There are approximately 5,400 mammalian species, ranging in size from the tiny 1-2 in
(30-40mm) bumblebee bat to the 1,083ft (330m) blue whale. These are distributed in about
1,200genera,153familiesand29orders. (seehttp://users.tamuk.edu/kfjab02/Biology/
mammalogy/mammal_classification.htm).
Reproduction in Mammals
Keep in mind what you have learned about reptiles and birds and see how mammals might
be both similar and different to these groups. The egg-laying monotremes, such as echidnas
(Figure14.11) and platypuses (Figure14.12), use one opening, the cloacae, to urinate,
deficate and reproduce, just as lizards and birds do. They lay leathery eggs, similar to those
of lizards, turtles and crocodilians. Monochromes feed their young by “sweating” milk from
patches on their bellies, since they lack nipples, unlike other mammals.
Figure 14.11: The echidna is a member of the monotremes, the most primitive order of
mammals. ( 25 )
Allothermammalsgivebirthtoliveyoungandareeithermarsupialorplacental. Thefemales
ofmostmarsupialshaveanabdominalpouchorskinfoldwithinwhicharemammaryglands
and a place for raising the young (Figure14.13).Placentalmammals have a placenta that
nourishes the fetus and removes waste products.
Some mammals are solitary except for brief periods when the female comes intoestrus,the
optimal time for a female to get pregnant. Others form social groups where a pronounced
difference between sexes (sexual dimorphism) is frequently extreme. Dominant males are
often those that are largest or best-armed. These males usually have an advantage in mating
or may exclude other males from access to females within a group, such as in elephant seals
(Figure14.14). This group of females forms aharem. Think back to what you learned
about courtship displays in birds. How are such systems in mammals similar or different?