Geometric morphometric analysis of the head of Microlophus atacamensis (Tropiduridae) in a latitudinal gradient

The genus Microlophus (Squamata: Tropiduridae) has a disjunct distribution, rare in terrestrial vertebrates
(Benavides et al. 2007]). Twenty-one species are recognized; nine of these are found only in the Galapagos
Islands, while the remainder are distributed along the coast and interior deserts
of western South America, from southern Ecuador to northern Chile (Etheridge and De
Queiroz 1988]; Frost and Etheridge 1989]; Frost 1992]; Benavides et al., 2007]). The species of the genus Microlophus form a monophyletic group of lizards, characterized by apical disks in the hemipenis
(Frost 1992]). According to Dixon and Wright (1975]), two groups are recognized in the genus whose monophyly is justified by characters
of body scales and skin folds, occipitalis and peruvianus; this was later corroborated
by Frost (1992]) with osteological characters. Two subgroups are recognized within the peruvianus
group; the first group groups interior species which feed on insects and terrestrial
plants with no direct ecological relation to the intertidal zone: M. peruvianus, M. thoracicus, M. theresioides, M. tigris, M. yanezi, M. theresiae, and M. tarapacensis (Ortiz and Serey 1979]); the second group is composed of intertidal species which feed on crustaceans and
algae: M. heterolepis, M. atacamensis, and M. quadrivittatus (Ortiz and Serey 1979]). In Chile, this genus is currently represented by M. atacamensis, M. quadrivittatus, M. yanezi, M. tarapacensis, M. theresioides, and M. heterolepis (Ortiz 1980a]), which are distributed from Arica to Arrayán (La Serena) (Donoso-Barros 1949]; Sepúlveda et al. 2006]).

According to Donoso-Barros (1960]), the origin of the genus Microlophus in Chile began with M. peruvianus, which would have descended southward along the coast and then extended its range
to the interior desert by way of the valleys. Ortiz (1980a]) followed this logic but added that the distribution to the interior desert would
have occurred through the Pampa del Tamarugal. Later, using isoenzyme data Victoriano
et al. (2003]) proposed that an ancestral lineage advanced from Peru towards the coast and later
diverged into two groups; one was the ancestor of the M. quadrivitattus–M. theresioides group (one of whose subgroups still inhabits the interior desert) and the other was
the ancestor of M. atacamensis. Based on a molecular phylogeny of data from many genes, Benavides et al. (2007]) proposed that the origin of the Chilean clade was a migration from Peru to the interior
desert; M. theresioides diverged and then dispersed along the Río Loa to the coast, giving rise to M. quadrivittatus in the north and M. atacamensis farther south. This last proposal is questionable, since the limit of distribution
between M. quadrivittatus and M. atacamensis is near Antofagasta (Victoriano et al. 2003]). Here, we describe some characteristics of M. atacamensis.

M. atacamensis is a lizard species which inhabits the intertidal zone of northern Chile from Antofagasta
(23° 39? S, 70° 22? W) to Arrayán, La Serena (29° 41? S, 71° 19? W) (Ortiz 1980b]; Heisig 1993], Sepúlveda et al. 2006]). This is a medium large (mean: 102.7 mm length) (Donoso-Barros 1966]; Ortiz 1980b]), corpulent species with dark brown color and black blotches on the dorsum; crest
tenuous in the vertebral zone and mouth narrow. According to Ortiz (1980a]) and Vidal et al. (2002]) there is a spatial segregation between juveniles and adults in the height of the
perch used for thermoregulation, as well as a difference between sexes; although males
are larger, there was no significant difference in their regulation, both were thermoconformers
(Vidal et al. 2002]). This species is omnivorous behavior, since they consumed mainly Diptera and algae
obtained from the intertidal area (Ortiz 1980a]; Vidal and Labra 2008]; Fariña et al. 2008]).

Clinal variation is defined as gradual variation of a character over geographic distance
(Futuyma 1998]). For example, Vidal et al. (2007]) found latitudinal variation in coloration in Liolaemus tenuis; green increased and brown decreased from north to south. Sepúlveda et al. (2008]) proposed a latitudinal gradient in the thermoregulatory ability of M. atacamensis, in which northern populations have low average to which lizards experience a Tb
outside the selected temperature, while south this ability increases. Fariña et al.
(2008]) found a clinal pattern in the diet of M. atacamensis, in which algal consumption decreases and consumption of Diptera increases from north
to south.

Many theoretical and empirical studies have focused on the adaptive significance of
morphological clinal variation (Trussell 2000]; Martínez-Freiría et al. 2009]). While some analyses of linear morphometry have been often used to test hypotheses
related to morphological polymorphism in a microevolutionary context (Endler 1977]; Scolaro and Cei 1987]; Quatrini et al. 2001]), others are more holistic and quantitative analyses are then required to identify
them and to appraise the selective forces responsible for their evolution (Adams and
Rohlf 2000]). In fact, due to scale problems with linear measures, these do not take into account
the morphological complexity of biological structures (Humphries et al. 1981]; Rohlf and Bookstein 1987]; Mousseau 1991]; Warheit 1992]). In the case of traditional (non-geometric) measurements, statistical techniques
for measuring distances, relations among distances, areas, volumes, or angles are
applied (Bookstein et al. 1999]). However, the geometric morphometry preserves the geometry configurations of landmarks,
by allowing a statistical representation of real forms or forms only (Rohlf and Slice
1990]; Rohlf et al. 1996]; Rohlf 1999]; Rohlf and Corti 2000]), whereas denotes the geometric shape properties of a structure that is independent
of size, position, and orientation of the same, while the form of an object includes
both size and shape (Rohlf and Slice 1990]; Adams and Rohlf 2000], Mitteroecker and Gunz 2009]). This information on shape allows a more complete biological interpretation than
morphological variation (Rohlf and Marcus 1993]). In relation to clinal variation, results obtained in Liolaemus show that, although there is not a clear pattern of latitudinal variation of the
shape in L. tenuis (Vidal et al. 2005]), there are significant differences between two geographical areas (arid Mediterranean
and oceanic with Mediterranean influence). These different geographic areas may act
as partial barriers to gene flow in this species. Vidal et al. (2006]) found divergence between L. pictus from the Isla de Chiloé and the Chilean continent in ocular extension and location
of the labial commissure. Individuals from the island had more extended ocular orbits
and a more posterior position of the labial commissure than individuals from the continent,
which may be attributed to differences in diet between these two localities (Vidal
et al. 2006]).

In this study, we explored the morphological adjustment to clinal environment variation
in the shape of the head of M. atacamensis in dorsal and lateral view in different localities, comparing over a latitudinal
distribution from 23° to 28° S. Considering that environmental temperature decreases
as latitude increases and previous data has shown variation in thermal and feeding
in this species, we specifically attempted to test the hypotheses that the shape of
the head varies latitudinally including more extended shape in the north for your
diet and more compacted shape the south.