Leptokurtic distributions of movement distances observed in field-release studies, in which some individuals move long distances while most remain at or near their release point, are a common feature of mobile animals. However, because leptokurtosis is predicted to be transient in homogeneous populations, persistent leptokurtosis suggests a population heterogeneity. We found evidence for a heterogeneity that may generate persistent leptokurtosis. We tested individuals of the Trinidad killifish Rivulus hartii for boldness in a tank test and released them back into their native stream. Boldness in the tank test predicted distance moved in the field releases, even after effects of size and sex were removed. Further, data from a 19-mo mark-recapture study showed that individual growth correlated positively with movement in a predator-threatened river zone where the Rivulus population is spatially fragmented and dispersal is likely to be a hazardous activity. In contrast, no such correlation existed in a predator-absent zone where the population is unfragmented. These results show that a behavioral trait, not discernible from body size or sex, contributes to dispersal and that a component of fitness of surviving "dispersers" is elevated above that of "stayers," a fundamental assumption or prediction of many models of the evolution of dispersal through hazardous habitat.
An experiment was conducted to compare the nutritive value of a range of ensiled forage legumes. Silages were prepared from late second‐cut lotus (Lotus corniculatus), first‐cut sainfoin (Onobrychis viciifolia) and both early and late second‐cut red clover (Trifolium pratense) and lucerne (Medicago sativa). Each experimental silage was offered to six Suffolk‐cross wether lambs, aged 10 months, housed in metabolism crates. Voluntary intakes of dry matter ranged from 71 to 81 g kg−1 liveweight0·75 d−1. Voluntary intakes were similar on the lotus, sainfoin and late‐cut red clover silages, but the voluntary intake on the lotus silage was significantly higher than that on the lucerne silages and early‐cut red clover silage. Digestibility of organic matter in the dry matter was highest for the lotus silage (0·650), and lowest for the sainfoin silage (0·527). Although most of the N in the sainfoin silage appeared to be in an indigestible form, N digestibility was approximately 0·70 for the other legume silages. The highest loss of N in urine, 0·75 of N intake, was recorded for lambs offered the lucerne silage. Differences in N intake, N loss in faeces and N loss in urine led to statistically significant differences in the amount of N retained, with the highest and lowest N balances recorded for the lotus (16 g N d−1) and sainfoin (−2 g N d−1) silages respectively. The results confirm that these high protein forages have high intake potential. While low N digestibility appears to limit the nutritional value of sainfoin, further research could formulate feeding strategies that improve the efficiency with which the protein from red clover, lucerne and lotus is utilized.
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