Middle Cauca Research Articles

Outils bifaciaux multifonctionnels emmanchés comme marqueurs des premiers habitants des forêts de montagne en Colombie occidentale

The sub-Andean forests in the Middle Cauca River Valley (Central West of Colombia) show evidences of preceramic lithic industry, recovered from paleo-soils buried by volcanic debris. Their production and use were dated by 12,000 to 4000 BP. Lithic assemblages included cobbles with modified edges with surface modification by use. According to the shape, size or raw material those instruments were used as hammers or ground-stones. Other flint knapped products, such as cores and flakes, or products of bipolar percussion, are present. Additionally, in those cultural contexts there were some particular instruments with bifacial manufacture, standardized shape and preparation for hafting. Those instruments were used in different tasks, basically for preparation of soils toward domestication and harvest of plants. Lithic assemblages were localized on rolling hills, alluvial or colluvial terraces. Artifacts demonstrated careful selection of raw material, from volcanic or metamorphic origin. A relative stability can be proposed during the Early and Middle Holocene in the inland basin of Cauca, taking into account the similar typological and technological characteristics of the lithic ensembles, attesting to the role of this region in the origins of horticulture.

Assessing microhabitat, landscape features and intraguild relationships in the occupancy of the enigmatic and threatened Andean tiger cat (Leopardus tigrinus pardinoides) in the cloud forests of northwestern Colombia.

Mesocarnivores play a key role in ecosystem dynamics through the regulation of prey populations and are sensitive to environmental changes; thus, they are often considered good model organisms for conservation planning. However, data regarding the factors that influence the habitat use of threatened small wild felids such as the Andean tiger cat (Leopardus tigrinus pardinoides) are scarce. We conducted a two-year survey with 58 camera trap stations to evaluate the determinants of Andean tiger cat habitat use in three protected areas in the Middle Cauca, Colombia. We developed site occupancy models and found that Andean tiger cat habitat use increased with leaf litter depth at intermediate elevations and far from human settlements. Through conditional cooccurrence models, we found that Andean tiger cat habitat use was invariant to the presence of prey or potential intraguild competitors and killers/predators, but its detectability increased when they were present and detected. This suggests that Andean tiger cats may be more likely to be detected in sites with high prey availability. We found that Andean tiger cats preferred sites with deep leaf litter, which is a particular feature of cloud forests that provides suitable conditions for ambush hunting and hiding from intraguild enemies. Our results showed that Andean tiger cats avoided human settlements, which may minimize potential mortality risks in those areas. Moreover, the restricted use of middle elevations by Andean tiger cats suggested that they could be used as a sentinel species to track the effects of climate change since their suitable habitat is likely to be projected upward in elevation. Future conservation actions must be focused on identifying and mitigating human-related threats close to the Andean tiger cat habitat while preserving microhabitat conditions and the existing networks of protected areas.

First records of the Truandó Toad, Rhaebo haematiticus (Cope, 1862) (Anura, Bufonidae) in the Middle Cauca river valley, Caldas, Colombia

We present the first records of the Truandó Toad, Rhaebo haematiticus (Cope, 1862), in the inter-Andean valley of the Middle Cauca River, Colombia, the second largest inter-Andean basin of the country. In Colombia, this species has been extensively documented in the Chocó biogeographic region, on the western slopes of the Cordillera Occidental, in the valley of Magdalena River, and the foothills of the western slope of the northern Cordillera Oriental. Our records help fill an expansive hiatus in the known distribution of this species and increases the number of amphibians known to occur in the basin of the Cauca River.

