Original cartoon by Alex Martin
Source: Original cartoon by Alex Martin

As is well known, for any given body size, birds and mammals have bigger brains than reptiles, amphibians, and fish. This distinction is surely linked to another major difference. Birds and mammals are “warm-blooded”, with adaptations of their energy household (metabolism) to maintain a fairly constant body temperature, typically well above that of their surroundings. Humans, for example, maintain an average core body temperature of about 98.6°F (37°C) even in icy cold environments. Reptiles, amphibians, and fish, by contrast, are “cold-blooded”, lacking metabolic adaptations to keep their body temperatures raised at a constant level. Their body temperatures are therefore usually quite low and generally fluctuate, tracking swings in environmental temperature. Less obviously, an interesting third factor shows a parallel distribution. Birds and mammals commonly show quite intensive parental care of offspring, which is a rarity among reptiles, amphibians, and fish. So it seems quite likely that constant high body temperatures, big brains, and parental care may all be connected.

Developmental State of Offspring

Anyone who has kept breeding pairs of hamsters, hedgehogs, or mice at home knows that the mothers give birth to litters of poorly developed offspring. Newborn infants are pink, hairless little grubs at birth, with their eyes and ears sealed with membranes. By contrast, many other mammal mothers such as horses, cows, dolphins, and chimpanzees give birth to a single, well-developed infant. Their offspring are typically born with a coat of hair already present, and their eyes and ears are open at birth. Largely thanks to Swiss zoologist Adolf Portmann, the crucial distinction between poorly-developed altricial offspring and well-developed precocial newborns is now widely recognized. As a rule, altricial infants are born in a nursery nest in which they develop until they are able to move around independently. The eyes and ears open while they are developing in the nest. Most precocial infants, on the other hand, can move around independently from birth onwards, and they usually have little need for a nest. Evolutionary reconstructions clearly indicate that the common ancestor of today’s live-bearing mammals (marsupials and placentals) gave birth to poorly-developed offspring. One striking clue is evident part of the way through fetal development. In precocial mammals—including humans—the eyes and ears initially become sealed with membranes and then re-open again before birth. This suggests that, during their evolution, an original nest-living phase was replaced by extended development in the mother’s womb.

Author’s illustration based on a 1952 diagram by Portmann, republished in 1990.
Distinction between development of poorly-developed, altricial offspring (top row) and well-developed, precocial offspring (bottom row) in mammals. Altricial offspring are born after a relatively short pregnancy (pink zone) and typically live in a nest for some time before emerging. Precocial offspring are born after a relatively long pregnancy that encompasses the original nest phase, as is indicated by formation and then loss of membranes sealing the eyes and ears.
Source: Author’s illustration based on a 1952 diagram by Portmann, republished in 1990.

The same distinction between poorly-developed altricial offspring and well-developed precocial infants is also found in birds. In this case, it is the state of the offspring when hatching from an egg, rather than emerging from a mother’s womb, that is at stake. Song-birds, for instance, all produce altricial offspring that spend some time in the nest before and after hatching, whereas members of the chicken group (order Galliformes) typically have precocial offspring that hatch relatively quickly and become independent soon after hatching. Curiously, however, the evolutionary sequence in birds is the polar opposite of that seen in mammals. Reconstructions indicate that the common ancestor of all modern birds laid eggs from which well-developed offspring hatched. As Portman astutely realized, during the development of any precocial chick within the egg the eyes and ears are never sealed with membranes at any stage. There is hence no indication that during the evolution of precocial birds an original nest-living phase was incorporated into development inside the egg.

Big-footed mound builders

 File licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.
Left: Artist’s representation of the malleefowl. Right: Malleefowl at Yongergnow Malleefowl Centre (Ongerup, Western Australia).
Source: Images from Wikimedia Commons. Left: Plate by Keulemans from The Birds of Australia by Gregory Mathews. Right: File licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Natural selection has spawned some really strange adaptations, not least when it comes to parenting. A bizarre example is provided by a group of big-footed birds known as incubator birds or mound-builders. Members of this family of more than 20 living species (aptly named Megapodiidae) include scrubfowl, brushturkeys and malleefowl. A prime example is the malleefowl (Leipoa ocellata), a stocky, chicken-sized bird with huge feet that lives most of its life scurrying around dry mallee scrub areas of southern Australia. Although male and female malleefowl commonly pair for life and share an extensive home-range covering about one-and-a-half square miles (4 square kilometers), they largely avoid one another except when breeding and do not feed or roost together. The female deposits her eggs in a large mound of decomposing vegetation, relying on heat generated by this avian compost heap to incubate her eggs. When the extremely precocial offspring emerge from their makeshift incubator, they are to all intents and purposes independent.

