I have a litter of 15 day old gerbils. Three of them are strong and healthy. Shiny fur, eyes still sealed shut, very active and try to chase after momma when she leaves them. The last however, is now a third of their, size, not much bigger then at birth, and doesn't seem to be growing. The size difference keeps escalating between her and her brothers. Her black and white fur is dull and thin, and her head isn't the round bullet like shape of a typical baby gerbil; it is more pear like. She is not nearly as active and can't compete well with the others to eat. She just gets pushed away in the "feed me" frenzy, and doesn't have the strength to squeeze in. She's already made it past two weeks, but I'm becoming increasingly worried about her. Any suggestions? Is there anything I can do?

You can call the vet and ask him or her to take a look at the baby, you can also ask them if there's some way you could maybe feed the baby with a dropper to help it gain weight and get stronger since it apparently cannot compete with the other babies.

Well you should seperat them acouple times a day and just leave her and momma and see if it gets any healthyer sometimes they just can't fin for themselves so you half to give them time alone with momma