Reviews 



Reflections on Slime 



by Steven Austad 



In his essay Possible Worlds, the late 

 British geneticist J.B.S. Haldane imagines 

 the moral and religious sense we might 

 find in dogs, honeybees, and barnacles and 

 concludes: "My own suspicion is that the 

 universe is not only queerer than we sup- 

 pose, but queerer than we can suppose." It 

 is difficult not to be reminded of Haldane's 

 remark when entering the believe-it-or-not 

 world of microscopic invertebrates, where 

 John Tyler Bonner has spent his scientific 

 life. 



In Bonner's world, the rules that our 

 five senses have taught us govern animal 

 life simply do not apply. When an amoeba 

 reproduces by splitting in two, is it then 

 both parent and offspring of the same age, 

 or two offspring and no parent? What do 



Life Cycles: Reflections of an Evo- 

 lutionary Biologist, by John Tyler 

 Bonner. Princeton University Press, 

 $19.95; 206 pp. 



gender and age mean? What constitutes an 

 individual? For instance, we normally 

 think of individuals as large, multicellular 

 animals, such as ourselves, that arise from 

 repeated cell divisions of a single fertilized 

 egg. But some organisms become large 

 without becoming multicellular. The 

 "true" slime molds, such as those bright, 

 slimy orange gobs we see on rotten logs, 

 can, under good condiUons, grow to the 

 size of a human hand, yet still consist of a 

 single cell. 



Other organisms become multicellular 

 by aggregating rather than repeatedly di- 

 viding. In this category are the "cellular" 

 slime molds, the organisms to which, says 

 Bonner, in his new book's opening sen- 

 tence, he has devoted his life. They can 

 exist as self-sufficient, single-celled 

 "amoebas" or as multicelled "slugs." 



Ubiquitous in soil and decaying wood, 

 these creatures emerge from spores and 

 assume a solitary existence, slithering 

 about, eating (by engulfing bacteria), and 

 reproducing (by simply splitting). 



When the going gets tough, however, 

 slime molds stick together — literally. That 

 is, food shortages cause certain amoebas 

 to begin secreting attraction chemicals, 

 thus drawing surrounding individuals to- 

 ward them. The resultant social aggrega- 

 tion of these previously independent cells 

 forms the sausage-shaped "slug." This 

 new individual develops distinctive cell 

 types at its front and back, migrates toward 

 light, and upon finding a suitable spot, 

 forms an erect fruifing body crowned by 

 spores from some of the original amoebas. 

 These spores depend, however, on the al- 

 truism of many other amoebas that died in 

 forming the fruiting body's sturdy sup- 

 porting stalk. 



We could delve deeper into what an in- 

 dividual slime mold is, but this book isn't 

 about slime molds. It is about the unusual 

 perspective that a lifetime study of shme 

 molds can give to large biological ques- 

 tions. Bonner says he has "an inordinate 

 fondness for grand ideas." This isn't sur- 

 prising. Generally, the smaller and less 

 charismatic the study animal, the more we 

 focus on grand ideas. And because our 

 ideas are as heavily influenced by the par- 

 ticular organisms we study as by the envi- 

 ronment in which we were raised, thinking 

 about these bizarre creatures has led Bon- 

 ner to a succession of unusually absorbing 

 ideas and trenchant observations. 



For instance, he argues that biologists 

 generally are overly fixated on adult or- 

 ganisms, probably because humans spend 

 so much of their existence as adults. When 

 we think of a dog, we immediately picture 

 some generic adult dog. But a puppy is just 

 as much of a dog as an adult dog — so is 

 the fetus, embryo, and even the fertilized 



egg. "Organisms are not just adults — they 

 are life cycles," Bonner says. Focusing on 

 the period of the life cycle between fertil- 

 ization and first production of offspring al- 

 lowed Bonner to realize that generation 

 length will, to a certain extent, be limited 

 by how much growth an organism requires 

 before reproducing. To demonstrate this, 

 he gathered data, now reproduced in virtu- 

 ally all introductory biology texts, show- 

 ing that body size and generation length 

 have a consistent relationship whether the 

 organism is a bacterium or a sequoia. He 

 also emphasizes that an adult cannot be al- 

 tered without altering the process by 

 which adults are created. 



Bonner's focus on size and the various 

 routes to multicellularity have led him to 

 additional insights. He notes that increas- 

 ing cell number is intimately related to in- 

 creasing division of labor, a theory appli- 

 cable to microscopic organisms or the 

 workings of a modem city or corporation. 

 A simple division of labor is seen even in 

 filament-shaped colonies of primitive 

 cyanobacteria, which may have some 

 sporelike cells speciahzed for surviving 

 hard times, other cells specialized for pho- 

 tosynthesis, and still others specialized for 

 chemically processing nitrogen. In case 

 we hadn't thought to ask, he also points 

 out that the multicellularity produced by 

 successive cell divisions is a phenomenon 

 of aquatic organisms, whereas multicellu- 

 larity by aggregation is primarily a terres- 

 trial phenomenon, thus reassuring us that 

 we did have an aquatic ancestry. 



In the latter half of the book, Bonner 

 tackles, in his understated way, the nature 

 of sociality, consciousness, and culture. 

 He considers many animal societies in 

 light of the twin forces holding shme mold 

 societies together, namely, division of 

 labor and communication among the 

 parts. If the terrain covered in this section 

 is less compelling, it is because other pop- 



62 Natural History 2/94 



