Discussion
Our results suggest that P. vaillanti and P. bicolor show
conspicuous colorations in some areas of the body are unpalatable, have
toxins and prefer to remain on the surface of the water column. These
strategies may be evolving in tandem, and represent an adaptive syndrome
that reduces the risk of predation (Relyea 2001). Most of the larvae
were depredated; the mortality of the naiads that consumed the P.
vaillanti larvae is an indication of the presence of toxicity from
tadpole’s skin. However, there is a great variation in the effect that a
toxin can cause to different predators and in how a predator can more
easily recognize a non-palatable prey than another (Adams et al. 2011).
For example, fish and amphibians can recognize and identify a toxic prey
faster than aquatic insects (Kats et al. 1988). This is because
vertebrates consume whole prey including skin, while insects use buccal
parts to suck the prey content (Brodie et al. 1978; Gunzburger and
Travis 2005). Therefore, it is necessary to corroborate the lack of
palatability using other predators that can coexist with the larvae.
Toxicity assays helped us to reinforce the hypothesis about the presence
of alkaloids in the skin extracts of these larvae. The presence of these
toxins as an aposematic signal in anuran larvae is a rare characteristic
in which few studies have focused (Toledo and Haddad 2009). Some
investigations have shown that various larvae of the genusAtelopus, Rhinella and Oophaga are fed with unfertilized
eggs the female deposits and they possess chemical defenses for the
tadpoles. Also, some eggs of these same species hatch with chemical
defenses the female has provided during oviposition as protection
against predators (Pavelka et al. 1977; Hayes et al. 2009; Stinosky et
al. 2014). In the Phyllomedusine larvae, the acquisition of chemical
defenses has not been reported; however, we can be sure the toxicity
assays are reliable to demonstrate these tadpoles possess toxic
substances, although we cannot prove their origin. The presence of
glands that secrete opioid peptides such as dermorphins, bradykinin and
delthorphine by some adult Phyllomedusa are highly unpleasant to
predators because they can modify cardiac function, induce
regurgitation, or produce catalepsy (Sazima 1974; Negri et al. 1992).
These same substances may be appearing along larval growth, as well as
serous glands, which could function as regulatory and defense mechanisms
in the aquatic environment (Delfino et al. 1998).
About the presence of conspicuous colorations in our model, we observed
that there is a bimodal pattern of coloration. On the dorsal area, the
tadpoles have conspicuous colorations, while ventrally the tadpoles can
be considered inconspicuous. This would suggest that dorsal coloration
would be targeted at aerial or terrestrial predators, whereas the
coloration of the ventral area makes them cryptic for predators living
underwater (Thibaudeau and Altig 2012). P. vaillanti was the most
conspicuous species in our experiment. It also had a very significant
metalized interocular patch. It is possible this conspicuous patch not
only gives it advantages to warning predators, but it can also serve as
a disruptive coloration, thus increasing their opportunity to avoid
predators (Eterovick et al. 2010). On the other hand, C.
tomopterna , presents a black spot at the end of the tail, which could
be used as a strategy to redirect the predator’s attack. Such an attack
would be directed toward a less vulnerable area and compensate the
absence of some conspicuous coloration (Caldwell 1982).
On the other hand, conspicuousness did not to have a correlation with
the toxicity in P. bicolor. This correlation was only observed in
the P. vaillanti larvae and only associated to the coloration of
the interocular spot, but not all tadpole coloration. The reason can be
that the conspicuousness and presence of toxins could operate decoupled.
For example, some species of dendrobatids may differ significantly in
toxicity, but their coloration is the same. Other individuals may
exhibit similar colorations, but their toxicity varies significantly
(Darst et al. 2006).
Conspicuous dorsal colorations of these larvae may work together with
schools towards the surface of the water. This would suggest a positive
effect on predator learning. Other phyllomedusine larvae (P.
gutatta, P. jandaia and P.vaillanti ), mainly present
congregation behaviors supposedly as an antipredatory mechanism. They
are predominately grouped on the water surface forming schools with
individuals of the same size during the day while at night they are
dispersed and located along the water column (Branch 1983; Caramaschi
and Jim 1983). This behavior towards the water surface could be strongly
related to environmental factors such as light and the different sensory
systems of the larvae, which would explain the behavior of P.
vaillanti and P. bicolor (Katz et al. 1981).
On the other hand, it has been evidenced the effectiveness of this
conspicuous coloration and the toxicity present in both P.
vaillanti and P. bicolor increases as the individuals’ size,
which produces greater aversion by predators (Hagman and Forsman 2003).
In addition, the palatability experiments support this hypothesis, since
it was observed that the larger larvae were not depredated. This has
suggested this aposematic signal may evolve favoring larger prey (Lever
2001, Gunzburger and Travis 2005). Depending on the stage of larval
development, it is possible for toxins or alkaloids to change their
composition and abundance, making the animals less palatable to
predators (Brodie and Formanowicz 1987; Peterson and Blaustein 1991;
Hayes et al. 2009; Stynoski et al., 2014). For example, there has been
an apparent change in the toxicity of Bufo during ontogeny. Large
mortality of giant water bugs (Belostomatidae) was observed when the
insects predated tadpoles in late stages of development (Crossland
1998). In other experiments, were offered to chickens and predatory
rejection of larger insects was observed (Gamberale and Tulberg 1998).
In conclusion, P. vaillanti and P. bicolor larvae may
present an antipredatory syndrome using various mechanisms to avoid
predators. These combined strategies can reduce the probability of
attack, confuse the predator, reinforce the alarm signal, and warn of
the presence of toxins in the skin of the tadpoles (Aronsson and
Gamberale 2012). Finally, the high cost of having a conspicuous
coloration associated with chemical defense and the formation of
congregations is a point that merits greater attention in future
investigations, specifically at the time the larvae complete their cycle
or survival rates associated with the use of these strategies.