Amphibians
Tree-hole Frog
Metaphrynella sundana · also called Bornean tree-hole frog
A male tree-hole frog does something almost no other animal is known to do: he listens to the acoustic properties of the exact water-filled tree cavity he is calling from and actively retunes his own voice to match it, turning a small hollow in a Bornean rainforest tree into a natural amplifier for his call.
Overview
The tree-hole frog, Metaphrynella sundana, is a tiny insectivore living almost entirely in and around water-filled tree cavities in the rainforest of Borneo. Small and cryptically coloured, it is far more often heard than seen.
Tuning a tree hole like an instrument
A calling male adjusts his pitch in real time to match the specific resonant frequency of his chosen cavity, using the hollow as a passive amplifier to project his call much farther than an untuned call could manage — and readjusting as rainfall or evaporation shifts the water level and, with it, the chamber’s acoustics.
Status
Least Concern but decreasing, since dependence on the water-filled cavities of mature trees, which take decades to form, makes the species especially vulnerable to logging and old-growth forest loss across its Bornean range.
Tree-hole Frog: common questions
How does a tree-hole frog actually adjust its call?
Researchers studying the species have found that a calling male can shift the pitch of his call to align with the specific natural resonant frequency of the water-filled tree cavity he is occupying, a frequency determined by the size and shape of the cavity and how much water it currently contains. By actively adjusting his vocal output based on apparent real-time feedback from the acoustic properties of his particular chamber, the male achieves significantly greater call amplification than a fixed, unadjusted call would produce in the same space.
Why is tuning a call to match a tree hole useful for this frog?
Matching the call frequency to the cavity's natural resonance allows the small chamber to function as a passive acoustic amplifier, making the male's call substantially louder and more likely to carry effectively through dense rainforest vegetation to reach a receptive female some distance away. Since a louder, farther-carrying call can improve a male's chances of attracting a mate, this tuning behaviour likely provides a meaningful reproductive advantage over calling without regard to the specific acoustic properties of the chamber being used.
Does the tree-hole frog have to keep readjusting its call over time?
Yes, because the resonant frequency of a given tree cavity depends partly on the water level inside it, which naturally rises and falls with rainfall and evaporation, a male calling from the same cavity over an extended period may need to continually fine-tune his call frequency to keep it matched to the chamber's changing acoustic properties. This ongoing, responsive adjustment is part of what makes the species' vocal tuning behaviour particularly notable to researchers studying animal communication.
Why is the tree-hole frog vulnerable to logging even though it is not currently highly threatened?
The species depends specifically on natural water-filled cavities that form in mature trees, structures that generally take many decades of tree growth and natural decay processes to develop and are far less common in young secondary forest or heavily logged areas. Because this specialised breeding habitat cannot be quickly replaced once mature, hollow-bearing trees are removed, sustained logging and deforestation pose a particularly serious long-term risk to the tree-hole frog's ability to find enough suitable breeding sites, even in areas where some forest cover remains.