The age and petrogenesis of reduced to weakly oxidized porphyry intrusions at the Marmato gold deposit, Colombia

The Marmato gold deposit is a hybrid between a porphyry gold and a reduced intrusion-hosted gold deposit with epithermal features at shallow levels. It is adjacent to the contemporaneous Aguas Claras porphyry gold deposit. They are located in one of the premier gold-copper belts of Colombia, the Middle Cauca Belt (MCB), and associated with a Late Miocene magmatic arc. The Marmato Porphyry Group (MPG), defined as the Marmato and Aguas Claras Porphyry intrusions, was dated at 6.58 ± 0.07 Ma to 5.74 ± 0.14 Ma by U-Pb LA-ICP-MS in zircon. The MPG is composed of andesitic-dacitic porphyry intrusions which belong to the I-type granite series and the reduced or weakly oxidized ilmenite-series suites, with a calc-alkaline to high K calc-alkaline signature. 87Sr/86Sr ratios between 0.70435 and 0.70513 and 143Nd/144Nd values of 0.512688 to 0.512898 from the MPG suggest partial melting of the mantle wedge as source of the magma. An additional contribution by partial melting involving garnet in the deep crust is suggested by a (Dy/Yb)N ratio of 1 to 1.5 and flat REE diagrams. Crystal fractionation during magma ascent is indicated by high Sr/Y >55 ratios with suppression of plagioclase on the liquidus until the appearance of amphibole shown by EuN/Eu*<1. The presence of ilmenite indicates relatively reduced or weakly oxidized conditions for the magma, probably generated by contamination with carbon-bearing crust in graphitic schists of the Arquía Complex. In addition, high manganese values in the intrusions (~1010 ppm) and in ilmenite (6.2–14.7 wt% Mn) suggest contamination by mafic to ultramafic rocks during magma ascent. The relatively reduced or weakly oxidized conditions created by the degree of interaction of the ascending magma with the crust produced variations in the amount of Mn-ilmenite and magnetite in the MPG porphyries. These conditions influenced the fugacity of the resulting rocks and magmatic-hydrothermal fluids involved in the genesis of different types of deposit in the adjacent Marmato and Aguas Claras gold deposits. This supports a model of interaction in the upper crust rather than a fundamental difference in the initial magma. Compositional similarities of the MPG with intrusions related to other gold deposits in the MCB reflect similar process for the generation of productive intrusions in the Late Miocene. This study highlights the important of the MPG as an example of the features of magmas capable of producing Au deposits in the MCB during the Late Miocene, such as high H2O content, high Sr/Y ratio, I-type granite series and reduced or weakly oxidized ilmenite-series granitoid suite. These features result in variable magnetic susceptibilities, which are important criteria for the interpretation of magnetic surveys for gold exploration in the Middle Cauca Belt.

Archaeological evidences on early peopling in the fluvio-volcanic Pereira-Armenia fan (Colombia): Volcanic activity influence on cultural adaptation and depopulation events

Archaeological research in the Middle Cauca region of west-central Colombia over the past 20 years has produced new information regarding early preceramic archaeology and environmental changes in the Central Cordillera landscapes. Stratigraphic, sedimentological, and geomorphic studies provided important contextual information for locating and interpreting the region's earliest archaeological sites, from a geoarchaeological perspective. This article presents results on the Pereira-Armenia Fluvio-Volcanic Fan studies and surrounding areas, during the Late Pleistocene through Middle Holocene periods. Twenty-five archaeological sites with preceramic deposits were tested and some have been excavated in a region measuring 30 b y 100 km. The region contains data with occupations ranging dates from approximately 13,540 to 3600 14C years BP (Aceituno, 2019; Dickau, 2015). These sites included cultural and natural deposits, within in situ tephras and polygenic soils with buried soil sequences. Cultural materials, sediments and soils from these sites have the potential to yield important data about the processes of site formation, environmental adaptation, early subsistence, depopulation and lithic technology in the Middle Cauca region. Layers of volcanic ashes demonstrated that in areas proximal to the Cerro Bravo-Cerro Machín Volcanic Complex, humans suffered impacts. However, depopulations was not generalized. In some places in the Fluvio-Volcanic Fan, far enough form volcanic impacts, people could stay.

