Honorary mammals

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So building complex circulatory systems, peering at the world through a camera-eye, and employing intelligence with a large brain have all evolved convergently. We are, however, in pursuit of the humanoid, and I shall assume that the galactic equivalent is in some sense mammal-like. This may seem too bold a claim, but let us see. So far as the Earth is concerned, there is a simple natural experiment. What we need to do is find a landmass which the mammals have failed to colonize. There is an excellent example, and it is called New Zealand. Indeed, Jared Diamond124 went so far as to say 'New Zealand is as close as we will get to the opportunity to study life on another planet.'125 For at least 85 million years126 these islands have remained isolated in the Pacific Ocean, too remote to be colonized by any of the terrestrial mammals, other than by the bats and much later the boat-travelling Polynesians who arrived about ad 1000. New Zealand, however, had plenty of other inhabitants. Particularly extraordinary are the giant wingless crickets, known as wetas, relatives of the grasshopper and locusts. These are ecologically convergent on mice and rats, 'resembling rodents not only in their biomass, but also in nocturnal foraging and diets, use of diurnal shelters, polygamy and even their droppings',127 and when surprised leaping across the forest floor.128

Nor are these the only animals in New Zealand to approach mousedom. Diamond remarks that the 'Stephens Island wren ... was ... the world's only known flightless songbird and functioned as an avian mouse.'129 So, too, one of the endemic bats (Mystacina) is also convergent on a mouse-like habit, being partially terrestrial and when walking on the ground folding its wings to protect the delicate membranes.130 Presumably like the bats the ancestors of these birds had originally flown there, but perhaps because of the absence of other ground-dwelling competitors many of them became flightless. These include an extraordinary nocturnal parrot, known as the kakapo, which grows as big as a turkey.131 The most famous are the kiwi and the much larger moa.132 These birds are not closely related and, arrived at flightlessness convergently.133 The moas have vanished, and their extinction was almost certainly by way of the stew-pots, roasts, and fricassees of the Maoris.134 The much smaller kiwis, however, avoided the category of a walking larder. If, as for the moa, we had only the bones, it is unlikely that we would ever have realized that in a land without mammals, the kiwi (Fig. 8.7)

figure 8.7 The kiwi, a flightless bird, emblem of New Zealand, but also an 'honorary mammal'. (Reproduced from the figure on p. 103 of Calder (1978; citation is in note 135), with the permission of the artist, Alan D. Iselin.)

has converged strongly in this direction.135 The feathers are rather atypical in being shaggy, in fact almost fur-like. In part this must be because these flightless birds live in burrows. Like many of the equivalent-sized mammals, kiwis are also nocturnal, are strongly dependent on a sense of smell, and unusually have nostrils at the tip of the beak. Moreover, the set of feathers around the mouth is modified into whisker-like structures.136

It is in its reproduction, however, that the kiwi (Fig. 8.7) is the most mammal-like. Not so far as giving birth to live young; like the monotreme mammals, such as the duck-billed platypus, the kiwi still lays eggs. Given the widespread occurrences of ovoviviparity, that is, where the egg is retained in the female who ultimately gives birth to live young (and as discussed below is strongly convergent), it is something of a mystery why, with one dubious exception concerning a budgerigar living in Dorking,137 no bird has managed to become capable of live births. There has been no shortage of suggestions, including potential problems of immunology (leading to rejection of the embryo), the type of sex determination (which in birds depends on the female, with a ZZ-ZW system of chromosomes as against our XX-XY

220 alien convergences?

male determination), and perhaps most plausibly the difficulty in dispensing with a shelled, calcareous egg.138 Even so, it is possible to predict that if ovoviviparity were ever to evolve in the birds the species concerned would show the following characteristics: flightlessness; a lower than average body temperature (one reason why internal incubation may be difficult for birds is their elevated body heat, at c. 40 °C); and a single egg rather than a clutch. And the closest candidate to this list? Yes, the kiwi.139

So, even though the kiwi is not ovoviviparous, it still remains the case that its reproduction is otherwise strongly mammal-like and unlike that of other birds. Thus, both ovaries remain fully functional, with the ovum released in the alternate fashion. The incubation period of the egg is exceptionally protracted, equivalent in time to a mammal of the same body weight. In reviewing this flightless denizen of New Zealand, William Calder III was in no doubt that this convergence was more than superficial when he wrote,

When one adds to this list [that is reproduction] the kiwi's burrow habitat, its furlike body feathers and its nocturnal foraging, highly dependent on its sense of smell, the evidence for convergence [with mammals] seems overpowering. Only half jokingly I would add to the list the kiwi's aggressive behavior ... When I intruded on his domain at night, [the male kiwi] would run up to me snarling like a fighting cat... and drive his claws repeatedly into my ankles until I went away. For this behavior and the many other reasons [given] ... I award this remarkable bird the status of an honorary mammal.140

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