Pregnant mommas must make a tough choice: preserve energy for themselves or invest it in their little ones. Now new research suggests that higher levels of the hormone leptin may fool a hamster’s brain into thinking there is energy to spare, promoting larger litters at the expense of the mother’s health. Researchers report the results in an upcoming Proceedings of the Royal Society B.
“I think the findings are compelling,” comments behavioral biologist Randy Nelson of Ohio State University in Columbus. He says this is the first evidence he has seen suggesting that leptin plays a part in how a mother invests her energy.
Secreted mostly by fat tissue, leptin seems to tell the body when to stop eating. But it has also been shown to affect rodents’ immune function and reproduction. When stressed mice have lowered immune defenses, doses of leptin can restore the activity to normal levels. A number of stressed mammal species treated with leptin will reproduce when they normally would not do so.
In this study, Susannah French of Utah State University in Logan, along with biologist Gregory Demas and colleagues from Indiana University in Bloomington, fitted pregnant Siberian hamsters with a pump that kept their leptin levels artificially high. Other pregnant Siberian hamsters got a pump with no leptin, as a control.
Pregnant hamsters receiving leptin had larger litters, between one and two more pups on average, than the pregnant hamsters without the leptin pumps. The pups also tended to be larger. French says the control mothers probably reabsorbed more embryos into the womb than the leptin-treated mothers (a common practice among some rodent species).
And whereas hamsters are known to pick off a newborn pup or two, presumably to cut down on the energy invested in rearing them, the leptin-treated hamsters did not eat a single one of their young.
“We generally see at least half of the hamsters eating one or two pups,” French says. The fact that the leptin-treated hamsters did not eat any of their newborns was surprising. In this study, 40 percent of the pregnant control animals consumed at least one of their offspring.
Hamsters that got the artificial leptin signal also paid a price for the larger broods. Blood tests showed that while pregnant, the leptin-treated hamsters could not fend off bacterial attacks as easily as their control counterparts. And once the animals were taken off the pumps, leptin-treated hamsters ate more than the untreated hamsters. French says this suggests that the rodents were making up for a dearth of energy, which high leptin levels had masked from their brains.
Unlike the baby-centric, leptin-treated moms, the control animals were responding naturally to the energy demands of pregnancy, French says. “They were not stretching themselves to the point where certain immune responses are suppressed.”
Nelson says that “it’s pretty clear to me that leptin is playing an important role in the physiological decision to invest in offspring.” He would like, however, to see the authors track the mechanisms by which leptin prevents cannibalism of the pups.
French says future studies will try to find out how leptin might be causing this tradeoff.