Conectando un territorio: simulación de rutas de movilidad entre cazadores-recolectores y primeros cultivadores. El caso del Cauca medio (Macizo Volcánico, Colombia)

La movilidad es una de las principales características de los grupos cazadores-recolectores e incipientes cultivadores. A partir de los años 90 y a nivel global, los estudios sobre movilidad experimentan un fuerte avance gracias a la aplicación de los Sistemas de Información Geográfica (SIG), con los análisis de los costes de desplazamiento y el modelado de rutas, así como la relación de estos elementos con el acceso a los recursos, la interacción entre grupos y la territorialidad. El presente artículo se basa en la simulación de rutas de movilidad en el Cauca medio, región andina localizada en el Macizo Volcánico colombiano (Cordillera Central), donde se cuenta con una secuencia muy completa de ocupaciones precerámicas entre el Pleistoceno final y el Holoceno medio. Esta tarea se ha abordado, en el marco de los SIG, mediante herramientas de análisis de costes, concretamente el análisis Modelo de Acumulación del Desplazamiento Óptimo desde un origen (MADO) y la generación de caminos óptimos. Los resultados muestran una asociación significativa entre yacimientos arqueológicos y rutas potenciales, así como una red de movilidad coherente y articulada.

La Colosa Au Porphyry Deposit, Colombia: Mineralization Styles, Structural Controls, and Age Constraints

The La Colosa porphyry Au deposit is located on the eastern flank of the Central Cordillera of Colombia, within the Middle Cauca metallogenic belt. The deposit contains more than 800 t (22.37 Moz) Au at grades up to 0.8 g/t and is hosted by a composite porphyry stock of dioritic to tonalitic composition, which was emplaced into Triassic-Cretaceous schists of the Cajamarca Complex in the late Miocene (~8 Ma). The country rocks underwent two ductile deformation events, including development of shear zones, folds, and penetrative foliation, prior to emplacement of the stock. Subsequent brittle deformation reactivated preexisting N- and NNE-trending structures and formed secondary faults due to a change from right- to left-lateral shear sense on regional faults. This switch in stress orientations is attributed to a new plate configuration in the mid-Miocene. The left-lateral movement along regional faults favored emplacement of intrusive centers in dilational pull-apart zones, including the La Colosa stock within the regional Palestina fault zone. Three gold mineralization events are recognized at La Colosa. The first was of porphyry style, during which hypersaline fluids (40–50 wt % NaCl equiv) formed predominantly A- and S-type veinlets and caused multistage wall-rock silicification accompanied by potassic and sodic-calcic alteration. The early-stage intrusions contain the highest gold grades varying from 0.75 to 1 g/t Au, associated with pyrite and minor chalcopyrite, molybdenite, and magnetite in the porphyries and with pyrrhotite-pyrite-melnikovite in the country rocks. In the intermineral-stage intrusions the gold grades drop to 0.5 to 0.75 g/t Au, and pyrrhotite and pyrite are the major sulfides. Gold grades reach low values of <0.3 g/t Au in the late-stage porphyries. The second gold-precipitating event formed sheeted veinlets of drusy quartz and pyrite with centimeter-wide halos of albite-sericite-pyrite overprinting all other alteration types at the deposit. A ~200°C hydrothermal brine (21–28 wt % NaCl equiv) deposited gold at high grades (>1.5 g/t Au over >10 m drill core intervals) within N-striking normal faults that developed during and after emplacement of the porphyry stock. The third mineralization event was supergene, with Au enrichment confined to late porphyries and characterized by sulfide boxworks, resulting in gold grade increases from 0.3 to 1.2 g/t Au.

High Sr/Y Magma Petrogenesis and the Link to Porphyry Mineralization as Revealed by Garnet-Bearing I-Type Granodiorite Porphyries of the Middle Cauca Au-Cu Belt, Colombia

