Many species of animal-pollinated flowers are known to vary widely in the nectar content of flowers. Some proportion of flowers in many species is apparently nectarless, and such flowers are believed to be ‘cheaters’. Cheating may explain a part of the variability in nectar content. If cheating exists as a qualitatively different strategy then we expect bimodality in the distribution of nectar content of flowers. It has been shown in a multispecies study that gregarious species have a higher proportion of cheater flowers. We studied the frequency distribution of total nectar sugar in two gregariously flowering species Lantana camara and Utricularia purpurascens, which differed in other floral and ecological characters. At the population level, both the species showed significant bimodality in the total sugar content of flowers. The obvious sources of heterogeneity in the data did not explain bimodality. In Lantana camara, bimodality was observed within flowers of some of the individual plants sampled. In Utricularia purpurascens the proportion of nectarless flowers was more in high-density patches, suggesting that the gregariousness hypothesis may work within a species as well. The results support the hypothesis of cheating as a distinct strategy since two distinct types of flowers were observed in both the species. The effect of density in Utricularia purpurascens also supports the gregariousness hypothesis.

Predatory protozoa are known to enhance biodegradation by bacteria in a variety of systems including rumen. This is apparently counterintuitive since many protozoa do not themselves produce extracellular degradative enzymes and prey upon bacterial degraders. We propose a mechanism of protozoal enhancement of bacterial biodegradation based on the sociobiology of biodegradation. Since extracellular enzyme production by degraders involves a cost to the bacterial cell, cheaters that do not make the enzyme will have a selective advantage. In the presence of cheaters, degraders that physically attach to water-insoluble substrate will have a selective advantage over free-floating degraders. On the other hand, cheaters will benefit by being free floaters since they consume the solubilized products of extracellular enzymes. Predatory ciliated protozoa are more likely to consume free-floating cheaters. Thus, due to protozoan predation a control is exerted on the cheater population. We illustrate the dynamics of such a system with the help of a computer simulation model. Available data on rumen and other biodegradation systems involving protozoa are compatible with the assumptions and predictions of the model.

In host–parasite co-evolution, parasites are assumed to have an advantage owing to their shorter generationtime. Evolution of pathogens within the lifetime of a host individual is implicated as a strong selective force inthe evolution of sex and aging in the host. However, this assumption or its testable predictions have not beenexamined empirically. We classified infectious bacteria and viruses into those that can have continued longtermexistence on the host body (group 1) versus those that have only a short-term interaction during an activeinfection (group 2). We surveyed the literature for age-specific incidence data about infections from both thegroups. The age trends of the two groups show contrasting patterns. The incidence of infections by all group 1pathogens showed a 2.28- to 28-fold increase in older ages. In group 2, 6 out of the 9 pathogens showed asignificant declining trend in incidence with age. In both groups, there was greater mortality or morbidityamong the infected in the old-age classes. These patterns are better explained by pathogen evolution than byage-related decline in immunity.