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Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (PhD) -- Macquarie University, School of Biological Sciences, 1990. / Thesis by publication. Bibliography: leaves 269-324.

I have investigated the social grooming in kinship, rank, age and seasonal change among adult female Formosan macaques (Macaca cyclopis) that inhabit Mt. Longevity, Kaohsiung. The major study groups were C and Cd groups. Field observations were conducted from August 2000 to February 2003 covering three mating seasons and two non-mating seasons. The observations covered a total of 188 work days including 1248.8 hours. I actually had recorded C group for 660.6 hours, and Cd group for 244.5 hours. During my study, C group consisted of 8-13 adult males and 14-15 adult females, while group Cd had 1-3 adult males and 2-4 adult females. In order to analyze grooming data, I divided 15 adult females into sub-groups such as dominant/ submissive groups, old (>13) /young age (5-12), relative higher/lower ranking and related/unrelated. I have also divided adult males into troop-males and periphery males. Adult female allo-grooming activities accounted for 37.62% ¡Ó 13.59 (n = 15) of the behaviors in the daytime. I also found that adult females grooming infants and juveniles were greater than received from them (p < 0.001). The social grooming among adult females occurred mainly during non-mating seasons and its frequency was 2.12 times of mating seasons. Regardless of mating or non-mating seasons, the frequency of grooming among related females was significantly higher than among unrelated females (both p < 0.001). In addition, kinship affected the grooming frequency among female macaques with relative lower or higher ranking females during mating seasons (p < 0.05), while the dominant rank did not have the effect. However, the highest grooming frequency occurred in the high-ranking females who groomed relative lower ranking females within relatives (0.38 ¡Ó 0.40 bouts / 100 scans, n = 7). During non-mating seasons, kinship affected the grooming frequency among female macaques with relative lower ranking females (p < 0.05); the highest grooming frequency occurred in the old females who groomed relative lower ranking females within relatives (1.57 ¡Ó 1.74 bouts / 100 scans, n = 8). Moreover, seasons (mating or non-mating) and kinship relationship had significant effects on grooming frequencies among female macaques (both giving and receiving p < 0.01), as well as on the grooming frequency of females groomed with relative lower ranking females (p < 0.01). The highest grooming frequency occurred on females groomed with relative lower ranking females within relatives during non-mating seasons (1.25 ¡Ó 1.48 bouts / 100 scans, n = 11). Without kinship relationship, low-ranking females groomed relative higher ranking females more frequent than high-ranking females did (p < 0.05). Within adult females, 65% of social grooming was among relatives. However, 40% of adult females groomed equally with related and unrelated females, while 20% disproportionately groomed more with unrelated females than with related females. The grooming was kin-biased for 40%. About 8.68% of social grooming among unrelated females was being reciprocated. On the other hand, the ratio of related grooming female partners to the total number of available related females was higher than that with unrelated females (p < 0.05). It also indicated that the ratio of each female received grooming from high-ranking grooming partners was higher than that from low-ranking females (p < 0.01).On the other hand, the ratio of the number of old or young grooming partners of adult females had similar values. The grooming frequency of adult females gave or received from adult males during mating seasons was higher than non-mating seasons (both p < 0.05) while high-rank females groomed adult males more than low-rank females did. The socionomic sex ratios of these two social groups were similar during mating seasons (AM:AF = 1:1.8). Both troop and periphery males had significantly higher frequency of social grooming with adult females in mating seasons than in non-mating seasons. The types of males and social groups had significant effects on the allogrooming frequency among adult males in mating seasons (p < 0.05) but not in non-mating seasons (p > 0.1). Moreover, troop males had higher grooming partners than periphery males. Major grooming partners of troop males were adult females regardless of the seasons. Subordinate males were mostly the receivers in the grooming dyads with dominant males in the mating seasons, but the relationships changed during non-mating seasons. Agonistic interactions occurred mainly during mating seasons and its frequency among periphery males was 1.8 times of troop males (p < 0.05). The preference grooming sites between allo-grooming and auto-grooming of C and Cd groups had varied significantly (p < 0.001). The back region was the preferred grooming site in allo-grooming of C and Cd group and the ano-genital region was the least groomed site. In auto-grooming, monkeys paid much attention to the legs ignoring the back and face. The result indicated that when the adult individuals groomed the head, back and face which showed significant difference in the frequency among AM and AF (p < 0.05). The related female adults groomed head more frequently than unrelated female adults (p < 0.05), but dominance rank and age-class did not apparently affect the corresponding values for the frequency of grooming sites among adult females (both p > 0.1). The results indicated that social grooming among adult females took place more often during non-mating seasons, and more often in kin-related females than unrelated females. Moreover, the dominant females were likely to groom related females. Therefore social grooming among kin-related females may reinforce relationships while reciprocal grooming of unrelated females may serve to form alliance or ranking promotion in the social group. On the other hand, social grooming between adult male and female macaques more frequent in mating seasons than in non-mating seasons. This showed that adult male Formosan macaques employed complex strategies to achieve reproductive success. Nonetheless, the periphery males had more male grooming partners than troop males did which seemingly to enhance male coalitions.

