Frogs jump. They're darned good at it. Because of this, they've been one of the key groups used to study the oh-so-important first phase of jumping – take off. But what goes up must come down and if you're a tree-dwelling frog, that down can be 10 or more meters below if you miss your intended landing site. While a handful of studies have examined how frogs and toads land, these studies have focused on landing on flat surfaces. Although this may be appropriate for land-lubbing jumpers, little is known about how the arboreal sort jump from branch to branch, let alone how they land.
Nienke Bijma and her collaborators at Kiel University in Germany wanted to find out not only how arboreal frogs land when they jump but also what role their toe pads play in helping the frogs stick to their landing location. They used high-speed video to record Amazon milk frogs (Trachycephalus resinifictrix), first as they hung from one front foot underneath a narrow horizontal rod, then as they landed after jumping from a flat landing surface 25 cm away.
The authors found that when the frogs were placed underneath a narrow rod and one front foot was placed on the rod, they were able to hold their body weight using only two toes – if they were the third and fourth toes. These findings supported earlier work on walking in tree frogs, which showed that the third and fourth toes are important for contact stabilization. And once the frogs had one foothold, they immediately placed another foot on the rod and attempted to pull themselves up: no one likes to be left hanging.
When it came to landing after a jump, the frogs used one of two strategies: the belly flop, or the reach-and-grab. When frogs landed with a belly flop, they impacted the rod perpendicular to their bodies and folded over it like a taco. Once the rod had abruptly halted their flight, the frogs quickly grabbed the rod with their feet and held on. The authors suggest this method of landing is effective and accurate, but could potentially result in internal damage.
For the reach-and-grab strategy, if the frogs overshot the rod, they would reach with a back leg to grab it; when the frogs undershot, they would reach up with a front leg to grab the rod. These reaching-while-flying landing strategies caused the frogs to cartwheel around – sometimes landing on top of the rod – or sway back and forth underneath the rod, as they pulled themselves up towards it. For all landing strategies, the researchers observed the frogs lean towards the rod as they got closer, in order to make contact more easily, though this happened less frequently with the belly flop.
In contrast to the ground, which is unlikely to move when frogs and toads are mid-jump, branches of trees shift in the wind and under the weight of the animals themselves. So, while previous research has shown that each species of terrestrial frog and toad has relatively stereotyped landing strategies, the authors suggest it makes sense that arboreal frogs need to be more adaptable when it comes to dealing with what's at the other end of the jump.
