Biology Reference
In-Depth Information
Rhesus macaques are classified as being either of eastern (i.e., China and
vicinity) or western groups (i.e., India and vicinity) without being given sub-
specific status (Fooden, 2000 ). The eastern group has much shorter tails (35.3
percent) than the western group (42.5 percent) (Hamada et al ., 2005 ). The
grouping has been supported by molecular phylogenetic analyses (Smith and
McDonough, 2005 , Smith et al ., 2007 ; Satkoski et al ., 2008 ). In the eastern half
of the Indochina Peninsula, including Laos, around the border zone of the two
species (14-20˚N), supposed hybrid rhesus macaques with tails longer than 45
percent were found (Fooden, 1964 , 1995 , 2000 ; Hamada et al ., 2006 , 2008 ).
Introgression from long-tailed macaques was suggested for these populations
(Malaivijitnond et al ., 2007 , 2008 ). However, definite molecular analyses have
not been reported because of the difficulty in accessing “hybrid” rhesus in the
Indochina Peninsula.
Our inspection of rhesus macaque pets in central and southern Laos showed
that relative tail length significantly correlated with latitude (Hamada et al ., in
preparation). Although this geographical cline could be explained by the cli-
mate cline, i.e., cold adaptation (i.e., Allen's rule), this is not the case because
both the eastern and western groups of rhesus macaques do not show intra-
group clinal variation (Fooden, 2000 ) and because rhesus macaques from
northern Laos did not have short enough tails to conform to the trend of length
against latitude found in rhesus macaques from central and southern Laos.
Therefore, hybridization would be the major cause of tail length variation in
rhesus macaques, and the introgression intensity would be clinal in central and
southern Laos. On the other hand, geographical variation in tail length or other
phenotypic characteristics was not depicted in northern long-tailed macaques.
Thus, conditions of the ancestral populations that received introgression may
be different in northern long-tailed and rhesus macaques in the contact zone.
Clinal variation in tail length of rhesus macaques could have been produced
by either one-way (i.e., introgression) or two-way (i.e., hybridization) genetic
flow. It is not impossible that the ancestors of rhesus, or rhesus-like, macaques
with longer tails could have been long-tailed macaques that received heavy
introgression from rhesus macaques. The heavy introgression altered the
phenotypic characteristics into those of rhesus macaques (e.g., tail length, pel-
age color pattern, and cranio-facial morphology). On the other hand, it is pre-
sumed that the ancestor of northern long-tailed macaques was once confined
to refugia in the glacier period with smaller populations and received intro-
gression from rhesus macaques. The introgression replaced the Y-chromosome
of the long-tailed macaque with that of rhesus macaque and slightly changed
its morphological characteristics. Although single nucleotide polymorphism
analysis showed complicated genetic relationships between the two species
(Street et al ., 2007 ), there is no data on the direction or strength of introgression
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