Posted by: barn owl | February 17, 2008

Monday Mimic: Beetles, Frogs, and Poisonous Birds

Neotropical Poison-dart Frogs of the family Dendrobatidae contain potent batrachotoxins-alkaloids that affect sodium channels in nerve and muscle cells-in their skins, and were thus used by native Americans to poison the tips of blow-darts. However, when raised in captivity, the Phyllobates species frogs do not contain detectable amounts of batrachotoxins, indicating that these amphibians obtain the alkaloids from dietary sources in the wild. In a different part of the world, passerine birds also use neurotoxins as a defensive mechanism: at least three species in the genus Pitohui, known by the indigenous people of New Guinea to be poisonous, contain homobatrachotoxin in skin and feathers (Dumbacher et al., 1992).

The six species of Pitohuis, endemic to New Guinea, are in the family Pachycephalidae, and the toxins are believed to provide protection against both ectoparasites and predators, including human hunters. In 2000, a second toxic bird genus, Ifrita kowaldi, was reported (Dumbacher et al., 2000); these birds live at higher altitudes in New Guinea, and were never collected in the same location as any of the Pitohui species. The single species, Ifrita kowaldi, is alternately placed in family Orthonychidae or family Muscicapidae, but nevertheless is not closely related to Pitohui. The concentrations of batrachotoxin alkaloids vary greatly among specimens of both Pitohui and Ifrita, and in some specimens no neurotoxins are detected. These observations are consistent with an environmental source of the batrachotoxins, and in 2004, Dumbacher and colleagues reported that Melyrid beetles of the genus Choresine are the likely toxin source for the New Guinea birds.

beetle

Figure 4. from Dumbacher et al., 2004

Two species of Pitohuis, Pitohui kirhocephalus and Pitohui dichrous, are demonstrably more toxic than the others, and both of these species have bolder plumage patterns, consistent with a “warning signal” for potential predators. Whereas P. dichrous birds always have a brick-red belly and back contrasting with black wings and head, P. kirhocephalous birds have quite variable plumage patterns across their geographic range (Rand and Gilliard, 1967). Dumbacher and Fleischer (2001) constructed a molecular phylogeny of Pitohuis, using DNA from museum skin specimens, to determine whether the contrasting plumage coloration of some P. kirhocephalus subspecies represented an example of Müllerian mimicry. In Müllerian mimicry, which occurs in some butterfly species, several chemically defended prey species resemble each other in coloration pattern. The phylogenetic evidence was consistent with the hypothesis of Müllerian mimicry for only one subspecies, P.k. dohertyi, with a range that overlaps that of P. dichrous; however, the remote and politically unstable nature of the study area makes direct testing of the mimicry hypothesis difficult. The remaining subspecies clade was determined instead to have evolved the plumage pattern from contrast-patterned ancestors, shared with P. dichrous. The authors point out that selection for mimicry in Pitohui species may be determined by variation in toxin levels, the degree of sympatry, and the abundance of visually hunting predators (Dumbacher and Fleischer, 2001).

pitohuis

Figure 1. Dumbacher and Fleischer (2001). P. kirhocephalus subspecies; potential mimics of P. dichrous are b, c, f, h, i, p, q.

References:

Dumbacher, J.P., Beehler, B.M., Spande, T.F., Garraffo, H.M., and Daly, J.W. (1992). Homobatrachotoxin in the genus Pitohui: chemical defense in birds? Science 258, 799-801.

Dumbacher, J.P., Spande, T., and Daly, J.W. (2000). Batrachotoxin alkaloids from passerine birds: a second toxic bird genus (Ifrita kowaldi). Proc. Natl. Acad. Sci. USA 97(24), 12970-12975.

Dumbacher, J.P. and Fleischer, R.C. (2001). Phylogenetic evidence for colour pattern convergence in toxic pitohuis: Müllerian mimicry in birds? Proc. R. Soc. Lond. B 268, 1971-1976.

Dumbacher, J.P., Wako, A., Derrickson, S.R., Samuelson, A., Spande, T.F., and Daly, J.W. (2004). Melyrid beetles (Choresine): A putative source for the batrachotoxin alkaloids found in poison-dart frogs and toxic passerine birds. Proc. Natl. Acad. Sci. USA 101(45), 15857-15860.

Rand, A.L., and Gilliard, E.T. (1967). Handbook of New Guinea Birds. London: Weidenfield and Nicolson.

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