Astronomically forced climate change in the Kenyan Rift Valley 2.7-2.55 Ma: implications for the evolution of early hominin ecosystems.

Global climate change, linked to astronomical forcing factors, has been implicated in faunal evolutionary change in equatorial Africa, including the origin and diversification of hominin lineages. Empirical terrestrial data demonstrating that orbital forcing has a significant effect, or is detectable, at early hominin sites in equatorial continental interiors during the Pliocene, however, remain limited. Sedimentation patterns in the Baringo Basin within the Central Kenyan Rift Valley between ca. 2.7 and 2.55 Ma, controlled by climatic factors, provide a detailed paleoenvironmental record spanning 35 fossil vertebrate localities, including three hominin sites. The succession includes a sequence of diatomites that record rhythmic cycling of major freshwater lake systems consistent with approximately 23-kyr Milankovitch precessional periodicity. The temporal framework of shifting precipitation patterns, relative to Pliocene insolation curves, implicate African monsoonal climatic control and indicate that climatic fluctuations in Rift Valley ecosystems were paced by global climatic change documented in marine cores. These data provide direct evidence of orbitally mediated environmental change at Pliocene Rift Valley hominin fossil localities, providing a unique opportunity to assess the evolutionary effect of short-term climatic flux on late Pliocene East African terrestrial communities.

A division of labour with role specialization in group-hunting bottlenose dolphins (Tursiops truncatus) off Cedar Key, Florida.

Individual role specialization during group hunting is extremely rare in mammals. Observations on two groups of bottlenose dolphins (Tursiops truncatus) in Cedar Key, Florida revealed distinctive behavioural roles during group feeding. In each group, one individual was consistently the 'driver', herding the fishes in a circle toward the remaining 'barrier' dolphins. Aerial fish-capture rates differed between groups, as well as between the driver and barrier dolphins, in one group but not in the other. These differences between the two groups may reflect differences in group stability or in prey school size.

Selective grazing by the mud snail Ilyanassa obsoleta.

Mud snails (Ilyanassa obsoleta) starved for 48 h were allowed to feed on sediments in laboratory microcosms. Sediment cores sliced at 2 mm intervals were compared to snail stomach contents for per cent carbon and nitrogen, plant pigment contents and species composition of benthic diatoms. Concentrations of carbon, nitrogen, phaeopigments, phycocyanin and chlorophyll were enriched in the top 2 mm of the sediments compared to 7-10 mm depth by a factor of 2-10. In turn, these materials were 20-40 times more concentrated in snail guts than in the surface sediments. Snail feces were enriched for carbon and nitrogen by 5-7 times over the surface sediments. Bacterial chlorophyll peaked at about 3-4 mm in the sediments and was not detectable in the snail stomach contents. The C/N ratio of the snail stomach contents was only 6 compared to a ratio of 8.5 for their feces and 12 for the surface sediments.The percentage of migratory diatoms (e.g. Nitzschia and Navicula) decreased with depth where non-migratory species, such as Fragilaria pinnata, dominated. These migratory species were more common in the snails than in the sediments on which they were feeding.A comparison of daily ingestion rates to the animal's energy budget shows that this selective ingestion is sufficient to meet Ilyanassa's energy needs.