Porphyry-style Cu (Au, Mo) mineralization is widely associated with igneous suites with Sr/Y > 40 and low heavy rare earth element (HREE) content. This geochemical signature is commonly attributed to garnet or amphibole fractionation in the lower crust where mantle derived melts evolve into intermediate composition arc suites. However, the presence of a residuum related to mineralized porphyry systems where amphibole and garnet are stable is generally inferred from geochemistry and has nowhere been documented directly. We here report on two occurrences of late Miocene garnet-bearing granodiorite porphyry at the Tesorito Au-Cu prospect in the Quinchia district and El Poma Au-Cu prospect in the Middle Cauca Au-Cu porphyry belt in Colombia. These rocks provide proof that garnet was stable in the lower crust at the time of porphyry mineralization. At both studied sites, garnet-bearing I-type granodiorite porphyries were intruded by intramineralization garnet-free plagioclase phyric granodiorite porphyries that were cut by quartz and K-feldspar veins and affected by biotite alteration. Garnet-bearing porphyries predate intramineralization porphyries by 0.4 to 2 m.y. Garnet occurs as unzoned grains, commonly included in large plagioclase phenocrysts, that in some cases also contain inclusions of igneous epidote; the latter indicating pressures of >1 GPa for early crystallized phases. Large plagioclase phenocrysts exhibit two growth stages separated by a resorption boundary. High-Al amphibole phenocrysts as well as titanite and magnetite formed after the first stage but prior to the second stage of plagioclase growth at pressures of ca. 0.9 GPa. Whole-rock geochemical compositions of garnet-bearing porphyries are consistent with only limited garnet fractionation whereas some of the intramineralization porphyries have Y 40, which is, together with other trace element indicators, taken as evidence for significant garnet fraction at depth during porphyry mineralization. HREE content of zircon in garnet-bearing porphyries are higher than that of intramineralization porphyries, which supports the interpretation that garnet fractionation in the lower crust was significant during the formation of the intramineralization porphyries. The phase relationships and geochemical data reflect a geodynamic change from an environment that permitted rapid ascent of oxidized garnet-bearing porphyries prior to Au-Cu mineralization to a regime that favored melt evolution in the lower crust where garnet was a residual phase and at the same time allowed the formation of large mid- to upper-crustal magma chambers. The latter are widely inferred to be essential for porphyry mineralization to occur.

The Nuevo Chaquiro Cu-Au-(Mo) Porphyry Deposit, Middle Cauca Belt, Colombia: Geology, Alteration, Mineralization

Nuevo Chaquiro is a newly discovered blind porphyry Cu-Au-(Mo) deposit located in the Middle Cauca belt, Colombia, approximately 60 km south-southeast of Medellin. The most recent resource estimate was stated as 566 M tonnes (t) averaging 0.64% Cu, 0.31 g/t Au, 4.34 g/t Ag, and 127 ppm Mo. The geology of Nuevo Chaquiro consists of a Miocene volcanic sequence of andesitic tuffs, agglomerates, and flows (Combia Formation) intruded by small stocks and dikes of diorite and quartz diorite also of Miocene age. Sparsely occurring outcrops at Nuevo Chaquiro consist of a 1.2 × 0.8 km zone of pervasive sericitic alteration with locally well developed quartz-Fe oxide stockworks. The sericitic alteration grades downward into sericite-chlorite alteration and then predominantly potassic (biotite-magnetite) alteration at depths of 250 to 400 m. Calcic-potassic alteration, containing actinolite and epidote, occurs at greater depths. A high-grade sheeted vein complex occurs immediately adjacent to the upper contact of an Early quartz diorite intrusion. Quartz veinlet density in this area reaches up to 80% quartz by volume, accompanied by up to 5 vol % chalcopyrite. Later Intermineral porphyries created a second orebody slightly to the west but contiguous with the Early orebody. Here Cu-Au-Mo mineralization in an inverted bowl shape is associated with potassic alteration that manifests itself as secondary flooding of the tuff wall rock matrix by hydrothermal biotite, accompanied by disseminations and hair veinlets of chalcopyrite, magnetite, pyrite, and molybdenite. The porphyry system is also accompanied by intermediate-sulfidation carbonate–quartz-base metal veins, typically with sericitic alteration selvages, which occur on the upper western flank of the deposit. At Nuevo Chaquiro, multiple intrusions of Early and Intermineral porphyry contain calcic-potassic and potassic alteration, with a later overprint by sericite-chlorite alteration centered on intrusion apexes. The amount of sericite-chlorite overprinting varies with each intrusion. Collectively, these intrusions are all overprinted by a single, later sercitic (phyllic) alteration event that is superimposed as a broad cap upon the entire system. There is a consistent paragenesis for the alteration/mineralization associated with each intrusion, suggesting a relatively stable underlying shallow magma chamber that fluxed geochemically similar pulses of magmatic/hydrothermal fluid with each porphyry. In many aspects, Nuevo Chaquiro is a “typical” Au-rich porphyry copper system. However, the contrast between the mineralization styles between the Early and Intermineral orebodies as well the timing differences in copper deposition suggest that differences in porphyry mineralization styles also occur at Nuevo Chaquiro.

