Growing out of a caste--reproduction and the making of the queen mole-rat.

Naked mole-rats have a eusocial colony structure consisting of non-reproductive workers and a reproductively active caste where a single, dominant queen and 1-3 males produce all of the offspring. Well-established queens have elongated bodies that characterize their caste. Worker females retain the ability to transform into queens, however the trigger and time course for this physical transformation remain a mystery. Here, we show a direct link between periods of pregnancy and vertebral lengthening in nascent queens. Adult female mole-rats were paired with a male and radiographed weekly for two and a half years to track the growth of the lumbar vertebrae as the mole-rats became sexually mature and experienced pregnancies. The lumbar vertebrae of breeding females grew at an increased rate during each pregnancy but growth rates returned to normal between pregnancies and during extended periods without reproduction. The rate of lumbar lengthening was reduced to normal rates in older, established queens experiencing pregnancies. Our results imply that the length of a new queen mole-rat is proportional to the number of pregnancies experienced and suggest that hormones related to pregnancy may play the critical role in bone growth associated with caste transformation.