Dens and Other Safe Places
By Fran Zak
Many organisms have dens and other refuges that protect them from predators. It is in these homes that adults can safely raise their young. This is true in all types of ecosystems, including temperate NJ (USA), tropical forests in Panama, tropical and subtropical forests in Queensland, Australia, and upland savannahs in Queensland. I have been to each of these and observed animal homes in all of these habitats.
The various habitats of Australia are famous for marsupials, which include a diversity of kangaroos, wallabies, wallaroos, pademelons, sugar gliders, koalas, and possums. We observed all of these species of marsupials and saw their young. These unique mammals, found mostly in Australia, provide a safe pouch "home" for their young, often for more than a year.
The young (joey) gets mother's milk from a nipple located inside her pouch and also gets protection from predators while in the pouch. This greatly improves marsupial survival rates. [See Australia Account #1 and #2 for more information on Marsupials.]
A very different kind of home is provided by scrub turkeys . These rainforest birds walk more than they fly. We often observed them scratching for insects, seeds, and fruits on the forest floor. The male uses a heap of leaves to incubate the females' eggs. He scrapes up leaves from the forest floor with his legs until a leaf mound is formed. This mound is often 3 meters wide and 1.5 meters high. After mating, several females lay single eggs in the mound. Now it is the male's responsibility to keep a constant close watch on the temperature of the decomposing leaf litter since that will be the only incubation that the eggs will receive. The temperature should remain very close to 33 degrees Celsius. Neither parent incubates the eggs by sitting on them. The male sticks his tongue and beak into the middle of his leaf mound where the eggs are incubating. His sensitive upper beak can detect the temperature to within 1 or 2 degrees Celsius. If the temperature is too cool for egg incubation, the male promptly adds more leaves to increase the temperature of the mound by increasing decomposition (which is an exothermic, or a heat-producing process). If the temperature is too warm, he removes some leaves, which aerates the mound, slows down decomposition, and decreases the temperature. After several weeks, when the young hatch, the male chases them off the mound and into the underbrush on the forest floor. The precocial young are on their own and must fend for themselves. This is a contrast to the extended parental care afforded to a joey by a mother kangaroo.
These are extremes in homes, and another is a home that has an entrance under water. Platypuses are unique to Australia. They are monotremes (egg-laying mammals) that have beaks like birds (ducks), fur like mammals, claws and venom glands like reptiles, eggs like reptiles and birds, and milk to feed their young. Platypuses swim and are usually found in rivers. They live in burrows along the river bank, making them aware of approaching land predators since they can detect the sound of feet walking on the ground above their homes. Their nests are dry and comfortable even though the entrance is under water. The opening to the nest is very narrow, so the platypus is squeezed to remove water from its fur as it enters the nest. They are nocturnal, being active from dusk to dawn. We went out before sunrise two mornings to observe platypuses who were foraging for food and heading back home for a good day's rest.
Take a look at this photo of a nest made by a scrub wren in a subtropical forest in Australia. A scrub wren is a small bird that spends a lot of time foraging on the forest floor. The nest hangs from a branch of a tree, only about 1.5 meters from the ground. The opening of the nest is concealed until you bend down and look at the nest horizontally. Then you can see the "front door" that is protected by an overhang to make it hard for predators to see inside, and to protect it from rain. Here is how inconspicuous this wren nest looks from the side view.
We observed an interesting bird called a logrunner. This species is very easy to locate and identify, not by its coloration and markings, but by its behavior. It looks for food on the forest floor by tossing leaf litter to either side of its body using its legs. It uses a very distinctive behavior of clearing patches on the forest floor by flinging its feet sideways. This is what a logrunner's nest looks like. We watched both the male and female logrunners work on this nest for a few days. You can distinguish the male from the female logrunner because the male has a white throat and the female has an orange throat. So, if you look carefully, you can see this is the female in front of her nest.
We also observed some special animal architecture that is not a home, but is certainly ornate enough to be one! Male bowerbirds build spectacular bowers using twigs that they glue together. These bowers are sometimes more that a meter high. This huge bower (notice it's almost as tall as Jackie) was made by a male Golden Bowerbird. In another species, the male Satin Bowerbird decorates his bower with blue objects. He'll use anything that is the right shade of blue. This included feathers from these beautiful Crimson Rosellas, blue flowers, and even blue household trash like plastic drinking straws, rings from plastic bottle caps, the caps themselves, and even blue pen caps that they manage to find near their bower. These bowers are built by the males to attract females for mating. Once mating has occurred, the female bowerbird, who is much more camouflage-colored and can hide better during egg incubation, builds a nest to lay and incubate her eggs and care for the young chicks. These bowerbirds show great sexual dimorphism, meaning a big difference in coloration between the male and female of each of the species.
Robbins, C. et al. Birds of North America. Racine, WI: Western Publishing Co., 1966. Simpson, K. and N. Day. Birds of Australia. Princeton: Princeton University Press, 1999.
Pascack Valley HS
Jackie Willis, Ph.D.
Montclair State University
Upper Montclair, NJ
Special thanks to Professional Resources in Science and Mathematics (PRISM) at Montclair State University and Dr. Jackie Willis for making these ecology trips possible and for sharing her wealth of knowledge, her expertise, and her photographs with us.