Biology Of Ratites

The flat, raftlike (ratis) sternum provided the name for the family. There is no keel and the pectoral muscles are absent or vestigial. In all except rheas, the body feathers lack barbicels, so the plumage is loose and fluffy. The feathers of the emu have two shafts. The rhea and the ostrich have longer wings than the emu and they use them during elaborate displays (Fig. 1). Female emus and rheas are larger than males, but the male ostrich is the largest.[1] All extant ratites are endemic to the Southern Hemisphere, whereas their ancestors were found in both hemi-spheres.[1] The ostrich, emu, and rhea are found in temperate and Mediterranean regions, but can survive in a wide range of climates.[1,2]

The ratites have very strong legs and their muscles have a specific distribution and physiology due to the mechanic constraints of bipedal locomotion. Ratites walk most of the day and can run at considerable speed (Table 1). They are nomadic and follow food availability, but are territorial during the breeding season. Ratites can also crouch, a posture between standing and sitting (Fig. 1). The females lay eggs in this position.[1,2]

Vigilant ratites stand with the neck stretched upward; their heads are very mobile and can turn almost 360 degrees. Vision is believed to be very efficient because of the elevated position of the eyes and also because of acuity. Thus, they are able to see and detect from long distances (few kilometers). Ratites are not active at night and spend most of the dark phase lying.

Rheas, emus, and ostriches can be found in groups of 30 or more, but also in smaller family groups. Ratites are very defensive when eggs or young chicks are present. Agonistic behaviors include vocalizations, body postures, and eventual charging.

The reproductive biology of ratites presents some unique features. The males have a large penile organ that erects from the cloaca and penetrates the female's cloaca during copulation. In females, sperm storage tubules in the reproductive tract allow the female to remain fertile for several days after copulation.[3] The mating system varies between species. In the wild, male ostriches and rheas form harems, but also copulate with females from other groups, whereas emus form pairs that are stable during the mating period.[1,4] Courtship is based on vocalizations and displays or postures from both sexes (Fig. 1). Ostriches reproduce during summer, but rheas and emus reproduce mainly during winter. Photoperiod is essential for the emu,[5] but is not that critical for the other ratites because, when nutrition is not limited, they breed at anytime.[1] Ratites nest on the ground in very simple nests (Fig. 1). Females lay large eggs at 2 3 day intervals. The total number of eggs laid by one female varies between species and individuals (Table 1). The number of eggs laid over a season seems to be strongly influenced by the level of fat reserves and nutrition of the female before the start of the laying period. With the exception of the ostrich, male ratites are solely responsible for incubating the clutch and raising the young (Fig. 1). Female emus leave their partner during incubation and mate with other males. Ratite chicks are precocious and the nest is usually abandoned within 48 h after hatching.[1]

The digestive system is simple and in most respects similar to that of other plant-eating birds, but ratites also consume insects and small animals.[1,2,6] The esophagus is mobile and expandable and ratites swallow their food whole. The crop is absent in all ratites, but the structure of the stomach varies among species.[6] Their appetite varies dramatically between the breeding and the nonbreeding seasons, leading to large variations in body weight, mostly due to variation in fat reserves.

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Fig. 1 Clockwise from the top left corner. Male ostrich with his harem; one female is incubating. Male ostrich displaying courtship. Male emu displaying courtship. (View this art in color at www.dekker.com.)
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