Ratite Evolution: An Overview
*As seen in Emu Today & Tomorrow, Issue 1, Volume 7, January 1997*
The many studies of avian biology and behaviour which have been
conducted over the past century have concentrated mostly on carinates
(flying birds) with relatively little attention being paid to "the
more primitive ratites". Until recently, most of the information
which has been collected was either based on casual observations in
the wild or on captive specimens.(1) The recent inception of the ratite
industry has sparked a great deal of interest in this family of birds.
A veritable explosion of studies has occurred as people strive to
discover new information on all aspects of the birds' lifestyles and
physiologies. Currently information on the behaviour and lifestyle
of the various ratites, which was considered ground breaking in 1968,
can be found in the most basic "How to Farm Ratites" manual. While
this knowledge is certainly vital to the industry, it is interesting
to note that a great deal of information has escaped notice. The
areas of evolution and species relations, while not perhaps directly
related to the farming of these animals, does give one a feel for how
truly ancient and special ratites are. Belonging to the oldest Order
of living birds, they reached virtually every landmass in the Southern
Hemisphere (and some in the North) and there became important figures
in many primitive cultures both as supernatural entities and as food
staples.
The origin of ratites has long been a point of dispute in the
scientific community. Despite numerous papers written on the subject
over the years, there still remain some areas of contention which are
unresolved. These areas involve the following questions: 1. did
ratites evolve from flying or flightless ancestors, 2. was the power
of flight lost at one point in their evolution or did it occur
independently for several different families, 3. did all ratites
have a common ancestor or did several species happen to evolve along
the same lines, and 4. how are the different species of ratites
existing today related.(2) It has also been a subject of debate as to
whether the tinamous (a family of South American birds which resemble
partridges) should be included in the same order as the rest of the
ratites, that being the Order Palaeognathiformes, or if they should
be classified in an Order by themselves.
In 1973, Joel Cracraft closely examined many aspects of the skull,
pelvis and leg bones of ratites and devised a theory of ratite
evolution which remains widely accepted today. He concluded that
ratites share a common ancestor with all other birds, which was
capable of flight, and that flightlessness was acquired only once
early in their evolution.(3) Thus they had to travel overland to reach
their present day ranges. This would have happened 80 to 100 million
years ago before the super-continent, Gondwanaland, began to break
apart into the separate continents we know today.(3)
Cracraft also concluded that the tinamous belonged to the same Order,
Palaeognathiformes, as the rest of the ratites. The
term palaeognathiformes is derived from the unique structure of their
palate.(3) The various bones in the ratite skull and jaw are not fused
as they are in the flying birds. In flying birds, this fusion is
necessary to strengthen the skull in order to withstand the stresses
associated with sustained flight. It is probable that this fusion
had not yet occurred when the ratites diverged from the main line of
birds. This could mean that sustained flight had not yet been
achieved when the ratites began their evolution. However, since
tinamous also possess this type of palate and yet are capable of
flight it is reasonable to assume that the ancestors of the tinamous
and ratites could fly to a certain extent.(3) In order to simplify
terminology, from this point on the term "ratite" will be used in
reference to all members of the Order, including the tinamous.
Family: TINAMIDAE
The ancestors of ratites were flying birds living somewhere
in Gondwanaland during the Cretaceous period. Early in their history
the lineage that gave rise to the tinamous branched off and was
isolated in what is now South America.(2) The name "Tinamou" is a
corruption of the Argentinean name "inambu", which is probably derived
from the sound of the bird's call.(4)
Today this family consists of some 47 species found throughout
South America and in parts of Mexico. They primarily occupy forest
floors with a few species preferring more open habitat. They are very
much like a pheasant in size and appearance and are generally quite
drab, coloured in shades of grey, brown and black. They are good at
escaping notice by remaining still and are adept runners being among
the most terrestrial of all flying birds.(4) When frightened, they
prefer to freeze, walk or run away rather than resort to flight.(4)
The tinamous retained the ability to fly and thus, have well
developed wings and a keeled breastbone along with several other
adaptations associated with flight. The wings are small for a bird
of its size and so flight, although fast, cannot be kept up for long. They also have poor circulation, due to small blood vessels, which is aggravated by the fact that the lungs and heart are also small for a flying bird.4 This means that a fleeing bird tires rapidly, rarely flying more than 1500 feet before being forced to land.4 The tail is not well developed with rather weak feathers and thus is not very functional as a rudder, as is seen in most birds. They have been known to lose control upon take- off and fly into obstacles, sometimes killing themselves.4
The tinamous still occupy much of their original range, however,
several species are threatened. Massive habitat destruction and
overhunting have devastated some populations pushing them closer and
closer to extinction. For example, in 1971 the total population of
the Solitary Tinamou, located in coastal forests of eastern Brazil,
was estimated at only 100 individuals (information on their current
status was not available for this article).(4)
FLIGHTLESS BIRDS
Soon after the divergence of the tinamou line the ratites underwent
some major evolutionary changes.3 First, and most well known, was the
loss of flight. This is reflected today in the absence of a keeled
sternum and the reduction of wings relative to body size. Although
reduced, the wings of these birds were probably still of significant
size and were most likely used in social displays as seen in the
ostrich and rhea today.3 Since the ostrich/rhea lineage was the most
recent to develop, sizable wings were most likely present in the parent
stock.3 The wing elements in the other lineages would have become
reduced independently in each case by convergent evolution.
