Many animals are parthenogenetic, meaning that they reproduce by eggs that are not fertilized and produce only females. This sounds weird to us mammals, but many invertebrates, such as rotifers and aphids, have kinds where this is common. It's even known to have occurred in birds that normally require fertilization for the development of their eggs.
But why parthenogenesis, when obviously bisexual reproduction has been
selected for in so many animals? One suggested reason is the ability to quickly turn
out large numbers of offspring with a given genetic makeup. Many of the parthenogenetic
species reproduce by that method during the benign parts of the year, but begin to
produce males as well as females as the weather degenerates. Mating scrambles the
genetic makeup in a variety of ways in eggs that lie dormant until favorable conditions
again appear. Hopefully, at least a few of those eggs will produce offspring suitable
for the new conditions, and these can quickly repopulate the area without having to put
up with the presence of males—useless creatures until bad times return. 
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.