Influence of temporal fluctuations in seed abundance on the foraging behaviour of harvester ants (Pogonomyrmex spp.) in the central Monte desert,Argentina

The simultaneous study of the temporal dynamics of foraging behaviour, diet and seed abundance is essential to assess the way in which resources affect the behaviour and ecology of harvester ants. Here, we evaluate how fluctuations in grass seed abundance during three consecutive growing seasons influenced the foraging behaviour and diet of the harvester ants Pogonomyrmex rastratus, P. mendozanus and P. inermis in the central Monte desert, Argentina. Seed abundance of the most consumed grasses varied greatly through ant activity season, and ants altered their foraging behaviour in response to those changes. Foragers spent more time travelling and searching for food, and their foraging trips took longer during the low seed availability season. Foraging distance was very similar among species and, contrary to our expectations, did not vary between seasons. Foraging success of P. rastratus and P. inermis increased during the high availability season. This matched the seasonal pattern of foraging activity, suggesting that colonies may detect seed abundance and regulate their foraging effort with the rate of forager success. Although grass seeds were the main item in the diet of the three species, P. mendozanus, and to a lesser extent P. rastratus, turned more generalist when grass seeds were scarce. In contrast, P. inermis showed a very narrow diet breadth, only harvesting grass seeds in both seasons. Our results indicate the relevance of seed availability on foraging behaviour of harvester ants, which should be taken into account when predicting and evaluating the effect of ants on seed resources as well as numerical responses of harvester ant populations to the temporal and spatial variations in grass seed abundance.     

Vagabond males and sedentary females: spatial ecology and mating system of the speckled rattlesnake (Crotalus mitchellii)

We used radiotelemetric data and behavioural observations to characterize seasonal (mating versus post‐mating seasons) and sexual variation in movement patterns, as well as to examine some of the ecological factors contributing to the evolution of the mating system in a venomous predator from the Mojave Desert of North America, the speckled rattlesnake, Crotalus mitchellii. Mating occurs in spring from late April to early June, shortly after emergence from hibernation, when snakes are predictably aggregated around the dens. Males and females travelled further per unit time in the mating season compared to the post‐mating season. Males also travelled longer distances per unit time than females in the mating and post‐mating seasons, and males with larger home ranges during the mating season had more potential mating partners. The results obtained suggest that males actively locate females during the mating season, and that the drastic increase in distance travelled by males during the mating season may be caused by strong male–male competition for access to females, probably because of the limited availability of sexually receptive females. Furthermore, males fight for access to females, and males of larger size are more likely to acquire females. Therefore, sexual selection apparently acts on two different male phenotypic traits: investment in mate‐searching activities and male body size. The present study demonstrates that combining quantitative spatial analyses and behavioural observations in an explicit temporal context can significantly advance our understanding of the ecology and evolution of organismal mating systems. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 681–695.     

Sex and rank in competitive brood hierarchies influence stress levels in nestlings of a sexually dimorphic bird

Studies of sibling competition within brood hierarchies have rarely assessed simultaneously the effects of sex and rank in the brood hierarchy on traits other than offspring mortality and differential growth. We studied the expression of heat-shock proteins (Hsps) to assess the physiological stress response to different combinations of sex and position within competitive brood hierarchies in the black kite Milvus migrans (Bodd.), a sexually dimorphic raptor showing facultative siblicide. Senior males showed higher stress levels than did senior females and younger siblings of each sex as revealed by Hsp60 values. The analysis of Hsp70 levels indicated that nestlings from broods in which the senior chick was a male showed higher stress levels than did nestlings from broods in which the senior chick was a female. In addition, levels of Hsp60 were related negatively to nutritional condition expressed as levels of plasmatic albumin. This suggests that the sex of senior chicks may be key in determining their stress level and that of their siblings, which is probably associated with sibling competition by fighting within brood hierarchies. The comparatively higher stress levels of senior males (and their siblings) may be a consequence of their ability to exploit their potential advantage from being the head start while avoiding a possible competitive disadvantage from being the smaller sex, independent of environmental conditions determining the probability of brood reduction. Differential stress levels depending on sex and rank in the brood hierarchy may be a consequence of parental control of offspring behaviour through differential resource allocation (e.g. yolk androgens) or it may reflect adaptations of particular chicks (senior males) to enhance their competitive ability within brood hierarchies.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 383–390.     

Early Cambrian coelobiontic communities in tectonically unstable crevices developed in Neoproterozoic andesites, Ossa-Morena, southern Spain

A well-preserved biota of Lower Cambrian cavity-dwelling organisms is recorded within fissures in Neoproterozoic andesites in Ossa-Morena (southern Spain). The cavities are unique among described Lower Cambrian coelobiontic communities due to the igneous character of the host rock. Coelobiontic habitat was episodically enlarged by synsedimentary tectonic fracturing reflecting polyphase infill of recurrent facies. The pioneer coelobiontic biota was diverse, and consisted of encrusting stromatolites and thromboids (dominated by Epiphyton and Renalcis), attached to walls and ceilings of the cavities, associated with archaeocyaths. Sponge spicules and chancelloriid sclerites occur as dense clusters indicating in situ growth, death and decay of spiculate sponges and coeloscleritophorans. Other organisms, such as echinoderms, trilobites and brachiopods, are also found within the cavities as reworked skeletons, indicating that they were washed in from the overlying, open seafloor. The main feature of the coelobiontic biota is the dominance of a sessile, chemosynthetic and filter-feeding epibenthos, composed of microbial communities, archaeocyaths, spiculate sponges (demosponges and rarer hexactinellides) and coeloscleritophorans.     