Thesis by publication. / Thesis (PhD) -- Macquarie University, School of Biological Sciences, 1990. / Bibliography: leaves 269-324. / Introduction -- Models of parental allocation -- Sex expression in homosporous pteridophytes -- The origin of heterospory -- Pollination and the origin of the seed habit -- Brood reduction in gymnosperms -- Pollination: costs and consequences -- Adaptive explanations for the rise of the angiosperms -- Parent-specific gene expression and the triploid endosperm -- New perspectives on the angiosperm female gametophyte -- Overview -- Glossary -- Kinship terms in plants -- Literature Cited. / Among vascular plants/ different life cycles are associated with characteristic ranges of propagule size. In the modern flora, isospores of homosporous pteridophytes are almost all smaller than 150 urn diameter, megaspores of heterosporous pteridophytes fall in the range 100-1000 urn diameter, gymnosperm seeds are possibly all larger than the largest megaspores, but the smallest angiosperm seeds are of comparable size to large isospores. -- Propagule size is one of the most important features of a sporophyte's reproductive strategy. Roughly speaking, larger propagules have larger food reserves, and a greater probability of successful establishment, than smaller propagules, but a sporophyte can produce more smaller propagules from the same quantity of resources. Different species have adopted very different size-versus-number compromises. The characteristic ranges of propagule size, in each of the major groups of vascular plants, suggest that some life cycles are incompatible with particular size-versus-number compromises. -- Sex expression in homosporous plants is a property of gametophytes (homosporous sporophytes are essentially asexual). Gametophytes should produce either eggs or sperm depending on which course of action gives the greatest chance of reproductive success. A maternal gametophyte must contribute much greater resources to a young sporophyte than the paternal gametophyte. Therefore, smaller gametophytes should tend to reproduce as males, and gametophytes with abundant resources should tend to reproduce as females. Consistent with these predictions, large female gametophytes release substances (antheridiogens) which induce smaller neighbouring ametophytes to produce sperm. -- The mechanism of sex determination in heterosporous species appears to be fundamentally different. Large megaspores develop into female gametophytes, and small icrospores develop into male gametophytes. Sex expression appears to be determined by the sporophyte generation. This is misleading. As argued above, the optimal sex expression of a homosporous gametophyte is influenced by its access to resources. This is determined by (1) the quantity of food reserves in its spore and (2) the quantity of resources accumulated by the gametophyte's own activities. If a sporophyte produced spores of two sizes, gametophytes developing from the larger spores' would be more likely to reproduce as females than gametophytes developing from the smaller spores, because the pre-existing mechanisms of sex determination would favor production of archegonia by larger gametophytes. Thus, the predicted mechanisms of sex determination in homosporous species could also explain the differences in sex expression of gametophytes developing from large and small spores in heterosporous species. / Megaspores of living heterosporous pteridophytes contain sufficient resources for female reproduction without photosynthesis by the gametophyte (Platyzoma excepted), whereas microspores only contain sufficient resources for male reproduction. Furthermore, many more microspores are produced than megaspores. A gametophyte's optimal sex expression is overwhelmingly determined by the amount of resources supplied in its spore by the sporophyte, and is little influenced by the particular environmental conditions where the spore lands. Gametophytes determine sex expression in heterosporous species, as well as homosporous species. A satisfactory model for the evolution of heterospory needs to explain under what circumstances sporophytes will benefit from producing spores of two distinct sizes. -- In Chapter 4, I present a model for the origin of heterospory that predicts the existence of a "heterospory threshold". For propagule sizes below the threshold, homosporous reproduction is evolutionarily stable because gametophytes must rely on their own activities to accumulate sufficient resources for successful female reproduction. Whether a gametophyte can accumulate sufficient resources before its competitors is strongly influenced by environmental conditions. Gametophytes benefit from being able to adjust their sex expression in response to these conditions. For propagule sizes above the threshold, homosporous reproduction is evolutionarily unstable, because the propagule's food reserves are more than sufficient for a "male" gametophyte to fertilize all eggs within its neighbourhood. A population of homosporous sporophytes can be invaded by sporophytes that produce