Rapid morphological change in an introduced bird

TREE

bower complexity should be associated with variation in the ecological conditions, and if Gilliard3 is correct, with variation in male colour as well. Comparative studies of bowerbuilding are hampered by the fact that it is only bowerbirds that build bowers. However, there are many species of court-clearing birds3 which seem to occur in a wide range of ecological conditions and are not just restricted to New Guinea and Australia. A comparative study of the circumstances under which court-clearing occurs

References 1 Diamond, J. (1988)Am.

THE

LAYSAN

FINCH

(Telespyza

canfans)

is an endangered honeycreeper with a native range restricted to the island of Laysan, an atoll on the northwestern Hawaiian Islands. In 1967, the United States Fish and Wildlife Service introduced 59 male and 51 female finches to Southeast Island, a small atoll in the Pearl and Hermes Reef, some 500 km northwest of Laysan. This introduced population now numbers about 800 birds. As Conant shows in a new paperl, the introduced population has been morphologically distinct from the founder population on Laysan since at least 1984. Pearl and Hermes Reef consists of four vegetated islands. About 15 km north of Southeast Island is North Island; about 8 km west of Southeast is Grass Island, and a further 5 km west is Seal-Kittery Island. With human assistance and by natural dispersal, Laysan finches now occur on all these islands. Conant also shows that there are morphological differences between the finches on the different islands of the Pearl and Hermes Reef. North Island was first observed to have an unbanded pair of birds in 1973, and now has a population of about 200. Six birds were moved to Grass in 1970 from Southeast; the population on Grass is now about 50. Seal-Kittery had six birds in 1972 and now also has about 50. The original population on Southeast dropped to below 50 within a couple of months of the birds being introduced there, Stuart Pimm is at the Department of Zoology and Graduate Program in Ecology, University of Tennessee, Knoxville, TN 37996, USA. 290

Nat.

3, no.

of Paradise

7 1,

November

and Bower

1988

Birds,

Collins 5 Diamond, J.M. (1986) Annu.

Rev. Ecol. syst. 17, 17-37 6 Diamond, J.M. (1987) Ethology74, 177-204 7 Borgia, G., Pruett-Jones, S. and PruettJones, M. (1985) Z. Tierpsychol. 67, 225-236 8 Borgia, G. (1986) Sci. Am. 254,70-79 9 Zahavi, A. (1975) J. Theor. Biol. 53, 205-214 10 Kirkpatrick, M. (19871 Annu. Rev. Ecol. Syst. 18,43-70

131,

631-653 2 Marshall,

A.J. (1954) Bower-birds, Clarendon Press 3 Gilliard, E.T. (1969) BirdsofParadise and Bower Birds, Natural History Press 4 Cooper, W.T. and Forshaw, J.M. (1977)

Rapid Morphological Change inan Introduced Bird Stuart L. Pimm

The Birds

may help determine whether this behaviour and bowerbuilding are just a peculiar form of male showiness, or whether they actually represent something more.

vol.

11 Borgia, G., Kaatz, I. and Condit, R (I 987) Anim.

Behav.

35,1129-l

139

First, environmental differences might lead to differences in bill growth or wear between birds on the three islands. However, this seems unlikely for the populations within the Pearl and Hermes Reef. Differences in bill wear might explain the variation in bill length, but not in width and depth. Secondly, the morphological variation might be caused by founder effects: the few founding individuals might be a highly non-random selection of genotypes. The population on North may have been founded by as few as two birds and so founder effects are an entirely reasonable explanation for the difference of this population. The population on Southeast, however, was founded by 100 birds. And, even though the population dropped to under 50 within a couple of months, it had risen to 270 within a year. The population would not seem to have been small enough for long enough for founder effects to have been important. For the morphological differences to have a genetic explanation, the bill dimensions would have to be heritable. Conant is still in the process of assembling enough data to test this requirement. But if the dimensions are heritable there would at least be a precedent in Grant’s studies of the Geospiza finches on the Galapagos’ Thirdly, the differences might reflect very rapid natural selection; if so, then what are its mechanisms? The Laysan finch is omnivorous, feeding sometimes on insects, carrion and birds’ eggs. Seeds, however, form a major proportion of the diet. The seeds taken by the finch range from nearly microscopic Portulaca and Sesuvium to the large and hard Tribulus cistoides (which have mericarps up to 12 mm in length). A number of soft seeds and fruit are also taken. The variety of seeds avail able on Pearl and Hermes is less than that on Laysan. From feeding

but had climbed back to 270 the following year. Pearl and Hermes Reef can be an extraordinarily difficult place to reach; Laysan is days away by boat, and it, too, is remote. The idea of a tropical paradise is quickly dispelled by the lack of shade, the need to take all supplies, including water, and by the tiger sharks (Galeocerdo cuviera) that patrol the shores preying on recently-fledged seabirds. Despite these difficulties, Conant has mounted several month-long trips to the islands. The birds are tame and readily caught by hand nets, and Conant has banded about 50% of the individuals on the Reef. In addition, she has taken various morphological measurements: wing and tarsus lengths, and bill widths, lengths and depths. There are differences in size between the sexes, and Conant restricted her analyses to birds with adult plumage. (Adult plumage is obtained by the time the birds are two years old.) Only the sample sizes from Southeast and North were adequate for testing the morphological variation. There were no significant differences in tarsus length between Southeast, North and the original population on Laysan. But there were significant differences in bill shape between Southeast and Laysan, and even between Southeast and North. Birds on North have deeper and shorter bills than those on Laysan and Southeast. Southeast birds have longer bills than those on Laysan. So how have these differences arisen? Conant considers three possible explanations. c I, ;, i

