Circadian Rhythms of Locomotor Activity in Solitary and Social Species of African Mole-Rats (Family: Bathyergidae)

Abstract

Mole-rats are strictly subterranean and hardly, if ever, come into contact with external light. As a result, their classical visual system is severely regressed and the circadian system proportionally expanded. The family Bathyergidae presents a unique opportunity to study the circadian system in the absence of the classical visual system in a range of species. Daily patterns of activity were studied in the laboratory under constant temperature but variable lighting regimes in individually housed animals from 3 species of mole-rat exhibiting markedly different degrees of sociality. All 3 species possessed individuals that exhibited endogenous circadian rhythms under constant darkness that entrained to a light-dark cycle. In the solitary species, Georychus capensis, 9 animals exhibited greater activity during the dark phase of the light cycle, while 2 individuals expressed more activity in the light phase of the light cycle. In the social, Cryptomys hottentotus pretoriae, 5 animals displayed the majority of their activity during the dark phase of the light cycle and the remaining 2 exhibited more activity during the light phase of the light cycle. Finally in the eusocial Cryptomys damarensis, 6 animals displayed more activity during the light phase of the light cycle, and the other 2 animals displayed more activity during the dark phase of the light cycle. Since all three mole-rat species are able to entrain their locomotor activity to an external light source, light must reach the SCN, suggesting a functional circadian clock. In comparison to the solitary species, the 2 social species display a markedly poorer response to light in all aspects. Thus, in parallel with the sociality continuum, there exists a continuum of sensitivity of the circadian clock to light.

Distribution of orexinergic neurons and their terminal networks in the brains of two species of African mole rats

“…Both species studied have a greatly reduced visual system (Oelschlager et al, 2000;Cernuda-Cernuda et al, 2003;Nemec et al, 2004), are subterranean, and appear to have a free-running circadian activity oscillator (Lovegrove and Papenfus, 1995;Lovegrove and Muir, 1996;Negroni et al, 2003;Oosthuizen et al, 2003;Gutjahr et al, 2004). These atypical rodent features combined with the distant (but familial) relation to each other and to the non-familial laboratory rat (Adkins et al, 2003), provide an interesting model to examine changes in nuclear organization and terminal network patterns that may be related to phenotype, life history and behaviour.…”

Orexinergic neuron numbers in three species of African mole rats with rhythmic and arrhythmic chronotypes

“…It is well known that mole rats, subterranean, visually regressed rodents, have unusual patterns of circadian rhythmicity (Lovegrove and Papenfus, 1995;Lovegrove and Muir, 1996;Oosthuizen et al, 2003). Locomotor activity studies have shown that within a species of mole rat there are individuals that have a rhythmic chronotype, and others that are distinctly arrhythmic and this is seen in both social and solitary species of mole rat (Oosthuizen et al, 2003).…”

“…Locomotor activity studies have shown that within a species of mole rat there are individuals that have a rhythmic chronotype, and others that are distinctly arrhythmic and this is seen in both social and solitary species of mole rat (Oosthuizen et al, 2003). A study of sleep patterns in the giant Zambian mole rat (Cryptomys mechowi) showed some differences in sleep patterns between rhythmic and arrhythmic chronotypes of this species, where arrhythmic individuals spend more time in waking with a longer average duration of a waking episode, and less time in nonREM (NREM) sleep with a shorter average duration of a NREM episode though with a greater NREM sleep intensity (Bhagwandin et al, 2011b).…”

“…It should be noted that Ansel"s and Damaraland mole rats were previously considered to be members of the genus Cryptomys. All three species of mole rat have a reduced visual system, live a subterranean lifestyle and have unusual patterns of circadian rhythmicity (Lovegrove and Papenfus, 1995;Lovegrove and Muir, 1996;Oelschlager et al, 2000;Cernuda-Cernuda et al, 2003;Negroni et al, 2003;Oosthuizen et al, 2003;Gutjahr et al, 2004;Nemec et al, 2004;Nemec et al, 2008). Brains from circadian distinct rhythmic and arrhythmic individuals of each species were examined.…”

Distributional range, ecology, and mating system of the Cape mole-rat (Georychus capensis) family Bathyergidae

“…The onset of this drumming breaks down territoriality between individuals and courtship is accompanied by increased testosterone levels (but see Oosthuizen and Bennett 2009) and enlargement of the testes and reproductive glands in males (Bennett 1988;Bennett et al 2006). Previous research on G. capensis has focused on its physiology (Oosthuizen et al 2003, Oosthuizen andBennett 2007;Oosthuizen et al 2008;Oosthuizen and Bennett 2009), reproductive biology (Taylor et al 1985;Bennett 1988;Bennett and Jarvis 1988;Kinahan et al 2008), intergeneric relationships (Honeycutt et al 1987;Nevo et al 1987;Allard and Honeycutt 1992;Faulkes et al 2004;Ingram et al 2004), age determination (Taylor et al 1985), seismic communication (Narins et al 1992), energetic cost of digging (Du Toit et al 1985; Scantlebury et al 2006), influence on plant communities (Hagenah and Bennett 2012) and burrowing dynamics (Romañach 2005;Thomas et al 2012). Invariably, sample sizes in these studies were small and biased towards single regions within the distributional range, especially the western Cape Province.…”