a greater number of smaller spores which could land in additional locations and fertilize additional eggs. Such'spores would be male-specialists on account of their size. Therefore, both spore types would be maintained in the population because of frequency-dependent selection. -- The earliest vascular plants were homosporous. Several homosporous groups gave rise to heterosporous lineages, at least one of which was the progeniture of the seed plants. The first heterosporous species appear in the Devonian. During the Devonian, there was a gradual increase in maximum spore size, possibly associated with the evolution of trees and the appearance of the first forests. As the heterospory threshold was approached, the optimal spore size for female reproduction diverged from the optimal spore size for male reproduction. Below the threshold, a compromise spore size gave the highest fitness returns to sporophytes, but above the threshold, sporophytes could attain higher fitness by producing two types of spores. -- The evolution of heterospory had profound consequences. Once a sporophyte produced two types of spores, microspores and megaspores could become specialized for male and female function respectively. The most successful heterosporous lineage (or lineages) is that of the seed plants. The feature that distinguishes seed plants from other heterosporous lineages is pollination, the capture of microspores before, rather than after, propagule dispersal. Traditionally, pollination has been considered to be a major adaptive advance because it frees sexual reproduction from dependence on external fertilization by freeswimming sperm, but pollination has a more important advantage. In heterosporous pteridophytes, a megaspore is provisioned whether or not it will be fertilized whereas seeds are only provisioned if they are pollinated. / The total cost per seed cannot be assessed solely from the seed's energy and nutrient content. Rather, each seed also has an associated supplementary cost of adaptations for pollen capture and of resources committed to ovules that remain unpollinated. The supplementary cost per seed has important consequences for understanding reproductive strategies. First, supplementary costs are expected to be proportionally greater for smaller seeds. Thus, the benefits of decreasing seed size (in order to produce more seeds) are reduced for species with small seeds. This effect may explain minimum seed sizes. Second, supplementary costs are greater for populations at lower density. Thus, there is a minimum density below which a species cannot maintain its numbers. -- By far the most successful group of seed plants in the modern flora are the angiosperms. Two types of evidence suggest that early angiosperms had a lower supplementary cost per seed than contemporary gymnosperms. First, the minimum size of angiosperm seeds was much smaller than the minimum size of gymnosperm seeds. This suggests that angiosperms could produce small seeds more cheaply than could gymnosperms. Second, angiosperm-dominated floras were more speciose than the gymnosperm-dominated floras they replaced. This suggests that the supplementary cost per seed of angiosperms does not increase as rapidly as that of gymnosperms, as population density decreases. In consequence, angiosperms were able to displace gymnosperms from many habitats, because the angiosperms had a lower cost of rarity. -- Angiosperm embryology has a number of distinctive features that may be related to the group's success. In gymnosperms, the nutrient storage tissue of the seed is the female gametophyte. In most angiosperms, this role is taken by the endosperm. Endosperm is initiated by the fertilization of two female gametophyte nuclei by a second sperm that is genetically identical to the sperm which fertilizes the egg. Endosperm has identical genes to its associated embryo, except that there are two copies of maternal genes for every copy of a paternal gene. -- Chapter 9 presents a hypothesis to explain the unusual genetic constitution of endosperm. Paternal genes benefit from their endosperm receiving more resources than the amount which maximizes the fitness of maternal genes, and this conflict is expressed as parent-specific gene expression in endosperm. The effect of the second maternal genome is to increase maternal control of nutrient acquisition. -- Female gametophytes of angiosperms are traditionally classified as monosporic, bisporic or tetrasporic. Bisporic and tetrasporic embryo sacs contain the derivatives of more than one megaspore nucleus. Therefore, there is potential for conflict between the different nuclear types within an embryo sac, but this possibility has not been recognized by plant embryologists. In Chapter 10, I show that many previously inexplicable observations can be understood in terms of genetic conflicts within the embryo sac. / Mode of access: World Wide Web. / 324 leaves ill