Convergence simply means that similar features may evolve independently
in different animals as a result of similar selection forces within
their environment.
It is probable that this was also the point when the ancestral
ratites attained their great size.(3) This is indicated by the fact
that almost all of their descendants, especially the earliest ones
(moas and Elephant bird), retained large body size. There is much
speculation as to what may have brought about an increase in body size
at this time. Large size could be used as a means of escaping
predation, either by increased running or fighting ability.(3) The
only Cretaceous predators that could have been important to the ratites
were dinosaurs but it is doubtful that this was the case since the
ratites would probably never have become flightless if predation
pressures were high.(3) However, large body size may have developed
initially to improve metabolic regulation in response to cooler
climates which developed during the Middle to Late Cretaceous period.(3)
In addition to this, large size would have made a broader range of
food items available to those early ratites.
These flightless birds were mainly centred in South America.
The ancestors of the next group to emerge made their way across
Western Antarctica to New Zealand before the two landmasses separated
around 70 million years ago.(2) There they eventually gave rise to the
various species of Moas (extinct 1500-1850) and to the Kiwi.
Family: DINORNITHIDAE
The Moas became the dominant life forms of New Zealand
eventually evolving into as many as 25 different species.(5) These
ranged in size from the Giant Moa, which was 13 feet tall and weighed
about 600 lbs, to six species of 'Pygmy Moas' which stood 3-4 feet
tall.(5) One of the most distinguishing characteristics of these birds
was that they possessed no external wing elements whatsoever.
About 15 species of Moas were present when the islands were
first colonized by Polynesians. These people soon became a Moa-hunting
culture and called themselves Maori.5 It was the first time that
these birds were threatened by predation for millions of years.
This spelled doom for the moas and by the time European explorers
began to arrive in numbers, most species were extinct with a few
surviving until about 1850.(5)
Family: APTERYGIDAE
The other ratite to develop on New Zealand was, of course,
the kiwi. In contrast to the many species of Moa, there evolved only
three species of kiwi, all of which still survive. These are the
Little Spotted Kiwi, the Great Spotted Kiwi, and the more well known
Brown Kiwi. Their optimal habitat is rain forest where they can find
the large numbers of invertebrates which make up most of their diet.(4)
The smallest of the ratites, their body length ranges from one to
two feet. This lineage probably developed small size at some point
after its divergence from the moas.(3) This theory is supported by the
fact that the kiwi lays an egg four times the size expected for a bird
of its stature (a five pound kiwi can lay a one pound egg). It would
seem that as the kiwi lineage became smaller a large egg remained
advantageous.(3) The ancestral kiwi was probably the size of the
smaller moas.
In addition to their small size, these birds differ from all
other ratites in several ways. Kiwis are monogamous and sedentary,
sharing a fixed territory with a single mate in a bond which lasts a
lifetime.(4) They are nocturnal, coming out at night to find food and
spending the day resting in burrows which they dig for themselves.(4)
They have a highly developed sense of smell, unusual in birds, as
well as sharp hearing (they can hear the movements of large
invertebrates underground). However, their eyesight is very poor as
they can only see 2 feet in front of them in the daylight and 6 feet
at night.(4)
The Maori believed that the Kiwi received special protection from
Tane, God of the Forest.(4) Because of this, many rituals were centred
around the bird and people of rank wore ceremonial cloaks made from
their feathers.(4) By the time white colonists arrived they were
already extinct in some areas. The Europeans hunted the birds for
their meat and feathers (used in trout flies) and destroyed much of
their habitat. In addition to this they brought stoats, weasels, cats,
and dogs which caused much damage to remaining populations.(4) Today
the bird is the national emblem of New Zealand and is strictly
protected by law.
Family: AEPYORNITHIDAE
The next lineage to diverge was the ancestor of the Elephant
Bird (extinct 1700). They would have travelled from South America to
reach Africa 80 to 90 million years ago.(2) They were once widely
dispersed throughout Africa, as shown by fossil evidence recovered
from Egypt and the Canary Islands, with some birds becoming isolated
on Madagascar when it separated from the mainland sometime during the
Mid to Late Cretaceous period.(2) The largest bird that ever lived, it
stood as much as 10 feet tall and weighed over 1,100 lbs.(5) Their eggs
were 3 feet in circumference and held 2 gallons (9 litres) of fluid -
the equivalent of 200 chicken eggs.(5)
While they were hunted to a certain extent by primitive man, it
was more likely man's predation upon their eggs and destruction of
habitat that caused their fall. By the time European settlements were
being established on the coast of Madagascar in the early 1600's, the
birds had already removed themselves to the more remote regions of the
island. By the year 1700, they had almost certainly vanished.(5)