Syndrome of Eusociality in African Subterranean Mole-Rats (Bathyergidae, Rodentia), its Diagnosis and Aetiology

Abstract

Tbe problem of eusociality is analyzed (in an analogy to medical approacb) in several steps. First, cbaracteristic signs (basic symptoms necessary and sufficient for differential diagnosis) of eusociality and relevant examination metbods are pointed out. Secondly, tbe species of mole-rats exhibiting tbis pattern of qualities (syndrome) are identified. Tbirdly, proximate mecbanisms and ultimate causes (d., aetiology) of particular basic symptoms are analyzed using a comparative metbod (ecological comparison witb otber subterranean and otber cooperatively breeding mammals, and pbylogenetic comparison with other hystricognatb rodents). Tbe most parsimonious explanation is that eusociality in Heterocephalus glaber and at least three species of Cryptomys developed from cooperative monogamy. This breeding system is considered here an ancestral trait in bathyergid mole-rats, linking them to otber hystricognatb rodents. Common features of cooperative monogamy and eusociality in mammals are social tolerance of mate and weaned offspring, and reproductive division of labor (monopolization of reproduction by one female and reproductive altruism, i.e., sterility and belping, by offspring). Sterility is due to incest avoidance (based on individual recognition) in smaller and stable families of Cryptomys, and due to bebavioral suppression through tbe "queen" in larger families witb high turnover of H. glaber. Hormonal regulation of reproductive suppression, social tolerance, and (a1lo)parental bebavior may correspond to common mammalian patterns. Tbe only substantial difference between eusociality and simple cooperative monogamy is lifelong philopatry Oack of dispersal) in most offspring in eusocial families. Consequently, attention is focused on bow to examine and explain tbis trait. Pbilopatry can be estimated indirectly from tbe colony size in the field and net recruitment (assessed on the basis of laboratory data). Induction and/or suppression of dispersal in mole-rats bave not been studied but may correspond to normal mammalian patterns. Independent loss of social bebavior in subterranean mammals can be explained easier than its independent gain. The question is not why some mole-rats became 385 S. P. Wasser (ed.), Evolutionary Theory and Processes: Modern Perspectives