In this thesis, I explore four different ways in which relatedness affects sexual interactions in the red junglefowl Gallus gallus ssp., and the fruit fly Drosophila melanogaster. First, I show that in both species, inbreeding depression is sex-specific and modulated by parental age and gametic age. However, the sex that suffers higher inbreeding depression was trait- and species-dependent. Second, I examined patterns of inbreeding avoidance. I found no evidence of inbreeding avoidance in the fruit fly, but in the red junglefowl both males and females avoided mating with relatives, independently from sex-ratio of the social group. Third, I investigated whether relatedness amongst members of one sex affects mate choice in members of the opposite sex. Male fruit flies preferentially courted females unrelated to females with whom they had previously mated, while female flies displayed a weak preference for males related to their previous mates. In the red junglefowl, females exposed to male trios of two males related to each other and one unrelated male, displayed a marked preference for mating with the male unrelated to the other two males, and might also bias postcopulatory sperm utilization in favour of the unrelated male. Fourth, I explored the implications of male relatedness on the intensity of male-male competition. Male red junglefowl were less aggressive towards related competitors, but invested more sperm in females that had previously mated with a related male rather than with an unrelated male. In fruit flies, male relatedness had a strong impact on female life-history and offspring viability, although I found no evidence that these effects were modulated by changes in male-male competition. Collectively, the findings of these studies demonstrate the complex relationship between relatedness and other important biological phenomena as such senescence and sexual conflict.

Sociality is widespread across the animal kingdom and explanations for its incidence and persistence are numerous. Whilst various drivers of sociality have been identified and tested, controversies remain and we are still far from a complete understanding of the mechanisms underlying social structure. Here I use a combination of field observations on a free-living population of feral horses Equus caballus and laboratory behavioural experiments on the Pacific beetle roach Diploptera punctata to investigate the drivers of sociality in these species. I explore four key aspects of sociality: the influences of kinship on sociality and social development, the strength and persistence of social bonds, the relationship between inbreeding avoidance and dispersal and the potential influence of individuals on social structure. Whilst kinship is a major driver of social structure in most mammalian species, I present evidence in Chapter Three that horse society is not structured by levels of kinship; however, in Chapter Five, I show that kinship levels to potential mates are significant in female dispersal choices in this species. In Chapter Eight, I provide evidence for significant effects of kinship to companions upon social and physical development in D. punctata, indicating a clear potential benefit of kin-based associations. The stability of social bonds can have substantial effects upon social structure. In Chapter Three, I show that the bonds between female horses show significant stability and are formed independently to kinship levels, a rare result in a non-primate species. I also provide evidence consistent with the hypothesis that these bonds are driven by male harassment. Similarly, in D. punctata, I find in Chapter Eight that female clustering occurs within resting aggregations and that the most likely explanation is the avoidance of male harassment. I therefore propose that this driver of female sociality may be a highly prevalent force structuring animal societies. Inbreeding depression has been demonstrated in a variety of species and contexts. Here I show in Chapter Five that in horses, female dispersal is likely to be influenced by kinship levels with potential mates. In Chapter Four, I then show that more heterozygous males have a higher reproductive success, most likely due to their ability to utilise a larger home range. Finally, local population structure can be highly influenced by individual association choices and behaviour. In Chapter Two, I show that in horses, mothers may allow their sons to postpone dispersal by the maintenance of stronger mother-son bonds, permitting an extended period of social learning. In Chapter Seven, I demonstrate that consistent inter-individual variation in personality traits exists in D. punctata which is stable across life stages, despite age effects on the strength of boldness. This is a source of variation which may be extremely important for decision-making social groups. My main conclusion from this work is that male harassment is often a key driver of sociality which may frequently be overlooked. I also demonstrate that the effects of kinship are far-ranging but not omnipresent. This thesis therefore makes a major contribution to our understanding of the mechanisms underlying animal sociality and presents clear potential avenues for future research.