Radiocarbon chronology of terminal Pleistocene to middle Holocene human occupation in the Middle Cauca Valley, Colombia

Archaeological research over the past two decades in the Middle Cauca region of central Colombia has documented numerous preceramic sites dating from the terminal Pleistocene to middle Holocene, along with substantial artifactual and archaeobotanical evidence for early plant use and food production. We present a radiocarbon chronology of 26 sites, including dates previously available only in unpublished reports, and 36 new AMS dates from 11 sites. This chronology solidly establishes the preceramic (before 3600 14CBP) human occupation in the Middle Cauca. The earliest date clearly associated with cultural evidence of occupation is 10,619 ± 66 14CBP at the site of Cuba. Four sites show occupation before 10,000 14CBP, but between 10,000 and 9000 14CBP, this number increases to eleven sites. Thereafter, despite evidence of episodic volcanic activity, there is a relatively constant and continuous sequence of human occupation in the region, although small localized population movements may have occurred. The fertility of periodically renewed andisols likely attracted settlement and continued occupation of the region by people practicing early plant cultivation, based on the archaeobotanical evidence for the early adoption and use of domesticates.

A new endemic species of Bryconamericus (Characiformes, Characidae) from the Middle Cauca River Basin, Colombia

Bryconamericus caldasi, a new species, is described from the Middle Cauca River drainage, Andean versant of Colombia. The new species is distinguished from all congeners by: the number of predorsal scales (15–17 vs. 9–14); a wide anterior maxilla tooth, at least twice as wide as the posterior tooth, both of which are pentacuspid (vs. maxilla teeth of same size); a dark lateral stripe overlaid by a peduncular spot; and a reticulated pattern on the sides of body (vs. peduncular spot and other body pigments not superimposed over a dark lateral stripe). We found several differences that distinguish the new species from B. caucanus, the only sympatric congener: number of predorsal median scales (15–17 vs. 12–13); convex predorsal profile (vs. oblique); scale size and number of scale rows at caudal–fin base (small scales arranged in two or more rows vs. large scales in just one row); pectoral fins not or just reaching pelvic fin insertions (vs. pectoral fins reaching posterior to pelvic–fin insertions); and dorsal–fin origin position (at vertical through posterior tip of pelvic–fin vs. at a vertical anterior to pelvic–fin tip).

Early and Middle Holocene evidence for plant use and cultivation in the Middle Cauca River Basin, Cordillera Central (Colombia)

This paper presents the latest results of research done in the Colombian Andean region known as Middle Cauca River Basin, an important location for the study of the origins of plant use and the dispersal of domesticates throughout the Americas due to its geographical position in northwest South America. We discuss human–environment interactions during Pleistocene/Holocene transition to middle Holocene (ca 10,000–4000 BP), specifically human–plant interaction and environmental factors that led to the adoption of horticultural practices. Three lines of evidence are analyzed: archaeological stratigraphy, lithic technology, and microbotanical remains. Our results suggest that early Holocene environmental stability allowed Middle Cauca settlers to use the diverse local resources for several millennia, altering the local vegetation, and leading to the development of horticultural practices that included the use of both local and foreign plants. These results inform the ongoing debate about the antiquity and nature of plant domestication and dispersals in the Americas.

