Energetics of crop raiding

Elephants raid cultivated crops almost exclusively at night, thereby implying that they have knowledge of the location of crop fields and that raiding is usually deliberate. The movement pattern of raiding elephants during the crop season would thus reflect their foraging needs in the natural habitat during the day, alternating with visits to crop fields at night. While the energetics of such a foraging pattern have not been empirically studied or analyzed even theoretically, I made detailed calculations of quantities of millet plants consumed by solitary bulls, bull groups, and family groups in several villages during 110 nights

Hut Schematics

Figure 8.3

Average monthly frequency of crop raiding during 1992-1993 in the Waza-Logone region of Cameroon. The frequencies are shown for three zones (northern, central, and southern) in this region. (Based on Tchamba 1996.)

Figure 8.3

Average monthly frequency of crop raiding during 1992-1993 in the Waza-Logone region of Cameroon. The frequencies are shown for three zones (northern, central, and southern) in this region. (Based on Tchamba 1996.)

of raiding in 1981-1982. In some instances of raiding by solitary bulls, I was even able to compute the rate of feeding. When finger millet plants were in a vegetative stage, a bull consumed 1.5 kg (dry weight) of plants per hour. This increased substantially to 6.4 kg per hour as the plants grew and flowered. In one instance, a bull managed an intake of over 12 kg per hour when the finger millet had set grain. At this rate, an adult bull can meet its daily requirement of forage in just 5-6 hours of feeding.

The average quantities of finger millet plants consumed by an adult bull during a night of raiding were 44 kg and 30 kg in two sets of villages differentiated on the basis of crop productivity. The maximum quantity consumed was 70-75 kg per night, recorded several times in both finger millet and sorghum fields. For family herds, the average quantities consumed in these sets of villages were 24 kg and 11 kg of millet plant per elephant per night, with an upper limit of 52 kg.

Using data of elephant population density, population structure, and the frequency of raids in the study villages, I made two ecologically relevant estimates of the relative importance of crop raiding in the elephant's diet. The "average" bull in the study area raided crops on 49 nights in the year, while the average family group did so about 8.4 nights. In quantitative terms, the cultivated crops constituted 9.3% of the diet of adult bulls and 1.7% of the diet of family groups during the year.

There were obviously sharp seasonal differences in raiding, with a peak during October-December, when the staple crop, finger millet, was cultivated (fig. 8.2). During these months, the crops constituted 22%-30% of the diet of adult bulls and 4%-5% for family groups.

Averages also obscure other patterns of skewness in the data. Certain adult bulls carried out most of the raids, and presumably so did particular family groups, although this was difficult for me to determine. Two adult bulls (of about 20 bulls in the area) were particularly notorious raiders, one of them entering cultivation about 120 nights of the year. At least one in two of the larger bulls seemed to be regular raiders, while some of the others may have raided more sporadically. Christy Williams and A.J.T. Johnsingh also report from a recent study at Rajaji National Park in northern India that two of four adult bulls monitored through radiotelemetry were crop raiders. Of three radio-collared female elephants from different clans in the Nilgiris in southern India monitored by M. Balasubramanian and associates, only one indulged regularly in crop raiding over the 4-year period of the study. However, all four family groups being tracked through telemetry by my research team in the Buxa-Jaldapara Reserves (section 4.4.1) have indulged in raiding crops.

These patterns indicate that cultivated crops may be significant in the diet of some elephants in a population. In qualitative terms, crops may be even more important than the figures suggested by quantitative intake because of the superior nutritional attributes of crop plants compared to wild plants.

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