Monotremes are characterized by which reproductive trait?

Monotremes are a unique group of mammals that are specifically distinguished by their reproductive trait of laying eggs. Unlike most mammals that give birth to live young, monotremes reproduce by laying eggs that incubate outside their bodies. This trait is a remnant of the early evolutionary ancestry of mammals, as their ancestors were egg-laying reptiles.

Monotremes, such as the platypus and echidnas, provide an interesting contrast to other mammals, which typically have more complex reproductive processes. The eggs laid by monotremes are soft-shelled, much like those of reptiles. After the eggs hatch, the mother lactates to nourish the young, which showcases another unique aspect of monotreme reproduction, blending traits of both egg-laying and nurturing common in mammals. This unique reproductive characteristic is pivotal in understanding the evolutionary biology of mammals.