The San Nicolás succession of the Cauca paleolake: a late Holocene laminated ria lake record from the Neotropics

The stratigraphic, geochemical, and organic matter study of the late Holocene San Nicolas succession of the Cauca paleolake (Santa Fe–Sopetran pull-apart basin) in the middle Cauca Valley, northern Colombia, suggests that it was deposited in a ria lake environment, at sedimentary accumulation rates in excess of 600 cm ky−1 between ~3500 and ~500 yr BP. Laminated deposition occurred, first under igapo (black-water), and then under varzea (white-water) conditions. The transition occurred ca. 3000 yr BP, a time of major change in El Nino/Southern Oscillation (ENSO) behavior in the Cariaco Basin, thus reflecting the southern migration of the intertropical convergence zone and intensified rain upstream the Cauca Valley. A second, but less conspicuous change occurred ca. 2000 yr BP, which apparently corresponds to the intensified and/or more frequent ENSO activity in the Galapagos Islands. Our contribution describes, for the first time, a ria lake sedimentary succession from the northern Andes and demonstrates the high potential of these hitherto undervalued deposits for the reconstruction of the paleohydrological history of the northern Andes.

Palinomorfos del cuaternario sobre el valle del rio san eugenio (risaralda, colombia)

Un análisis cualitativo de las observaciones palinológicas realizadas sobre un yacimiento arqueológico entre el Pleistoceno tardío hasta el Holoceno temprano sobre Santa rosa de Cabal, Risaralda Colombia fue llevado a cabo como parte de una reconstrucción paleoecológica, dentro de la cual se analizaron un total de 71 muestras a partir de dos núcleos suelos volcánicos, La pochola 01 (47 muestras) y la pochola 02 (25 muestras).Con base en los datos de 14C obtenidos de la Pochola 01 se pudo establecer que la edad más antigua es de 13.540±60 años A.P, en los 105 cm (Pleistoceno tardío) y la edad más reciente 6743±45 años a.P en los 57 cm, (Holoceno medio). Los resultados muestran que la preservación de los granos de polen en suelos volcánicos es alta,la diversidad y buena preservación. Un total de 67 especies fueron descritas dentro de las cuales se encuentran representantes de clases, Pteridophytos (11 especies), Pinopsida (1especies) y Angiospermas (50 especies).Mediante este documento se pretende incrementar y contribuir al conocimiento palinológico del Cuaternario en la región del Cauca Medio.

Aportes al estudio paleodietario mediante el análisis de isótopos estables de δ13C y δ15N en restos óseos humanos de la región centro-oriental del Cauca medio

La dieta, considerada como hábito alimenticio, es un componente biocultural fundamental en todas las sociedades, en particular en las precolombinas. Se reconstruye mediante la combinación de indicadores arqueológicos y bioarqueológicos que hacen posible identificar el uso y consumo de diferentes tipos de alimentos y su relación con otras variables culturales (p. e., la jerarquía) y demográficas (p. e., el sexo y la edad). El objetivo del presente estudio es reconstruir la dieta de ocho individuos prehispánicos de yacimientos arqueológicos del centro-oriente del Cauca medio, entre los siglos IX y XIV d. C., mediante razones de isótopos estables de C y N ( δ13C y δ15N). Los resultados evidencian la importancia de los recursos alimenticios disponibles en la región, así como diferencias y similitudes en el uso y consumo de alimentos entre los sitios investigados.

Palynofacies analysis of the late Holocene San Nicolás terrace of the Cauca paleolake and paleohydrology of northern South America

The detailed palynofacies analysis of the late Holocene laminated succession of the San Nicolás terrace in the middle Cauca Valley, northern Colombia, is interpreted as the product of sedimentation in a lacustrine to fluvial dominated setting. Radiocarbon analyses reveal that the succession was deposited between ~3500 and a few hundreds of years with sedimentary rates in excess of ~600cm/ka. The milimetric laminae and the high frequency fluctuations in the content of the palynodebris of terrestrial origin, i.e., the high altered ligno-cellulosic debris/amorphous organic matter ratio, together with diatoms typical of fluvial conditions, such as Aulacoseira granulata do suggest an intermittent and continuous hydrological and biological connectivity with the Cauca River, akin to a varzea lacustrine environment. Drier to wetter conditions appear to occur, moving upward in the succession. This might reflect regional precipitation conditions all along the Cauca Valley and the southern migration of the intertropical convergence zone.

