Efstratios Valakos | National & Kapodistrian University of Athens (original) (raw)

Papers by Efstratios Valakos

Molecular Phylogenetics …, 2003

Erhard's wall lizard, Podarcis erhardii (Sauria: Lacertidae), is highly diversified in Gr... more Erhard's wall lizard, Podarcis erhardii (Sauria: Lacertidae), is highly diversified in Greece and especially in the southern Aegean region. Out of the 28 recognized subspecies, 27 are found in Greece from the North Sporades island-complex in the North Aegean (grossly south of the 39th parallel) to the island of Crete in the South. The species exhibits great morphological and ecological plasticity and inhabits many different habitats from rocky islets and sandy shores to mountaintops as high as 2000m. By examining intraspecific variability at a segment of the mitochondrial gene cytochrome b we have found that that extant populations of P. erhardii are paraphyletic. Furthermore, we have found that subspecies previously defined on the basis of morphological characteristics do not correspond to different molecular phylogenetic clades, so that their status should be reconsidered. The DNA based biogeographical and phylogenetic history of Podarcis in Southern Greece is congruent with available paleogeographic data of the region, which supports the view that DNA sequences may be a useful tool for the study of palaeogeography.

Summary: The purpose of this study is to assess sexual dimorphism in the palate and base of adult... more Summary: The purpose of this study is to assess sexual dimorphism in the palate and base of adult crania using three-dimensional geometric morphometric methods. The study sample consisted of 176 crania of known sex (94 males, 82 females) belonging to individuals who lived during the 20th century in Greece. The three-dimensional co-ordinates of 30 ectocranial landmarks were digitized using a MicroScribe 3DX contact digitizer. Generalized Procrustes Analysis (GPA) was used to obtain size and shape variables for statistical analysis.
Three discriminant function analyses were carried out: (1) using PC scores from Procrustes shape space, (2) centroid size alone, and (3) PC scores of GPA residuals which includes lnCS
for analysis in Procrustes form space. Results indicate that there are shape differences between sexes. In males, the palate is deepest and more elongated; the cranial base is shortened. Sex-specific shape differences for the cross-validated data give better classification results in the cranial base (77.2 %) compared with the palate (68.9 %). Size alone yielded better results for cranial base (82%) in opposition to palate (63.1 %). As anticipated, the classification accuracy improves when both size and shape are combined (90.4% for cranial base, and 74.8% for palate).

Caudal autotomy is an effective antipredator strategy widespread among lizards. The shed tail thr... more Caudal autotomy is an effective antipredator strategy widespread among lizards. The shed tail thrashes vigorously for long periods to distract the predator and facilitate the lizard's escape. This movement is maintained by energy supplied by the anaerobic conversion of glycogen into lactate. It has been suggested that lactate accumulation serves as an index for the vigor of tail thrashing. We made three predictions: (1) tail loss frequency should be higher under heavier predation regime, (2) the duration of postautotomy tail movement should be extended in populations under heavy predation pressure as an adaptation to the higher risk and the increased need for defense, and (3) as result, lactate in these tail tissues should be concentrated at higher levels. To eliminate the impact of phylogeny and environmental factors on the interpretation of our result, we focused exclusively on one species, the Balearic lizard (Podarcis lilfordi). We studied three populations under different predation pressure but sharing the same climatic conditions. We found no differences among the studied populations either in postautotomy duration of tail movement or in levels of final lactate accumulation while autotomy frequency was higher where predation pressure was more intense. Τail loss effectiveness is directly influenced by the level of predation, while secondary features of the trait appear to remain independent from the impact of environment.

Israel Journal of Zoology, 2000

Data on the thermal ecology of the African Chameleon Chamaeleo africanus Laurenti, 1768 and the C... more Data on the thermal ecology of the African Chameleon Chamaeleo africanus Laurenti, 1768 and the Common Chameleon Chamaeleo chamaeleon (Linnaeus, 1758) are reported from Greece. In the field the Tb values ranged from 10.4¡C to 31.6¡C for C. africanus and 23.5¡C to 31¡C for C. chamaeleon. There was a significant correlation between Tb and Ta in spring and summer for both species. There was also a significant correlation between Tb and Ts only in the spring and only for C. africanus. Cloacal temperatures differed significantly between spring and summer and so did substrate temperatures and air temperatures. As the months became hotter the animals reached higher temperatures. In a laboratory temperature gradient, the preferred body temperatures of C. africanus and C. chamaeleon were measured and compared with field body temperatures. The preferred body temperature in the laboratory gradient ranged from 26.0¡C to 36.0¡C for C. chamaeleon and from 25.0¡C to 35.0¡C for C. africanus. The me...

Amphibia-Reptilia, 2009

Caudal autotomy, the self-induced tail shedding from the body, is a common defensive strategy in ... more Caudal autotomy, the self-induced tail shedding from the body, is a common defensive strategy in lizards. Tail loss is followed rapidly by regeneration, revealing the importance of a fully functional tail. Predation pressure has been traditionally correlated with autotomy performance. However there is a lack of evidence regarding the impact of predation regime on tail regeneration. Another important factor that has been neglected is the population density. Though it is well established that crowding alters various life-history traits, the impact of density on caudal regeneration remains understudied. In this paper we compared three island populations of the Aegean Wall Lizard (Podarcis erhardii) that have evolved under different levels of predation pressure: whereas the Naxos population is exposed to numerous predators the Kopries and Daskalio islet populations experience low predation pressure. To evaluate the effect of density-induced stress on caudal regeneration, lizards were treated under two conditions of housing, in single and crowded (six individuals) terraria. Tail sheding occurred prior to this treatment following a standardised method. The length of regenerated tail was recorded weekly. Regeneration rate was higher in single terraria when compared to crowded, reflecting the negative impact of crowding. However we failed to detect any statistically significant difference between single and crowded terraria in the case of Naxos. We believe that the underlying reason must be the heavy predation pressure under which rapid tail regeneration, even under unfavourable conditions, is crucial for survival. It seems that the imperious need for regeneration counteracts density-induced stress.

