Counting, estimating sizes, and performing other tasks that require quantitative abilities are an important part of our daily life and have been long believed to be a uniquely human ability. However, there are situations in which also other animals can take advantage of discriminating between groups of objects with different numerosity or between objects that differ in size. For example, mammals, birds and fish, and even invertebrates discriminate between foraging patches with different quantities of food in order to feed on the most profitable one (Cross & Jackson, 2017; Lucon-Xiccato et al., 2015).
Some researchers believe that the ability to make quantity judgement as shown by animals is evolutionarily linked to that of humans; an early ancestor of all vertebrate species might have evolved the ability to discriminate quantities, and thus all the extant vertebrates would share a similar quantitative system to help them solve the problems of their environment (Beran, 2008; Feigenson et al., 2004). Under this hypothesis, researchers expect to find quantitative abilities in a large number of species, and that these abilities are involved in several behaviors; yet, there are still entire classes of vertebrates, such as Amphibia, for which information on quantitative abilities is few to date.
A recent study has investigated the microhabitat choice of young treefrogs (Hyla intermedia) to assess whether quantitative abilities play a role in this behavior. Treefrogs are arboreal anurans that spend most of their life on trees. However, their larval stage is aquatic, so treefrogs lay eggs in the water of ponds and lakes. After metamorphosis, juvenile treefrogs emerge from the water and look for microhabitats that are adapted to their development.
Researchers noticed that juvenile treefrogs of an Italian population live in a grassland close to their natal pond soon after metamorphosis, and individuals are often observed climbing to the top of tall and dense grass clumps (figure 1). Grass could help juvenile treefrogs in hiding from predators, in finding their prey (Walsh & Downie, 2005), and in maintaining homeostasis (Seebacher & Alford, 2002). It is therefore likely that they would prefer a microhabitat with dense and large grass clumps.
In a series of field experiments, the researchers tested whether treefrogs are attracted to large clumps of grass. They removed the glass from a circular portion of the grassland and then captured treefrogs and placed them in the middle of the portion with no grass. The animals immediately ran to reach the closest tall grass clumps, and, within 1 minute, they rapidly climbed the grass. Some treefrogs were also placed into a circular arena made of white plastic (figure 2a), and pairs of grass clumps were presented on the wall of the arena.
With this setting, treefrogs still showed a strong attraction to grass, and rapidly ran to climb it. Further, if the two grass clumps presented had a different size, treefrogs reliably climbed the larger one. These results suggest a general attraction among treefrogs to large grass stimuli. Yet, the study of quantitative abilities requires the use of highly-controlled stimuli, and the researchers wanted to manipulate the size and the number of every single grass leaf. Since stimuli control is hard to achieve using grass stimuli, the authors tried to “trick” the treefrogs by presenting green bars printed on paper as stimuli.
Treefrogs presented with real grass versus a printed bar chose randomly between the two stimuli, suggesting that they do not note that the bar is not made of grass, or that they do not care about being on a leaf or on another green object. This allows the researchers to use bars as stimuli in the subsequent experiments, and bars can be finely controlled for their quantitative attributes (figure 2b).
A second set of experiments was performed in the laboratory, a more “controlled environment,” with grass stimuli, which can be finely controlled for their quantitative attributes. Treefrogs were presented with two sets formed by different numbers of same-sized bars (figure 3a) or with pairs of bars with different sizes (both different height and different width; figure 3b).
In both cases, treefrogs mostly chose the stimulus simulating a larger quantity of grass (the set with more bars and larger bars, respectively). This result supports the idea that treefrogs exploit quantitative abilities to choose their microhabitat. Interestingly, subjects chose the set with more bars when presented with 1 versus 2 bars and 2 versus 4 bars, but not when presented with 2 versus 3 bars and 3 versus 4 bars, which indicated that they can tell the difference up to numerical ratio of 0.5; They preferred the larger bar only when it was 4 times larger than the other. Are treefrogs better at counting that at glimpsing plants’ size?
Before answering this question, it was important to exactly understand which cues the treefrogs use to choose between the stimuli. For example, if we look at a set of 2 bars and a set of 4 bars presented to the treefrogs, we will notice that the set with more bars also has more green surface (figure 3a); treefrogs can thus use the amount of green to choose the larger set. Similarly, the different sized bars used in the experiment differed in both height and width (figure 3b), and treefrogs can thus use both cues for their choice.
Control experiments allowed us to separate these two types of information. When treefrogs are presented, for example, with a set of 2 large bars versus a set of 4 small bars in order to equate the green surface of the sets, they still preferred the more numerous set; thus, they actually choose the microhabitat based on the number of bars. Also, when treefrogs where presented with bars with the same width and different height, they chose the taller bar up to a 0.5 ratio (the same quantity ratio discriminated when “counting” the number of bars); treefrogs instead did not show any preference between bars with different widths and equal height.
This study is one of the few reporting quantitative abilities in an amphibian. It indicates that these vertebrates with a simple nervous system can use quantitative information during microhabitat choice and that they possess the ability to discriminate equally well between different numbers of bars and bars of different size. It is possible that the use of quantitative information is more widespread among species and among situations than previously thought.
These findings are described in the article entitled Quantity discrimination by treefrogs, recently published in the journal Animal Behaviour. This work was conducted by Tyrone Lucon-Xiccato, Elia Gatto, and Angelo Bisazza from the Università di Padova.