Birds defend territories just as frequently as mammals. Reproduction and Development Viviparity is well developed in mammals. They also nurse young, another demand on energy. Therefore there is usually a period when females and males are not sexually active. This is especially true for those species living in zones which have great seasonal differences.
Most females go through an estrus cycle.
During a part of this cycle she is receptive of the male. Secondary sex characteristics change which make males more interested. In most mammals there is no sloughing of the uterine lining if fertilization does not occur. All but 3 species are live bearers, viviparous . Female echidnas incubate eggs in a ventral pouch.