Acta Oecologica, 2007

Climate change

Archives of Environmental Contamination and Toxicology, May 17, 2012

The present study investigates the accumulation of heavy metals [copper (Cu), lead (Pb), zinc (Zn... more The present study investigates the accumulation of heavy metals [copper (Cu), lead (Pb), zinc (Zn), magnesium (Mn), cadmium (Cd), nickel (Ni), and chromium (Cr)] in tadpoles inhabiting the metalliferous streams flowing within the Asprolakkas River basin (northeast Chalkidiki peninsula, Greece) and the effect of potentially harmful elements in stream water and sediment on the corresponding levels in their tissue. Animals were collected from six sampling sites influenced by a wide range of surface water and stream sediment trace ...

Naturwissenschaften, Sep 1, 2009

Resource availability, competition, and predation commonly drive body size evolution. We assess t... more Resource availability, competition, and predation commonly drive body size evolution. We assess the impact of high food availability and the consequent increased intraspecific competition, as expressed by tail injuries and cannibalism, on body size in Skyros wall lizards (Podarcis gaigeae). Lizard populations on islets surrounding Skyros (Aegean Sea) all have fewer predators and competitors than on Skyros but differ in the numbers of nesting seabirds. We predicted the following:(1) the presence of breeding ...

Molecular Phylogenetics and Evolution, 2005

Molecular Ecology, 2005

Wall lizards of the genus Podarcis (Sauria, Lacertidae) comprise 17 currently recognized species ... more Wall lizards of the genus Podarcis (Sauria, Lacertidae) comprise 17 currently recognized species in southern Europe, where they are the predominant reptile group. The taxonomy of Podarcis is complex and unstable. Based on DNA sequence data the species of Podarcis falls into four main groups that have substantial geographical conherence (western island group, southwestern group, Italian group and Balkan group). The Balkan species are divided in two subgroups: the subgroup of Podarcis taurica ( P. taurica, P. milensis, P. gaigeae and perhaps P. melisellensis ), and the subgroup of Podarcis erhardii ( P. erhardii and P. peloponnesiaca ). We addressed the question of phylogenetic relations among the species of the P. taurica subgroup encountered in Greece, as they can be inferred from partial mtDNA (cyt b and 16S) sequences. Our data support the monophyly of P. taurica subgroup and suggest that P. gaigeae, P. milensis and P. melisellensis form a clade, which thereinafter connects to P. taurica . Within the previous clade, P. gaigeae is more closely related to P. milensis than to P. melisellensis . However, the specimens of P. taurica were subdivided in two different groups. The first one includes the specimens from northeastern Greece, and the other group includes the specimens from the rest of continental Greece and Ionian islands. Because the molecular clock of the cyt b and 16 rRNA genes was not rejected in our model test, it is possible to estimate times of speciation events. Based on the splitting of the island of Crete from Peloponnisos [ c. 5 million years ago (Ma)], the evolutionary rate for the cyt b is 1.55% per million years (Myr) and for the 16S rRNA is 0.46% per Myr. These results suggest that the evolutionary history of P. taurica in Greece is more complex than a single evolutionary invasion. The data analysed, stress the need for a reconsideration of the evolutionary history of Greek Podarcis species and help overcome difficulties that classical taxonomy has encountered at both the species level.

Molecular Phylogenetics and Evolution, 2014

The existence of sexual dimorphism in individual elements of the human skeleton allows sex determ... more The existence of sexual dimorphism in individual elements of the human skeleton allows sex
determination and significantly enhances the identification of fragmentary skeletal material.
Traditionally, the pelvis and the skull are used for an accurate sex determination. Sometimes they
are not available and alternative methods have to be used. The aim of this work is to study the sexual
dimorphism of the proximal portion of the posterior femur by analyzing the biometric data of the
Purkait’s triangle. The studied sample comprises 203 individuals (112 males and 91 females) from the
Modern Human Skeletal Reference Collection (The Athens Collection) of the Department of Animal &
Human Physiology (National & Kapodistrian University of Athens). The biometric data were analyzed by
Discriminant Analysis and equations were generated for sex determination, which gave an overall
correct classification of 78.3%. For the right and the left femur the percentages of correct classification
were 77.8% and 75.9% respectively. These accuracy rates were compared to those of an Indian population
and were found to be lower (86.5% for males and 86.3% for females). The study of sexual dimorphism (SD)
in Greek, Indian, European-American and African-American populations gave interesting results. The AB
diameter in the Greek sample shows the lowest degree of SD while the AC diameter shows the highest. In
the other three population samples, the BC diameter shows the highest degree of SD. We assume that the
femur anatomy of the Greeks is the major cause that the Purkait method does not give high rates of
correct classification. It is therefore advisable that practitioners exercise caution when using this method
on skeletal material from Greece, especially in forensic contexts.