 Glen Fergus (own work). File licensed under the Creative Commons Attribution-Share Alike 2.5 Generic License.
Images of a malleefowl mound. Left: Archival image. Right: Recent photograph.
Source: Left: From National Audubon Society. Internet Archive Book Image from Flickr's The Commons. No known copyright restrictions exist. Right: From Wikimedia commons, Author: Glen Fergus (own work). File licensed under the Creative Commons Attribution-Share Alike 2.5 Generic License.
 Pengo). File licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license.
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Source: Figure from Wikimedia commons. Source: Encyclopedia of the Animal World, Volume 11, 1972, p1006. Photo credit: Peter Halasz (User: Pengo). File licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license.

As winter begins each year, an adult male malleefowl uses backward raking movements of his outsize feet to excavate a trench in the sandy soil some 10 feet across and three feet deep. Once the trench is ready, leaves, bark and twigs are gradually piled up to build a nest mound rising two feet above ground. When rainfall—an essential ingredient—sprinkles the makeshift incubator, the male plows through the ingredients to trigger decay. Eventually, as winter draws to a close, he digs out an egg chamber. The mound, including the egg chamber, is covered with a layer of sandy soil for insulation. During the following summer months, if his compost heap is rotting nicely, the female lays her eggs in the chamber, depositing about a dozen large, thin-shelled eggs (each about one-tenth of her body weight) over as many weeks. Thereafter, the male, who may be aided by his mate, regularly tends the mound, adding soil on top as needed and keeping the temperature of the egg chamber approximately constant. Malleefowl expert David Booth has reported that egg temperatures can vary between 82°F and 100°F (28°C to 38°C) during incubation, although development is optimal at 93°F (34°C). Mound temperature is regulated by opening and closing air vents above to balance heat rising from the compost beneath. It has been estimated that for opening and closing the mound for egg laying and for temperature regulation a malleefowl pair shifts a total of about 3.6 tons of sand. After a variable incubation period lasting two to three months, the fully feathered hatchlings use those multi-purpose big feet to break out of their eggs and then laboriously claw their way to the surface with backward thrusts. Once free, they totter down to the base of the mound and quickly vanish into the scrub. By the end of the first day, chicks can run fast and fly well. From hatching onwards they lead an essentially solitary life like their parents.

Lessons for Parental Care

Incubator birds such as the malleefowl are unusual because, unlike other birds and all mammals, they barely interact with their offspring after they have emerged. As noted, parental care is seemingly associated with brain size in birds and mammals, and diligent parenting is often associated with strong social bonds. Malleefowl also have particularly low energy turnover (metabolic rates). So it is hardly surprising that incubator birds have tiny brains, corresponding to their miserable social lives. Regrettably, all those strenuous efforts to build and regulate the incubator do not translate into big-brained offspring.

Adapted from a graph kindly provided by Karin Isler.
Plot of brain size against body size for a large sample of bird species. Note that members of the chicken group (order Galliformes), which have precocial offspring, generally have relatively small brains. Incubator birds (family Megapodidae) resemble other birds in the chicken group in having among the smallest brain sizes known for birds.
Source: Adapted from a graph kindly provided by Karin Isler.

Incubator birds are also unusual in another respect. As a rule, incubation temperature does not influence sex ratios in birds, although this is common in reptiles. Together with his colleague Ann Göth, David Booth showed that incubation temperature does affect sex ratios in incubator birds. The sex ratio is approximately balanced at the average temperature of natural mounds, but more males hatch at lower incubation temperatures while more females hatch at higher temperatures. Because sex is determined by chromosomes in incubator birds as in other birds, however, the skewed sex ratios presumably arise because mortality of male and female embryos differs at high and low temperatures.

The take-home lesson here is that the intensive parent care that we see in humans is presumably directly linked to our unusually large brains.


Slonecker, E.M. (2017) Altricial. pp. 1-4 in: Encyclopedia of Animal Cognition and Behavior (eds. Vonk, J. & Shackelford, T.K.). New York: Springer International Publishing.

Weathers, W.W., Seymour, R.S. & Baudinette, R.V. (1993) Energetics of mound-tending behaviour in the malleefowl, Leipoa ocellata (Megapodiidae). Animal Behaviour 45:333-341.

Portmann, A. (1990) A Zoologist Looks at Human Kind. New York: Columbia University Press.

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