TREE vol. 3, no. 11, November

1988

observations Conant found that Tribulus is an important food source on Pearl and Hermes, but is much less important on Laysan. On Laysan only 4% of the birds were feeding on Tribulus, whereas 78% of the birds on Pearl and Hermes fed on Tribulus. Tribulus is much more abundant on Pearl and Hermes (25% ground cover on Southeast, more than 50% on North) than on Laysan (less than 5% cover). These observations on Tribulus are consistent with the observation that birds on North Island have significantly deeper bills. They would need deep bills to crack the Tribulus that form such an important food source there. Moreover, Conant measured the Tribulus mericarps and found that they were significantly larger on North than elsewhere. Birds on Southeast do not have deeper bills than the birds on Laysan, even though Tribulus is more important there. (However, the mericarps are smaller on Southeast than on North.) Why birds on Southeast should have the longest bills is not obvious. Could the morphological changes

Llost of tkr ~~gulaks Ihoofed mammalsI ‘Clal survive today belong to the orders Wiodactyla (even-toed unglflates) or 7erissodacfyla (odd-toed ungulates), and Ire known for their herbivorous specializItions (e.g. fhc rlrminant type of stomach I, or [heir large 6ody size (e.g. hippos or 4inos) or for their fleetness of foot 1e.g. Ilntelope or horsesl. Yet these present-day (Txamples represent the specialized endiloi~~ts of a large Tertiary radiation of iloofed mammals. There was a Gewildering I~ariely of small generalized early Tertiary “Irrns, even including sofne carnivorous iaxa. In addition, some specialized island ioMtinent ungulate radiations are now tither entirely extinct lthe South AmJ rican ungulatesI, or are represented 6y ( nlg a few living members ithe African :ubungulates’). Recent fossil discoveries, r#rld advances in phylogenefic systematits, I ave reopened a nlrmber of issues in I ngltlate tlassifitaiion, whick have affected of(r views not only on lrngalates fhems 71\1es,but also on patferns of Tertiary Lroqeography and evolution.

C ilrlstine Jams is m the Graduate Program in EcolC:L and Evolutionaq Biology, Division of Biology a Id Medicine. Brown University, Prowdence, i; (IW?, USA

have taken place in such a short time? There are particularly interesting parallels here to the study by Grant on Geospiza fortis on the island of Daphne in the GalBpagos2 (see also Ref. 3). It, too, feeds on the same species of Tribulus. During a period of extreme drought when most food was particularly scarce, deep-billed birds survived much better than those with shallow bills. As a result, the population showed a highly significant increase in average bill depth over the course of two years. It seems very possible that the Laysan finch has undergone genetic changes in less than twenty years. The possibility of these rapid evolutionary changes has caused Conant to consider the implications of introducing endangered species to new areas4. She asks: ‘when we translocate endangered species are we indulging in evolutionary “tinkering”, taking the chance that we may save something that could eventually become so distinct from the original type, that biologists would call it a new race or species?’ And, even if we accept this risk, ‘how many indi-

viduals should be used to start the translocated population in order to avoid founder effects?’ As Conant points out, more work remains to be done, and her joint work with R.C. Fleischer on the genetics of these birds has yet to be published. There have been numerous recent extinctions of birds on Guam5 and Hawaii6 and more will certainly occur in Hawaii in the next decade unless species are brought into captive programs or translocated. So, the issues Conant raises are important ones, and ones that conservation biologists have yet to consider in detail.

References 1 Conant, S. (1988) E vol. Ecol. 2, 270-282 2 Grant, P.R. (1986) Ecologyand Evolution of Darwin’s Finches, Princeton University Press 3 Pimm, S.L. (1987) Trends Ecol. Evol. 2, 22&229 4 Conant, S. (1988) BioSciencc: 38, 254257

5 Pimm, S.L. (1987) TrendsEcol. Evol. 2, 293-295 6 Freed, L.A., Conant. S. and Fleischer, R.C. (1987) Trends Ecol. Evol 12,196-203

NewIdeasin Ungulate Phylogeny andEvolution Christine M, Janis All hoofed mammals, with the few exceptions discussed later, are now considered to represent a monophyletic clade (i.e. to possess a single common ancestor1 I, although some controversy still exists’. The phylogeny of mammals was discussed recently in TREE’, and in this review I will concentrate on relationships between ungulate lineages. It is now widely realized that the evolutionary pattern, i.e. a valid phylogeny, is an essential prerequisite of interpretations of the evolutionary process”. I present here some examples of recent changes in taxonomy that have changed our ideas about evolutionary processes in ungulates, and also present an example of how a study of evolutionary processes in ungulate evolution eventually led to the establishment of a new phylogeny. Table I shows the present day distribution and time ranges of living ungulate orders. j’ ) 8:‘)

Traditional ideas of ungulate phylogeny The ‘basal order’ of ungulates is traditionally seen as the Condylarthra, a mainly Paleogene radiation that is held as being ‘primitive’ and generally ancestral to all later ungulate lineages. This is because they possessed the generalized herbivore features of bunodont cheek teeth (low-crowned with rounded cusps), and limbs that were not extensively modified in the direction of cursoriality (running adaptations), although they did possess hooves (rather than nails) on the terminal phalanges. Most condylarths were probably generalized omnivorous/herbivorous types of animals. Traditional views (see Fig. lal of the interrelationships of the later Tertiary ungulates to the condylarths hold that the Artiodactyla have their origin in the Hyopsodontidae5 or the Arctocyonidae”, while the Perissodactyla are derived from the Phenacodontidaei. 291