An unusual lizard (Reptilia: Squamata) from the Early Cretaceous of Las Hoyas, Spain

The Early Cretaceous (Barremian) lepidosaurian assemblage of Las Hoyas (Cuenca Province, Castilla-La Mancha), Spain, resembles that of roughly contemporaneous localities at Montsec (Catalonia), Una (Castilla-La Mancha) and Galve (Aragon) in being dominated by the scincomorph lizard Meyasaums ( Ilerdaesaurus ). It differs in the presence of two previously unknown taxa, one of which, described here, shows a strikingly unusual morphology in its ribs and distal limb skeleton suggestive of climbing specialization. This form is sufficiently distinct from other known taxa to warrant the erection of a new genus and species, Scandensia ciervensis gen. et sp. nov. Cladistic analysis of Scandensia suggests that it forms a sister taxon to living squamates (Iguania + Scleroglossa and all descendants of their most recent common ancestor).     

The importance of traditional farmland areas for steppe birds: a case study of migrant female Great Bustards Otis tarda in Spain

A detailed knowledge of the habitat requirements of steppe birds living in farmland habitats is necessary to identify agricultural practices compatible with their conservation. The globally threatened Great Bustard Otis tarda is a partial migrant in central Iberia, but factors affecting its winter habitat use have not been identified. We assessed habitat differences between breeding and wintering areas and winter habitat selection of radiotagged migrant female Great Bustards in central Spain. Of 68 tagged females, 35% moved to wintering areas located 64.3 ± 24.0 km south of their breeding areas, and 80% wintered in a single area of c. 236 km². A census of the population in this area identified it as one of the most important wintering areas of this species in the world, holding c. 1500 individuals. There were significant differences between breeding and wintering habitats of individually marked migrant females. Compared with breeding areas, wintering areas of migrant females were located further from roads and urban nuclei, had lower human population densities and area of urban developments, and a higher diversity of land‐use types, with less cover of cereals and more vineyards and olive groves. Within this area, radiotracked migrant females preferred sites with more vineyards and a lower land‐use diversity. Our results highlight the importance of traditional Mediterranean dry farmland mosaics, and suggest that different conservation strategies are needed for migrant and resident populations in winter to secure the conservation of suitable wintering habitat for Great Bustards in the Iberian Peninsula.     

Use of an encounter model to predict size-selective predation by a stream-dwelling cyprinid

1. The increased probability of detection of larger prey in the drift appeared to be the major determinant of prey consumption by red roach (Rutilus arcasi) in an Iberian stream. We developed a model of prey encounter incorporating this factor that explained 68% of the overall variation in the consumption of prey of different size. When the same model was applied to benthic size distributions the variance explained was 16%. 2. Gape-limited ingestion also was a significant factor influencing prey consumption and, when added to the model, the variance explained increased to 85% (32% for the benthos), but its importance was strongly dependent on both: (i) the maximum prey size available and (ii) fish size. 3. The exact shape of red roach reactive fields and, as a consequence, their degree of size-selectivity appeared to vary seasonally and ontogenetically. In summer, younger fish exhibited apparently reduced reactive fields. We hypothesize that this can be the result of factors such as microhabitat shifts or a reduced range of prey sizes.     

Understanding down-regulation of photosynthesis under water stress: future prospects and searching for physiological tools for irrigation management

Photosynthetic down-regulation and/or inhibition under water stress conditions are determinants for plant growth, survival and yield in drought-prone areas. Current knowledge about the sequence of metabolic events that leads to complete inhibition of photosynthesis under severe water stress is reviewed. An analysis of published data reveals that a key regulatory role for Rubisco in photosynthesis is improbable under water stress conditions. By contrast, the little data available for other Calvin cycle enzymes suggest the possibility of a key regulatory role for some enzymes involved in the regeneration of RuBP. There are insufficient data to determine the role of photophosphorylation. Several important gaps in our knowledge of this field are highlighted. The most important is the remarkable scarcity of data about the regulation/inhibition of photosynthetic enzymes other than Rubisco under water stress. Consequently, new experiments are urgently needed to improve our current understanding of photosynthetic down-regulation under water stress. A second gap is the lack of knowledge of photosynthetic recovery after irrigation of plants which have been subjected to different stages of water stress. This knowledge is necessary in order to match physiological down-regulation by water stress with controlled irrigation programmes.     

Cl‐ homeostasis in includer and excluder citrus rootstocks: transport mechanisms and identification of candidate genes

To reveal specific Cl^‐ transport activities in the symplastic pathway, uptake, long‐distance transport and distribution of Cl^‐ have been investigated in the citrus rootstocks Carrizo citrange (CC, Cl^‐ includer) and Cleopatra mandarin (CM, Cl^‐ excluder). Using an external concentration of 4.5 mm Cl^‐, both species actively transported Cl^‐ to levels that exceeded the critical requirement concentration by one and two orders of magnitude in the excluder and the includer rootstocks, respectively. Both CC and CM modulated Cl^‐ influx according to the availability of the nutrient as uptake capacity was induced by Cl^‐ starvation, but inhibited after Cl^‐ resupply. Net Cl^‐ uptake was higher in the includer CC, an observation that correlated with a lower root‐to‐shoot transport capacity in the excluder CM. The patterns of tissue Cl^‐ accumulation indicated that chloride exclusion in the salt‐tolerant rootstock CM was caused by a reduced net Cl^‐ loading into the root xylem. Genes CcCCC1, CcSLAH1 and CcICln1 putatively involved in the regulation of chloride transport were isolated and their expression analysed in response to both changes in the nutritional status of Cl^‐ and salt stress. The previously uncharacterized ICln gene exhibited a strong repression to Cl^‐ application in the excluder rootstock, suggesting a role in regulating Cl^‐ homeostasis in plants.