SAN JOSE, CALIFORNIA--Sugar and spice may be what little girls are made of, but rhinoceroses appear to use a different cookbook. A new study shows that black rhinos stressed early in their pregnancies are almost 3 times as likely to have male babies as female ones. The imbalance may be due to a spike in blood sugar that selectively dooms female offspring. The finding could help attempts to rebuild wild rhino herds.
Wildlife managers have tried to expand the range of critically endangered wild rhinos by transferring some of the animals to new game parks. Their efforts have been stymied, however; relocated females have given birth to too many males and not enough females. Studies in other mammals have suggested that high blood sugar harms female embryos at the earliest stages of pregnancy. Because stressful events such as capture trigger an acute rise in blood sugar, wildlife biologist Wayne Linklater of Victoria University in Wellington, New Zealand, wondered whether the stress of relocation was killing off females before they had a chance to be born.
Black rhinoceros births are rare, so to answer the question, Linklater and colleagues from the Zoological Society of San Diego in California turned to the rhinoceros "stud book," a record of births that goes back 40 years. By noting when females gave birth, the team estimated the gestational stage for each female relocated while pregnant. Among females that were captured in the first 100 days of pregnancy--the length of time it takes a rhinoceros embryo to implant and the placenta to form--male births outnumbered female 2.7 to 1. Females transferred later in their 490-day pregnancies had equal numbers of male and female calves. Timing the captures to later stages of pregnancy could rebalance the sex ratio, says Linklater, who will present his team's findings here on Wednesday at the Society for Conservation Biology Meeting.
That may only solve half of the problem, however. Females that become pregnant in their lush new surroundings also give birth mostly to males, says Linklater. The reason may be high blood sugar that results from obesity: Mothers pack on the pounds and can take half of their reproductive lives to lose the weight, he says.
"I think he's onto something," says conservation biologist Joel Berger of the Wildlife Conservation Society in Bozeman, Montana, who has worked with African rhinos in the past. Berger notes that captivity combines stress with abundant food, a potential double-whammy that conservationists should counter by reducing stress or altering the animals' diets. Wildlife biologist Elissa Cameron of the University of Pretoria in South Africa says she is also pleased with the study, but to be convinced by the results, "I would want to see some [direct] experimental evidence," such as measurements of glucose levels.