Despite the crucial importance of Hamilton's (1964) kin selection theory in evolutionary behavioral biology, psychological studies of family relationships have been relatively slow to incorporate a Darwinian perspective. One practical reason may be that existing evolutionary models of animal families, such as the honest signaling models, are applicable only if all family members fall into the same class in terms of age, sex, or health. The animal models are thus of limited use for investigating human families, in which the relative age of the child, as a corollary of birth order, may have played a pivotal role in shaping evolved family psychology. My dissertation has two main objectives: 1) to construct evolutionary mathematical models of family interactions that fully take into account the role of reproductive value and hence can be directly applied to human families; 2) to characterize the design features of evolved psychological mechanisms of human kinship by empirically testing a priori predictions derived from the models. I first examine how parents are expected to allocate their limited resources among offspring of differing ages. I show that the optimal strategy that serves parental interests is to bias parental resources toward the older offspring (chapter 2). I then empirically test the predictions derived from the first study, in comparison with previous evolutionary hypotheses of parental favoritism. The empirical results confirmed the predictions derived from the first study: in hypothetical allocation tasks, participants allocated more tangible resources toward older children (chapter 3). Next, I investigate how intrafamilial conflict over the allocation of parental resources occur when each family member (a parent, its senior offspring, and its junior offspring) are allowed to differ in age. The results gained in this study may require a substantial revision of Trivers' (1974) classical theory of parent-offspring conflict. Moreover, it will open a fruitful avenue for inferring the adaptive design of psychological mechanisms dealing with sibling relationships (chapter 4). I then show that evolutionary insights can be also applied to the psychological study of distant kin relationships such as cousins (chapter 5).

Androphilia refers to sexual attraction and arousal to adult males, whereas gynephilia refers to sexual attraction and arousal to adult females. Prehistoric artifacts such as art and pottery indicate that male-male same sex behaviour has existed for millennia. Bearing this in mind, and considering that male androphilia has a genetic component yet androphilic males reproduce at a fraction of the rate than do gynephilic males, how the genes for male androphilia have been maintained in the population presents an evolutionary puzzle. This thesis tests two hypotheses that attempt to address this Darwinian paradox. Chapter one reviews the current literature on the kin selection hypothesis and the sexually antagonistic gene hypothesis. In addition, rationales for testing these hypotheses in Canada are provided. Chapter two tests the kin selection hypothesis for male androphilia within a Canadian population. Results and implications are discussed. Chapter three tests the sexually antagonistic gene hypothesis within a Canadian population. Results and implications are discussed. Chapter four summarizes the results of the two studies and discusses how these findings may be interpreted from an evolutionary perspective. The impacts of gene-environment interaction on the functional behavioral expression of traits are emphasized. / viii, 113 leaves ; 29 cm

