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There has been debate over the nutritional significance of the fig in the diet of frugivores1,2 and over its importance as a ‘keystone’ species3,4,5,6. We compared the mineral composition of figs and other fruits from Belize, Indonesia and Uganda, and found that there was more than three times as much calcium in figs as in other fruits.

We analysed the mineral content of figs and other non-domestic fruit species from the neotropical, African and Asian regions (Table 1), and 11 tropical and 13 temperate domestic fruit species. Regional differences were evident in the calcium, magnesium, iron, manganese, phosphorus and sodium content of all the fruits. This may have reflected varying soil fertility between collection sites.

Table 1 Mineral analysis data
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However, between fig and non-fig fruits, differences in mineral concentrations were restricted to calcium. On average, figs contained calcium levels 3.2 times higher than other fruits — levels high enough to promote eggshell deposition in birds, and bone growth in birds and mammals7,8. Also, the ratio of calcium to phosphorus (a measure of calcium availability7,9) was 3.7 times higher in figs than in other fruits (Fig. 1). Figs from Sulawesi, Indonesia contained more calcium relative to the calcium availability in the soils, indicating that fig trees may selectively absorb calcium or allocate calcium to fruits.

Figure 1: Mean (±sd.) calcium-to-phosphorus ratios for fig and wild non-fig fruits from Belize, Indonesia, and Uganda, and temperate and tropical domestic fruits.
figure 1

Numbers indicate species per sample.

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Previous studies have shown that the protein, carbohydrate and lipid content of figs1,2 are variable and not exceptionally high. Our study indicates that calcium concentration relative to phosphorus may be an important criterion for selection of the fruit.

Growth processes and egg-laying are accompanied by a rise in requirements for calcium and phosphorus7,8 to lay down eggshell, aid metabolism, construct nucleic acids and form bone. Because most non-fig fruits, as well as seeds and invertebrates, are poor sources of calcium, many birds and mammals rely on calcium supplements such as mollusc shells, bone or soil to ensure adequate dietary calcium7,10,11. Others consume large quantities of figs throughout the year5,6,12. The biological availability of calcium in figs has not been determined, but animals whose diets are rich in figs are unlikely to suffer from calcium deficiency.

Terborgh3 and others5,6,12 suggest that figs constitute a ‘keystone’ plant resource for fruit-eating birds and mammals throughout the tropics. Figs display inter- and intraspecific asynchrony in fruiting, tend to produce large crops, and show low interannual variation in fruit production13. These fruiting patterns make figs a reliable food source during times of general fruit scarcity.

Our results indicate that figs may be important throughout the year for maintaining an adequate balance of calcium among fruit-eating animals, thus fulfilling the role of a keystone plant resource for many animal species. These findings also suggest that the concentrations of specific minerals represent an important nutritional criterion for evaluating dietary choices across taxonomic groups and pantropical ecosystems.