methods of amphibians. The other half of the world’s frog species are groups that are new to me. These include most of the frog species that surround us. They have a whole mess of tweaks they've added to this traditional reproductive strategy. All are designed to help them cope in unique environments.
These frogs lessen their need for a stable and consistent water source by laying their eggs on land. Amphibians, even terrestrial egg layers, lay eggs devout of a hard outer membrane and thus, are extremely vulnerable to water loss. Amniotes, such as birds and reptiles, lay eggs on land with a hard outer shell to prevent dehydration. Amphibians who laid their eggs in water had no need to expend the energy needed to create an impermeable membrane. So to lay eggs on land, amphibians needed to come up with a way to overcome this hurdle. They also needed to get the hatchling tadpoles back to the water. The cloud forests of Ecuador is certainly not a bad place to attempt this feat.
An analysis of many of the worlds frog species has shown terrestrial egg layers lay larger eggs. The larger surface to volume ratio helps decrease water loss5 (as elephants can retain heat better than mice). Furthermore, most terrestrial eggs are packed closer together, further decreasing the area for water to dissipate. Seems simple, but it has come at a cost. Because of their larger size, more nutrients and energy has to be allocated for the development of these eggs. So the clutch size is often less and the average adult size of terrestrial egg-laying frogs is also smaller compared to aquatic egg-layers.
An example of one of the frogs who lays their eggs on land is the Emerald Glassfrog. A very common frog found at Las Tangaras along the river embankments. Males will vocalize at night with the hopes of attracting a female. Following this attraction and the deeds that come with it, females will lay their eggs on vegetation just above the water edge. When the eggs hatch, out come the tadpoles who fall directly into the water to begin their stent in the Cloud Forests of Mindo.
Frog eggs are not only susceptible to dehydration, but also predation. The decreased clutch sizes of terrestrial eggs are especially vulnerable to predation because there are so few eggs to spare. In many species this has led to advanced forms of parental care. Keeping watch over your eggs helps prevent tampering or consumption. Many species of Glassfrogs will watch over their eggs (LINK TO PE). But in Mindo, it is the Darwin Wallace Poison-frog (Epipedobates darwinwallacei) that wins the best parent award. This frog and many other dendrobatids (poison frogs) will lay their eggs in a damp area on land. When the eggs hatch, the tadpoles will ride on their mother's back until she can find a suitable water hole for them to reside in6.
It is only natural for this metamorphic saga to continue with the largest family of frogs in Mindo: the Rainfrogs (Craugastoridae).
Carrying your offspring to the river can be a bit of a hassle. Especially so if you have already invested so much in laying big whopping eggs and also have to deal with the unpredictability of the environment around you. So, why depend on water at all? Do frogs have to go through a tadpole stage? Rainfrogs have shown that, no, frogs do not have to start out as tadpoles. Rainfrogs do not lay eggs in the water. In fact, many species never step foot in a body of water. Rain frogs emerge from their terrestrial eggs as fully formed frogs, skipping the tadpole stage entirely. This is what we call direct development7.
In order to do this, Rainfrogs lay even larger eggs. Inside these eggs you will find a large yolk sac used to provide a surplus of nutrients facilitating the transition into the adult form. Looking closely at the embryos' development, you will see these frogs begin growing legs very early. So they truly skip the tadpole stage and emerge as fully formed froglets. In embryonic development they do retain a tail, but this is speculated to aid in respiration8.
Upon hatching, the juveniles enter a terrestrial world. They enter the same world their adult conspecifics inhabit. Competing in the same environment for similar food items is definitely not beneficial for a species. However, many species seem to have found a work-around to ease this notion. Field notes of many herpetologist, and our own observations, indicate that younger and smaller members of the Pastures Rainfrog (Pristimantis achatinus) are most active during the daylight hours. On the other hand, the adults often restrict their activity to dawn, dusk, and night. The juvenile’s small size perhaps allows the frog to rapidly cool itself under leaf litter on hot Ecuadorian days. The larger adults do not have this privilege and thus are limited to foraging during the cooler night9.
The Andes Mountains are characterized by heavy and unpredictable rainfalls, fast moving water ways, and drastic elevation changes. Mudslides and directional changes in rivers are common occurrences, and there are often extended drought periods brought on by El Ninio. It is no wonder this group of frogs found more success abandoning the old ways of amphibians and releasing themselves from the restrictions of aquatic reproduction. In doing so, they have opened up a new door for Team Frogs—They now have the ability to populate previously un-amphibian populated habitats.
The genus Pristimantis— Which is the current genus given to Rainfrogs— is the most specious genus (akamost diverse) of terrestrial vertebrates on the planet. Spreading to new heights and new environments and rapidly evolving characteristics to be successful in these areas, has resulted in over 500 species of Rainfrog—Probably by the time you read this sentence the exact number will have changed. It is estimated that frog species are being found at a rate of 15 species per year10. The sheer diversity amongst the species is seemingly endless. To give you a taste, these are the few I have had the privilege of photographing in the parish of Mindo:
Pastures Rainfrog (Pristimantis acuritis): This frog is perhaps the most common anuran found on the reserve. As its name suggests, it populates and dominates open habitats such as pastures. Their vocalizations often dominate the night sounds right outside Reserva las Tangaras lodge.
