Two populations of X-ray pulsars produced by two types of supernova.

Two types of supernova are thought to produce the overwhelming majority of neutron stars in the Universe. The first type, iron-core-collapse supernovae, occurs when a high-mass star develops a degenerate iron core that exceeds the Chandrasekhar limit. The second type, electron-capture supernovae, is associated with the collapse of a lower-mass oxygen-neon-magnesium core as it loses pressure support owing to the sudden capture of electrons by neon and/or magnesium nuclei. It has hitherto been impossible to identify the two distinct families of neutron stars produced in these formation channels. Here we report that a large, well-known class of neutron-star-hosting X-ray pulsars is actually composed of two distinct subpopulations with different characteristic spin periods, orbital periods and orbital eccentricities. This class, the Be/X-ray binaries, contains neutron stars that accrete material from a more massive companion star. The two subpopulations are most probably associated with the two distinct types of neutron-star-forming supernova, with electron-capture supernovae preferentially producing systems with short spin periods, short orbital periods and low eccentricities. Intriguingly, the split between the two subpopulations is clearest in the distribution of the logarithm of spin period, a result that had not been predicted and which still remains to be explained.

The relationships between soil factors, grass nutrients and the foraging behaviour of wildebeest and zebra.

We examined the relationships between soil factors, nutrients in grasses and foraging behaviour of wildebeest (Connochaetes taurinus) and zebra (Equus burchelli) in a semi-arid nature reserve in South Africa. We tested the hypotheses that: (1) Soil nutrient levels determine the abundance and distribution of grass species; (2) nutrient levels within grass species are correlated with soil nutrient levels; (3) the spatial distribution and diet composition of ungulates is influenced by the nutrient availability in grasses. The distribution of soil factors in upper ground levels did explain the differential abundance of grass species in the reserve. Ordination of nutrient levels in grass species showed high levels of particular nutrients in certain species, but no one species showed uniformly high levels of all nutrients. Moreover, grasses on fertile soils did not necessarily accumulate higher nutrient levels than grasses on poor soils. Thus, nutrient levels in grasses were not correlated with soil nutrient levels. Wildebeest and zebra responded to monthly variations in the levels of N and P in grasses by moving seasonally to habitat types characterized by grass communities containing a high proportion of nutritional species, rather than by selecting particularly nutritious species within communities. We suggest that within semiarid savannas, areas with a higher diversity of grass communities will be more likely to have some of these communities containing high nutrient levels at any given season, than a lower diversity area. Therefore, the higher-diversity area would be likely to support more herbivores, and thus diversity would control carrying capacity.