by Whit Gibbons

September 14, 2014

True or false? All birds lay eggs. Mammals give live birth to their young. Only male deer have antlers. All owls are nocturnal. Hummingbirds and warblers are always diurnal. These statements are true. Mostly. But there are exceptions, and some of them are utterly fascinating.

Learning about the ecological and behavioral traits of different animals is one of the most enjoyable pursuits in the field of biology. We have literally millions of living species on earth, and the human tendency is to categorize groups into more manageable units. For example, salamanders, oak trees, and starfish have little in common with each other, other than being living things. But within any one group similarities and consistencies exist. And each group has attributes peculiar to it that can be used in most instances to differentiate it from other groups, even more closely related ones, such as frogs from salamanders, pine trees from oaks, and sea urchins from starfish. But our rules for distinguishing among biological groups often dissolve in the face of scrutiny. Exceptions are rampant.

The once commonly held belief that no mammals lay eggs is one of those exceptions. Which brings us to the aquatic duckbill platypus of Australia. Platypuses easily qualify as one of the strangest mammals in the world. The female lays one to three eggs that she curls around in a burrow for about 10 days before they hatch. The mother has mammary glands that produce milk for the babies for more than three months. The males are one of the few venomous mammals in the world. The venom glands are located on the hind feet, which have a sharp, protruding, grooved spur. Platypuses also use special organs on the flat, duck-like bill to detect electrical impulses created by prey such as worms, insect larvae, and crayfish in dark, murky waters.

The other egg-laying mammals, found in Australia and New Guinea, are spiny anteaters, also called echidnas. With their rounded little bodies, which are covered with spines, these terrestrial creatures look pretty much like hedgehogs. Depending on which scientist you talk to, two to five species exist. Echidnas typically lay one egg, which is incubated in a body pouch. Not to be outdone by their duck-billed, web-footed relatives in being strange, echidnas have no teeth.

The mammal group most commonly encountered by us on an everyday basis is composed of the placental mammals, which are most of those outside of Australia. Most people are also familiar with marsupials, such as kangaroos and possums, which raise their young in pouches. Platypuses and echidnas qualify as an exception among mammals. They fall into an evolutionary tangent distinct from either placental mammals or marsupials and are placed in a third, less commonly known category of mammals known as monotremes.

As for the other animals mentioned above, the general worldwide rule for deer is that only males have antlers. These are shed and regrown each year. However, one type of deer, reindeer (caribou), is distinctive in that females as well as males have antlers, a single exception to a rule that holds for more than 90 other species.

Likewise for how we categorize the time of day or night that birds fly. While owls are indeed characteristically nocturnal hunters, barred owls in the Southeast and the northern pygmy owl in the West can be active and capture prey in the daytime. But the northern hawk owl, found in the upper regions of North America from Alaska across Canada, is the undisputed daytime owl. As residents in the land of the midnight sun, during the summer they must either be active during the day or starve. As for birds that we normally think of as active in the daytime, among hummingbirds, warblers, and many other migratory species, nighttime flights are common.

And what about the assertion that all birds lay eggs? Is there an exception to that rule? Yes, but it’s not like the exceptions discussed above. Male birds do not lay eggs.

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