Yellow-groined Rainfrog (Pristimantis luteolateralis): Another common Rainfrog on the reserve seen most commonly on vegetation only a couple feet off the ground. It will hop from leaf to leaf looking for mouth sized insects to consume. The very similar Pristimantis walkeri looks identical to luteolateralis. However, walkeri occurs at elevations below 1240m, while luteolateralis occurs above.
Rio Faisanes Rainfrog (Pristimantis muricatus): A rare species endemic to the lower montane forests of Ecuador. We encountered this species on the Bosque Trail at Las Tangaras. This species, along with 16 other species of herps, are yet to be included in the field guide to the reptiles and amphibians of Mindo Ecuador due to their recent discoveries in the parish
Lonely Rainfrog (Pristimantis eremites): This frog occurs at elevations of roughly 1700m – 2500m. The Bosque trail is the only place at Las Tangaras this frog could potentially be seen. However, to my knowledge it has not yet been sighted. However, a trip to Bellavista may prove successful. This photo was taken at the Bellavista lodge.
Fern-loving Rainfrog (Pristimantis pteridophilus): “a nocturnal fern specialist” as described in the ‘Amphibians and Reptiles of Mindo’. Though the photo is not of this frog on a fern, these cryptic frogs have a preference for perching on ferns.
Spring Rainfrog (Pristimantis crenunguis): The largest of all Mindoan Rainfrogs. Though it is an endangered species, the Spring Rainfrog is very common in the localized areas in which it occurs. To my knowledge this frog has not been seen a Las Tangaras, but is often sighted at our neighboring property. They have unforgettably unique vocalizations.
Watchful Rainfrog (Pristimantis nyctophylax): An aptly named species seemingly for its big bulging eyes. These frogs are also commonly seen perched on low lying vegetation at night. This particular individual was rather fortuitous in our presence. Our head lamps attracted a good number of bugs and we witnessed this frog reach up and snag a large bug mid-flight. The hind toe pads kept this frog attached to the leaf.
Blue-thighed Rainfrog (Pristimantis crucifer): Perhaps my most favorite. The piercing red eyes of this species are like nothing I have ever seen. These are secretive frogs, with no known vocalization. They inhabit the deep primary forests of the reserve, such as the Bosque Trail.
Mutable Rainfrog (Pristimantis mutabilis): This is a frog that has thrown everyone for a loop. This species was first collected in 2006. However it was challenging for herpetologists to determine whether it was a new species, or a morph of an existing species, for this frog has the ability to change its skin texture. The spiky looking structures (turbercles) covering the frog can vanish in a matter of minutes. So people would collect this frog in the field, and look at it in the lab and think they had accidentally collected a completely different frog. It was finally described as its own species in 2015 and is often referred to as the punk rock frog. Since its description, several other Rainfrogs have been found to have this ability.
Mindo Rainfrog (Pristimantis mindo): A rather new species described in 2012. We hear this species calling frequently around Reserva Las Tangaras, but due to their highly arboreal and cryptic nature they are only seldomly seen. They were provided the species name ‘mindo’ after the anuran-diverse region they are endemic to.
So to conclude: frogs in the cloud forests of Ecuador are diverse! I have attempted to provide a framework as to why. Yet there certainly are many more reasons behind this diversity and it extends far beyond just amphibians. The birds are unlike anything I have encountered. The bugs are 10 times as plentiful and as colorful. The flowers.. well read Juliana's post. Mindo is an incredible place, and Juliana and I both have seen so much and learned far more than we could have expected.
Footnotes:
(1) This transformation can be initiated by a number of factors. And as they get older, or as food in the environment decreases, or as predation increases, or for whatever reason (WILBUR COLLINS model, 1973)
(2) Some amphibians even return to the water for a third life stage such as Notophthalmus viridescens
(4) Yes that is the actual common name of this frog. The entomology of the species name ‘carnifex’ translates to hangman. This is in relation to herpetologist John Lynch, who collected many of the early specimens leading to the description of this species. I believe common name is in relation to this as well.
(6) darwinwallacei may carry their young to suitable water holes. However, if food is scarce the tadpoles will often consume each other. Best parent award, yes, but best sibling? I don’t think so.
(7) Though I have laid this post out in a seemingly flawless transitional sequence, it turns out direct development in frogs did not evolve from frogs more prone to laying their eggs on land and taking their tadpoles to the water. Rather, taxonomical research suggest this form of development evolved from the ‘traditional’ method of laying eggs in the water. Gomez-Mestre et al.
(8) https://www.sdbcore.org/object?ObjectID=316
(9) I speculate this work-around could only work in the tropics. The closer to the equator, the less fluctuation in photoperiods and temperatures. Animals outside of the tropics are required to cope with an array of weather conditions, thus restrictions would be tough to impose. Adults would feed during the winter days because they would match the summer nights, and vice versa for juveniles. Even though there is a clear divide between activity periods in adult and juvenile Rainfrogs, this is not collectively agreed upon. If there is food to be eaten and you can eat it, you do. That is life.
(10) S. BLAIR HEDGES et al. New World direct-developing frogs (Anura: Terrarana): Molecular phylogeny, classification, biogeog- raphy, and conservation