Holocene soft-sediment deformation of the Santa Fe–Sopetrán Basin, northern Colombian Andes: Evidence for pre-Hispanic seismic activity?

The detailed study of four deformed intervals from the Holocene fluvio-lacustrine deposits of the Santa Fe–Sopetrán Basin in northern Colombia shows 17 types of soft-sediment deformation (SSD) structures. Evidence indicates that seismic activity was responsible for the SSD structures, a conclusion reached after considering the environmental conditions at the time of sediment deposition and shortly after, and the detailed analysis of the driving force systems. Other triggers (i.e. overloading and rapid sedimentation), however, are not discarded. Intervals showing SSD structures occurred at centennial frequencies and apparently resulted from Mw 6–7 earthquakes. The Holocene age of these major shaking events should be seriously considered when evaluating the seismic hazard and risk for the middle Cauca Valley and the nearby city of Medellín with 3 million inhabitants.

Hormigas: relaciones especies-área en fragmentos de bosque seco tropical

We analyzed the effect of fragmentation on ant species-area and specimen frequency-area relationships in nine patches of tropical dry forest in the middle Cauca river basin in Colombia. Species richness and specimen relative frequency of ants were positively correlated with area, whereas no significant correlation was found between species richness and the degree of isolation calculated for the forest patches. As the fragmentation affects different functional groups in different ways, we analyzed the species-area relationship for separate functional groups of ants. According to the habitat requirements we found that the species richness increased faster as area increased for ants inhabiting decomposing wood, followed by ants associated with trees, while species richness of ants living under dead leaves did not correlate with area. According to the food preference, species richness was positively correlated with area for the army ant group, while no significant correlation was found for solitary hunters or for leaf-cutting ants. Ant species richness and specimen density were calculated from equal size samples and examined in relation to the habitat area. An inverse correlation was found only for specimen density, the opposite of what was expected. To our knowledge this is one of the first studies showing differential responses of functional groups of ants to habitat loss. Moreover, it emphasizes the conservation value of small forest fragments for ants in a tropical dry forest.

Two new species of Hemibrycon (Characiformes, Characidae) from the Magdalena River, Colombia

Hemibrycon brevispini n. sp. can be distinguished from other species of the genus by the presence of hooks on all fins, and by an elongate projection on the fourth ventral neural arc near the first neural post-zygotic apophysis. Hemibrycon cairoense n. sp. can be distinguished from congeners by having nine proximal pteryigiophores in the dorsal fins including the terminal piece (vs. &gt; 10). It can be distinguished from Hemibrycon species in the Upper and Middle Cauca Rivers by the number of pored lateral-line scales (43-46 vs. &gt; 46 o &lt; 43; F = 13.67; p &lt; 0.000). Ecological data concerning the aquatic habitat of the taxa are presented.

Mobility strategies in Colombia's middle mountain range between the early and middle Holocene

This article is about the mobilityof hunter-gatherers and cultivators, who inhabited two regions of mountain rainforest in the Cordillera Central of Colombia between the early and middle Holocene. It aims to demonstrate that in tropical rainforests mobility is a more effective economic and social strategy than sedentary settlement. Given the poor soils and low carrying capacity of these environments, mobility provides a strategy which allows for periodic regeneration of resources. We argue that the hunter-gatherer cultivators who lived in the valleys of the Porce and middle Cauca maintained mobility even though the first evidence for cultivars appears between 7000 BP and 6000 BP. The later introduction of sedentary settlement was not due to plant cultivation, but rather to other factors such as the necessity to control territorial resources. Social factors thus played their part in influencing the reduction of mobility to produce the settlement patterns described by the Spanish chroniclers across the stud...