Foraging in wild and captive colonies of the common mole-rat Cryptomys hottentotus hottentotus (Rodentia: Bathyergidae)

Abstract

The foraging behaviour of wild and captive colonies of the common mole-rat Cryptomys hottentotus hottentotus was investigated. Field studies were undertaken at Steinkopf, an arid site in the Northern Cape, South Africa. The resource biomass and the total available energy here were amongst the highest recorded for any bathyergids, but geophytes were sparse and widely distributed. The low probability of locating these widely dispersed resources should promote co-operative foraging in these mole-rats. All common geophyte species were consumed, probably because high burrowing costs favour dietary generalism. The mole-rat biomass per metre of burrow system (0.5–1.5 g.m^-1) remains the lowest of any subterranean mammal. Burrow depths and diameters were positively correlated to the average mass of the mole-rats in each colony, apparently minimizing energetic costs and maximizing foraging success. Bulbs were stored in central food caches, and both field and laboratory data indicated size-based selectivity in their storage and consumption. As predicted by central place foraging theory, large bulbs were preferentially stored, and small bulbs preferentially consumed. However, factors other than energetic considerations may also influence these storage decisions. Total consumption time of bulbs was negatively correlated to animal size, and furthermore smaller bulbs were consumed more rapidly than larger ones. In addition to geophyte-storing, C. h. hottentotus also exhibited an alternative foraging strategy, termed ‘geophyte farming’, in which large bulbs were left and eaten in situ, and served as a renewable resource. Together with these foraging specializations, the evolution of sociality in mole-rats is a key factor which enables them to live in some of the most formidable habitats in Africa.