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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 123. No. 3. September 2011 
Leapfrog patterns may be an important aspect 
of allopatric speciation, because some document¬ 
ed cases refer to clades in which the dissimilar 
central form is no longer thought to exhibit gene 
flow with either one or both of the terminal forms 
and has therefore completed speciation. Most of 
the cases reported, however, refer to complexes in 
which species limits are still unclear. This 
suggests leapfrog patterns may be an important 
factor in differentiation of these birds and merit 
the close attention of evolutionary biologists. 
More research is needed before we can be sure 
of the exact role leapfrog patterns have in 
diversification of some bird lineages. 
Little is known about the causes of leapfrog 
patterns. Remsen (1984b) hypothesized Ihey are 
due to stochastic phenotypic changes in geo¬ 
graphically intermediate populations that have not 
yet affected the terminal populations, and pre¬ 
sented evidence for why other hypotheses, such as 
convergent evolution or ancient corridors between 
terminal populations, arc less likely. Modern 
phylogenetic studies confirm that terminal popu¬ 
lations are not generally sister taxa, despite their 
greater character similarity (Weir et al. 2008, 
Mauck and Burns 2009, Cadcna and Cuervo 
2010 ). 
Remsen (1984b) showed that leapfrog patterns 
are most prevalent in Andean birds, where they 
occur in about one fifth of all species complexes 
that fulfil the minimum requirement of being 
represented by more than three different-looking 
taxa. Since Remsen (1984b) first recognized the 
generality of leapfrog patterns, this peculiar mode 
of character evolution has been carefully docu¬ 
mented in about a dozen bird species complexes, 
the overwhelming majority being from the Andes 
(e.g.. Remsen 1984a. Remsen and Graves 1995. 
Maijer and Fjeldsa 1997. Hayes 2001, Johnson 
2002, Weir et al. 2008. Mauck and Burns 2009. 
Cadena and Cuervo 2010) and only a few from 
other tropical and subtropical areas (e.g., Norman 
et al. 2002). A number of candidates for leapfrog 
patterns may be present in the Australo-Papuan 
region (e.g.. Mayr and Diamond 2001), but most 
remain undocumented (see, however, Norman et 
al. 2002). The reason why the Andes seem to be 
so conducive to producing this pattern may be a 
combination of their geographic linearity, great 
topographic relief, and high species richness 
(Remsen 1984b). 
Most leapfrog patterns have been documented 
in morphological characters such as plumage 
traits, but Remsen (1984b) predicted they may 
occur in behavioral traits, such as vocalizations, as 
well. However, such cases seem to have gone 
largely undetected. The only documentation of a 
vocal leapfrog pattern of which we are aware 
relates to the Stripe-headed Brush Finch ( Arremon 
torquatus) species complex (Cadena and Cuervo 
2010). This brush finch radiation displays a great 
mosaic of plumage and vocal trait variation along 
its Andean range with species from the northern 
and southern end of the distribution having similar 
simple-structured songs while geographically 
intervening species sound more complex. 
We report a second case of a vocal leapfrog 
pattern occurring in the Maroon-chinned Fruit 
Dove (Pfilinopus subgularis), and use the new 
insights into geographic variation in vocal traits to 
address species boundaries in the complex. P. 
subgularis is a little known WaJlacean endemic 
that is divided into three subspecies: (I) epia from 
Sulawesi, (2) nominate subgularis from the 
islands of Banggai and Peleng in the Banggai 
Archipelago, and (3) mango/iensis from the Sula 
Islands of TaJiabu, Seho, and Mangole (Fig. 1; 
While and Bruce 1986, Baptista et al. 1997, 
Coates and Bishop 1997, Gibbs et al. 2001). The 
species inhabits secondary and primary lowland 
and hill forest, but becomes rare at higher 
elevations (Coates and Bishop 1997, Gibbs et al. 
2001). It has been reported to range to 800 m asl 
on Sulawesi (Coates and Bishop 1997, Gibbs et al. 
2001). to 900 m on Peleng (Rheindt et al. 2010). 
and to 1.100 m on Taliabu (Rheindt 2010). It 
seems to be replaced at higher elevations on 
Sulawesi by the Red-eared Fruit Dove (P. 
fischeri ). 
The taxon epia trom the Sulawesi mainland is 
well-known in terms of life-history, but the two 
small-island taxa ( subgularis and mango/iensis ) 
are little known in life and their vocalizations 
have remained undescribed (Coates and Bishop 
1997. Gibbs et al. 2001). Our poor understanding 
ot their field biology is probably at the root of 
differences in taxonomic classification between 
original descriptions and current treatments. Quoy 
and Gaimard (1830) first described the species 
from Manado (North Sulawesi) as Columba 
gularis. but Obcrholser (1918) found the latter 
name to be pre-occupied and replaced it with 
Leucotreron epia. Meyer and Wiglesworth 
(1896). before Oberholser's (1918) name replace¬ 
ment, described a specimen from the Banggai 
Islands as a new species Ptilopus [sic] subgularis. 
