Preference for Outbreeding in Inbred Littledale’s Whistling Rats Parotomys littledalei

Abstract

Inbreeding is sometimes tolerated in species occupying patchy, unpredictable habitats, when it becomes too costly to outbreed. Although traits leading to inbreeding depression may be selected against, the accumulation of deleterious alleles from constant inbreeding might promote outbreeding. Littledale’s whistling rat Parotomys littledalei occupies patchy habitats in arid western South Africa, and inbreeds in captivity without suffering inbreeding depression. We tested whether female whistling rats make different mate choice decisions based on their inbreeding status, by providing inbred and outbred females with male odours in four pairwise combinations: unrelated outbred (r = 0) versus either familiar littermate sibling (r = 0.5), half-sibling (r = 0.25), first cousin (r = 0.125), or unrelated inbred (r = 0). Inbred females generally preferred unrelated males, but did not show a preference between unrelated males and first cousins. In contrast, outbred females did not generally show a preference, but preferred first cousins to unrelated males. In breeding trials, reproductive success (output and viability of young) of outbred females paired with unrelated or related males was equally high, whereas inbred females suffered reduced reproductive success when paired with closely-related males. Our results indicate that the inbreeding status of female whistling rats influences their decision to outbreed or inbreed, which might influence their fitness. While outbred whistling rats do not suffer inbreeding depression, the costs of constant inbreeding could promote outbreeding by inbred females. We propose that the choice to inbreed or outbreed in female whistling rats depends on their inbreeding status and the genetic relatedness of available mates, with outbreeding more likely to occur unless unrelated mates are not available.