Morphological castes in a vertebrate

Abstract

Morphological specialization for a specific role has, until now, been assumed to be restricted to social invertebrates. Herein we show that complete physical dimorphism has evolved between reproductives and helpers in the eusocial naked mole-rat. Dimorphism is a consequence of the lumbar vertebrae lengthening after the onset of reproduction in females. This is the only known example of morphological castes in a vertebrate and is distinct from continuous size variation between breeders and helpers in other species of cooperatively breeding vertebrates. The evolution of castes in a mammal and insects represents a striking example of convergent evolution for enhanced fecundity in societies characterized by high reproductive skew. Similarities in the selective environment between naked mole-rats and eusocial insect species highlight the selective conditions under which queen/worker castes are predicted to evolve in animal societies.

Figure 1

Zygomatic arch width plotted against vertebral length (lumbar vertebra 5) for all individuals from colony 100 (A) and colony 1000 (B). ▫, breeding males; ▪, all nonbreeders of both sexes; ⧫, breeding females.

Figure 2

Plot of the regression line of vertebral length (lumbar vertebra L5) against zygomatic arch width for breeding females (; n = 10), adult nonbreeding females (; n = 10), and breeding males (□; n = 6) from the same colonies.

Figure 3

Zygomatic arch width (A) and vertebral length (lumbar vertebra L5) (B) in nonbreeding females, reproductively active virgins, and well-established (five litters or more) breeding females. Data are the mean and standard deviation.