Interspeci fi c combat in anurans : a case of Hypsiboas faber ( Wied-Neuwied , 1821 ) ( Hylidae ) and Rhinella pombali ( Baldissera-Jr , Caramaschi & Haddad , 2004 ) ( Bufonidae )

We report an interspecifi c combat involving the tree frog Hypsiboas faber and the toad Rhinella pombali in southeastern Brazil. Since the observation occurred after the beginning of the interaction, either one of the species may have started the fi ght. In general, intra-specifi c combats are more common for species in which the male is as large, as or larger than, the female, as observed in Hypsiboas faber. Thus, the male tree frog might have confused the toad with a conspecifi c opponent. On the other hand, bufonid males can use active searching for females, sometimes clasping other males or objects. Therefore, the toad might have clasped the male tree frog and it was this that provoked the subsequent wrestling.

M. R. de Moura et al.
In areas where several species of frogs breed simultaneously, interspecifi c interactions may occur, varying from shifts in acoustic signals to aggressive encounters (Schwartz and Wells, 1984).However, the occurrence of interspecifi c combats in anurans has been rarely reported, being more common among closely related species (Schwartz and Wells, 1984).In general, aggressive interactions among heterospecifi c males occur by the use of aggressive calls in the defense of calling sites (Wells, 2007).
During an anuran survey on September 9 th 2009 at around 19:30, we witnessed an interspecifi c combat between the tree frog Hypsiboas faber and the toad Rhinella pombali (Figure 1).The observation took place in a small temporary pond in the municipality of Viçosa,state of Minas Gerais,southeastern Brazil (20º48'01.4"S,42º51'45.6"W,elev. 711m).An adult male of H. faber was observed holding the body of an adult male of R. pombali, on the water surface near the edge of the pond.The male of H. faber remained clinging to its opponent, fi ghting at all times, while the bufonid struck with its forelimbs, also rolling the H. faber throughout the fi ght, apparently trying to keep the tree frog underwater.After about one minute, the opponents separated, distancing themeselves about one meter away from each other.We did not hear any acoustic signals being emitted during the combat.
The abundance of the resource is expected to modify the intensity of the competition among individuals (Wells, 1977;Duellman and Trueb, 1986).In anurans, such resources are almost always oviposition sites or calling sites, which are disputed mainly by males (Wells, 2007).Territorial defense of oviposition sites is particularly common in males of H. faber who frequently defend nests and the area around them, which represents simultaneously a calling, a courtship, and an oviposition site (Martins and Haddad, 1988).As a result, the wrestling in H. faber is more frequent in high density choruses, varying from a few seconds to more than 10 minutes (Martins et al., 1998).Another feature that can stimulate competition is the availability of females (Wells, 2007).Males of several Brazilian bufonids engage in active searching during situations of very dense choruses, sometimes clasping other males or objects (Pombal-Jr. and Haddad, 2007).On the same night and at the same pond where we observed the reported event, at least another ten males of H. faber were calling together with a similar or superior number of males of R. pombali.Since the observation was made after the beginning of the combat, it is diffi cult to know which individual started the wrestling, both being possible scenarios.Evidence for H. faber as the starter of the combat is provided by the several territorial and aggressive behaviors described for this species (Lutz, 1960;1973), including an escalated aggressive behavior among males, which can vary from territorial calls to physical fi ghts (Martins et al., 1998).Since the available area for nest building was limited, the high number of males calling on that night might have infl uenced the beginning of the interspecifi c battle.In the case reported here, the male of R. pombali might have entered the territory of the male H. faber, and the latter could have confused the toad with another co-specifi c calling male, jumping toward the R. pombali and then beginning the combat.The male of R. pombali almost certainly moved in response to aggression as a defensive tactic, whereas the opponent was still clinging to its body.On the other Interspecifi c combat in anurans hand, the possibility of R. pombali starting the combat cannot be rejected.Although the behavior of active search has not yet been reported for R. pombali, it is known for closely related species, such as R. ornata and R. henseli (Pombal-Jr. and Haddad, 2007).Thus, in the situation reported here, the male R. pombali could have confused the H. faber male with a co-specifi c female while searching for a mate, and attempted to amplex the latter by wrestling.
In the anuran species that present male combat, the males are as large as, or larger than, females in almost half of these combative forms, while in non-combative species the males are rarely as large as the females (Shine, 1979).This aspect of sexual dimorphism is observed in H. faber, a species in which males are as large as females (Martins, 1993) and is not verifi ed in most Rhinella species, in which females are larger than males (e.g.Caramaschi and Niemeyer, 2003;Baldissera-Júnior et al., 2004;Brandão et al., 2007;Chaparro et al., 2007;Fouquet et al., 2007;Lehr et al., 2007;Maciel, 2008;Narvaes and Trefaut, 2009).Although reports of aggressive behavior for the Rhinella species has been only rarely reported (e.g.Haddad et al., 2008), we have already observed males of R. pombali exhibiting territorial behaviors, such as territorial call and attempts to move a neighboring male, indicating that these interactions may not be uncommon for these toads.The occurrence of territorial behavior in males of R. pombali is another piece of evidence for the toad as the starter of the combat described here, although it was not observed in this reported event.

FIGURE 1 :
FIGURE 1: Adult males of Hypsiboas faber and Rhinella pombali wrestling in pond shallows (Viçosa, Minas Gerais, Brazil).Note the forelimbs of the H. faber clinging (white arrow) to the body of the R. pombali.Photo: M. R. Moura.