The feeding and ranging behaviour of Galapagos giant tortoises (Geochehne elephantopus) The Cambridge and London University Galapagos Expeditions, 1972 and 1973

Field observations on the behaviour of adult Galapagos giant tortoises ( Geochehne elephantopus ) on the islands of Santa Cruz (Indefatigable) and Pinzon (Duncan) were made by the 1972 Expedition, and on Santa Cruz by the 1973 Expedition. The animals (in all 14 on Santa Cruz and three on Pinzon) were observed continuously throughout the day for periods of 3–14 days so that their main non-reproductive activities (feeding and walking) could be quantitatively recorded. The period of potential activity was between about 08.00 hrs and 16.00–18.00 hrs, but the pattern of behaviour was extremely variable, both in a given individual and between one individual and another; it seemed little influenced by the relatively slight fluctuations of temperature and climate which occurred during the period of study (July-September) on Santa Cruz. The tortoises on Pinzon, a much more arid island, were less active, and one showed a strongly bimodal activity pattern with a resting period during the middle of the day. The animals studied in 1972 showed a strong disposition to return after several days to the same sleeping place, but this homing tendency was not observed in 1973. This discrepancy can perhaps be attributed to the fact that the two expeditions worked in different areas under different weather conditions. Observations on the plants eaten, on the association between tortoises and certain birds, and on some other aspects of tortoise behaviour such as walking speed and responses to sounds are also described.     

Periphyton responds differentially to nutrients recycled in dissolved or faecal pellet form by the snail grazer Theodoxus fluviatilis

1. We aimed to separate the effects of grazers on periphyton via grazing from that of nutrient recycling from their faecal pellets. 2. We set up three different experimental treatments (snails/no snails/faecal pellets) and sampled over 16 days. The ‘snail’ treatment contained a low density (snail biomass c. 14 g^?2) of the gastropod grazer Theodoxus fluviatilis and the ‘faecal pellet’ treatment received the same amount of faecal pellets as were produced in the ‘snail’ treatment. Whereas the ‘faecal pellet’ treatment provided extra nutrients to periphyton from the faeces, the ‘snail’ treatment provided nutrients in the form of both faeces and in excreta. There was also direct grazing on periphyton in the ‘snail’ treatment. The ‘no snail’ was not grazed and received no nutrients in faeces or excreta. 3. We measured periphyton C, N and P content, chlorophyll‐a (chl‐a), primary production, bacterial biomass, bacterial production and bacterial respiratory activity. In the water column we measured dissolved inorganic N and soluble reactive P. 4. Snails increased the amount of dissolved inorganic N in the water. On day 16, the periphyton N : P ratio in the ‘faecal pellet’ treatment was lower, and periphyton P content was higher, than in the other two treatments. N : P ratios decreased over time in the ‘faecal pellet’ treatment. Primary and bacterial production were positively correlated in all treatments. 5. Algal chl‐a and primary production of periphyton per unit area and periphyton chl‐a : C ratios increased over the 16 day in the ‘snail’ treatment, and thus excretion of dissolved N by snails had a stronger positive effect on the periphyton community than N and P in faecal pellets. 6. Our data show that excretion and egestion can have different effects on periphyton, probably because of the higher proportion of dissolved N in excreta and the higher proportion of P recycled in faecal pellets. The relative effect of nutrients recycled in egesta or in excretions, probably depends on the form of nutrient limitation of the periphyton. Further, the different components of the periphyton matrix could react differently to the different forms of nutrient recycling. 7. We conclude that direct grazing effects are less important than nutrient effects when nutrients are limiting and grazing pressure is low. Further, the spatial separation of different grazing effects can lead to differences in periphyton production and nutrient stoichiometry. This might be an explanation for the patchiness of periphyton in nature.