Rousettus aegyptiacus may also make migrations of hundreds of kilometres. An individual tattooed at caves in the Tzaneen area of Limpopo was subsequently recovered at Mtunzini on the northern coast of KwaZulu-Natal (Jacobsen and du Plessis 1976). Population numbers of the Mpumalanga colonies decrease during winter and increase again in summer, while, conversely, a known colony at Mission Rocks Bat Cave (Greater St Lucia Wetland Park) on the northern coast of KwaZulu-Natal decreases in size dramatically (from > 5,000 to < 300) at the beginning of spring, and increases again during winter. It seems highly likely that the same bats migrate from winter roosts on the warm KwaZulu-Natal coast to summer roosts in the Limpopo lowveld.
TORPOR AND HIBERNATION
Although bats are warm-blooded mammals and can maintain a constant body temperature, some show a remarkable degree of thermolability, which occurs on a daily or seasonal basis. The body temperature of torpid bats is colder than that of homeothermic mammals and fluctuates in accordance with the ambient temperature. Daily torpor and hibernation are common in the Vespertilionidae, Rhinolophidae and Hipposideridae, and in some Molossidae (Ransome 1990). Other southern African bats are more conventional thermoregulators and cannot enter torpor.
Bronner et al. (1999) have shown that in the South African lowveld, the Angolan free-tailed bat, Mops condylurus, routinely selects the hottest roosts under tin roofs, and by allowing its body temperature to rise, is able to save energy that would otherwise have been required for cooling (i.e. the reverse of torpor). These same bats readily undergo torpor to conserve energy during daytime roosting in both summer and winter (Vivier and van der Merwe 2007).
Although a few bats migrate to warmer climates for winter, as do some birds, many bats paradoxically move to colder climates for winter (Fleming and Eby 2003). Here, facultative torpor enables these small mammals to shut down their metabolism to a basic level in response to winter cooling, thus allowing their body temperature to fall to that of their environment, or to just above freezing in sub-zero temperatures. The colder the ambient temperature, the greater the energy saved through the lowering of their internal body temperature. By switching off their energy-expensive heating system during hibernation, bats can survive cold winters when insects (i.e. fuel reserves) are very scarce. Prior to hibernating, in late summer, bats will eat voraciously to build up their fat reserves, which may comprise up to 26% of their total body weight just before hibernation. These reserves may have to tide them over for up to six months during hibernation (Fleming and Eby 2003). Bats budget their fat reserves very precisely and forced arousal from hibernation consumes huge amounts of energy reserves, and can result in bats starving before the end of winter. Studies from the northern hemisphere show that one arousal costs the energy that could cover about 60 days of hibernation (Kunz 1982, Kunz and Fenton 2003).
Figure 12. Several species of bat carry their young while foraging. This female Eidolon helvum carrying her young pup was photographed at a large maternity roost on Bat Island, Lake Kivu, Rwanda (© Ian Little).
REPRODUCTION
The demands of hibernation also lead to interruptions of the reproductive process, and the reproductive patterns of bats show some unique features compared with mammals generally (Adams and Pedersen 2000, Crichton and Krutzsch 2000).
Worldwide, the average length of bat gestation varies from 44 days in Pipistrellus pipistrellus to 7 months in Desmodus rotundus, but typically it is around 2 months (Barclay and Harder 2004). Gestation in southern African bats varies from 60 days in Chaerephon pumilus to an effective 7 or 8 months in species with delayed reproduction.
Bats exhibit three patterns of delayed reproduction: sperm storage, delayed implantation, and retarded embryonic development (for reviews, see Racey 1982, Bernard 1989, Happold and Happold 1990a, Bernard and Cumming 1997). These have evolved in species in tropical as well as temperate biomes, which reveals that these adaptations have a tropical origin (Racey 1982).
Sperm storage: After mating occurs in autumn, viable spermatozoa are stored and nourished in the female’s uterus or oviducts throughout the hibernation period. Eggs are only fertilised once the females have ovulated after arousal from hibernation in spring. Most temperate-climate bats use this phenomenon, as do many southern African Vespertilionidae and Rhinolophidae. The burden of sperm storage might be shared or is confined to one of the sexes. In Neoromicia nana (Bernard et al. 1997), Pipistrellus rusticus (van der Merwe and Rautenbach 1990b), Myotis tricolor (Bernard 1982b), and most Rhinolophidae (Bernard 1983, 1989, Cotterill 1989, 1998), viable sperm is stored predominantly in the female reproductive tract, while in Neoromicia capensis (van der Merwe 1994a) and Nycticeinops schlieffeni (van der Merwe and Rautenbach 1990a), sperm is stored in both the male and female tracts. At the other extreme, only Rhinolophus capensis males store sperm (Bernard 1985), and mating, ovulation and fertilisation all occur after winter.
Delayed implantation: Mating, ovulation and fertilisation occur in autumn, but the implantation and subsequent development of the resulting blastocyst (fertilised ovum) is delayed until after hibernation is completed. In southern Africa, Miniopterus natalensis and M. fraterculus exhibit this phenomenon (Bernard 1980, 1994, Bernard et al. 1996), as does Scotophilus dinganii (van der Merwe et al. 2006).
Retarded embryonic development: Normal mammalian fertilisation and implantation occur, but the embryo’s growth is retarded during hibernation and only resumes after hibernation is over. In southern Africa, Hipposideros caffer exhibits this phenomenon (Bernard and Meester 1982), as does Rhinolophus simulator (Cotterill 1989, 1998). In Scotophilus viridis, delayed implantation occurs in combination with retarded development (van der Merwe et al. 1988).
In addition to these patterns of delayed reproduction in hibernating bats, two basic types of ‘normal’ mammalian reproduction – seasonal polyoestry and seasonal monoestry – are common in non-hibernating southern African bats (Happold and Happold 1990a, Bernard and Cumming 1997). An interesting life history has evolved in the polyoestrous molossid Tadarida fulminans, which breeds over the cool, dry season in southern Africa; this is interpreted as a response to predation on insect prey (aerial plankton) at high altitudes (Cotterill and Fergusson 1993, Cotterill 2001b).
Seasonal polyoestry: More than one pregnancy and birth season occurs during the restricted breeding season, for example, Epomophorus wahlbergi, Mops condylurus and Chaerephon pumilus (Happold and Happold 1989a, 1990a). The high-flying molossid Tadarida fulminans is unique among known bats, as females lactate over the cool, dry season (Cotterill and Fergusson 1993).
Seasonal monoestry: Only a single pregnancy and birth season occurs during the restricted summer breeding season (e.g. the majority of bats, including most Vespertilionidae).
Most bats give birth to a single young per litter (Barclay and Harder 2004), while some Vespertilionidae such as Neoromicia nana, N. capensis, Pipistrellus rusticus, Scotophilus species and Nycticeinops schlieffeni commonly give birth to twins or triplets, and sometimes even quadruplets in the case of Neoromicia capensis (see species accounts for details).
Species with widespread distributions may have multiple births in equatorial parts of their range in Africa, but only one or two birth periods in more temperate regions; this seems to be the case in Taphozous mauritianus (Happold and Happold 1990a).
In gregarious species, pregnant females typically form nursery (maternity) colonies in summer. Here, the babies are born, often with the mother clinging upside down to the substrate with her toe claws and one wing claw to maintain a semi-horizontal