Phylogeny of Parental Care in Darwin's Frog

Historical and Current Classification
How did parental care evolve?
How did parental care evolve in R. darwinii?

Darwin's Frog
Rhinoderma darwnii
Used with permission by Micro Sole

Historical and Current Classification


In the past the only reason Rhinodermatidae was considered a separate genus was due to its unusual form of parental care. R. darwinii was considered to be a branch of Leptodatylidae; however, as more techniques using DNA and RNA sequences have been developed, it has been possible to reevaluate their classification. Using analysis the 12S, 16S, and 28S gene sequences Rhinodermatidae has been placed as a branch of Cycloramphidae in the new Amphibian Tree of Life. Interestingly enough, no other member of Cycloramphidae uses a form of parental care like R. darwinii (Frost et al., 2006: Correra, 2006 #60). Even the other species in the genus Rhonodermatidae, Rhinoderma rufum, only has an intermediate form of vocal sac brooding. Larva are kept in the vocal sac until its jaws have developed. Then it is released to finish its development in an aquatic environment (Jorquera et al., 1982).

Phylogenetic tree

How did parental care evolve?


Parental care in general may have evolved in response to increased egg size. Increased eggs size can result from selection due to a number of environmental factors such as better oxygenation before hatching, colder climate leading to longer development, or decreased food supply after hatching. Larger egg size is strongly correlated with parental care, perhaps because larger egg size makes non-feeding larva and direct developing frogs possible by providing the nutrients required for metamorphosis (Summers et al., 2007). Once larger egg size made parental care possible, the benefits of parental care, such as protection from predators, dehydration, and infection from fungi and bacteria, would have caused parental behavior to be selected for (Wickramasinghe et al., 2004; Summers et al., 2007).

How did parental care evolve in R. darwinii?Vocal Sac


Considering the vast differences between R. darwinii and the members of its clade, it is thought that they may be relicts of lineages that were more diverse before a climate change at the end of the late Cenozoic (Correra, 2006).

It is likely that parental care in R. darwinii also developed after the selection for larger egg size. Brooding males behavior and physiology seem to be possible results of the same selective pressures that act on egg size. The vocal sac secretes a viscous fluid that provides aeration and nutrition to the larva and brooding males move to warmer microenvironments, perhaps to help speed the development of the larvae (Goicoechea et al., 1986; Crump, 2002). In addition, the behavior R. darwinii would have the same benefits as all other forms of parental care (prevention of dehydration, predation, and infection) (Wickramasinghe et al., 2004).

The case of the intermediate vocal sac brooding in R. rufum may provide a look at the changes that were necessary in order for brooding to develop in R. darwinii. For example, in R. rufum the physiology of its larva are more like free-living larva, but in R. darwinii larva remain largely undifferentiated until metamorphosis (Jorquera et al., 1982).