By combining laboratory experiments with field work, I have looked at the following aspects of cellular slime mould (CSM) biology: (a) the genetic structure of social groups (fruiting bodies) in the wild and its relation to the role of large mammals as dispersal agents; (b) social behaviour in clonal, intra-species polyclonal and interspecies social groups and (c) fitness-related trade-offs with respect to life history traits as a possible mechanism for coexistence and cooperative behaviour in CSMs. The major findings of this study are as follows: (a) individuals belonging to different strains of a species, different species and genera occur in close proximity, even on a speck of soil (250µm-1mm) or the same dung pat; (b) social groups formed in the wild by Dictyostelium giganteum and D. purpureum are generally multiclonal; (c) genetically diverse strains can co-aggregate and form chimaeric social groups; (d) in chimaeric social groups, strains differ in their relative sporulation efficiencies; (e) the fact that strains co-exist in spite of this may be attributable in part to trade-offs between various fitness-related traits as can be demonstrated in the case of wild isolates of D. giganteum in pair wise mixes. The Dictyostelids or CSMs are haploid, eukaryotic, soil dwelling social amoebae with an unusual life cycle (Bonner, 1967; Raper, 1984). They exist as single cells in the presence of food (bacteria, yeast, fungal spores). Once the food is exhausted, they enter the social phase of their life cycle. Approximately 102 to 106 amoebae aggregate at a common collection point and form a starvation resistant structure called the fruiting body. In many species a fruiting body is made up of an aerial stalk of dead cells and a ball of viable spores on top. In other CSM species (not part of this study), all amoebae in a fruiting body differentiate into spores and the stalk is an extracellular secretion. The CSM life cycle raises fundamental questions related to the evolution of an extreme form of 'altruism' in the form of reproductive division of labour in social groups. The spore-stalk distinction in the CSMs is analogous to the germ-soma distinction in metazoans, although, the CSMs achieve multicellularity not by repeated divisions of a zygote but via the aggregation of many cells which may or may not be clonally related (Bonner, 1982; Kaushik and Nanjundiah, 2003). Social behaviour in the CSMs offers interesting parallels to what is seen in the social insects (Gadagkar and Bonner, 1994). The origin and maintenance of 'altruism' has been a long-standing issue in sociobiology. Because of their simple life cycle and experimental tractability, the CSMs are ideal for studying the evolutionary origin and maintenance of social behaviour, in particular of 'altruistic' behaviour. By elevating spores above soil level, stalk cells, protect them from noxious compounds and predators present in soil and also facilitate their passive dispersal. In the course of doing so they die. The death of stalk cells appears to be an extreme form of altruism. Knowledge of the genetic structure of social groups and populations including patterns of kinship is essential for modelling the evolution of 'altruism'. Thus, it is important to understand the genetic structure of CSM social groups in the wild. For this, social groups (fruiting bodies) of CSMs were isolated from undisturbed forest soil of the Mudumalai forest reserve in South India. Soil and animal dung samples were brought to the laboratory and quasi-natural social groups were generated by inoculating the samples on non-nutrient agar. The fruiting bodies from various CSM species were formed by these isolates. Since soil and dung samples were not perturbed in any way, the fruiting bodies were formed as they would have in nature. When compared to soil, dung samples contained a higher CSM diversity and more CSM propagules. The presence of CSMs in fresh animal dung makes it likely that they were transported and dispersed over long distances through the gut of these animals. Such dispersal is likely to be preceded by a thorough mixing of spores in the gut. That increases the probability of co-occurrence of different genotypes in a social group. This possibility was confirmed by genetically characterizing spores in social groups of Dictyostelium giganteum and D. purpureum collected from the wild. Random amplification of polymorphic DNA (RAPD), a simple and reliable molecular technique, was used for genotyping spores within a fruiting body. 17 fruiting bodies (8 from animal dung and 9 from soil) were studied. 15 out of 17 (9 out of 11 of D. giganteum and 6 out of 6 D. purpureum) were polyclonal; the minimum number of distinct clones in a single fruiting body was 3 to 7 (animal dung) and 1 to 9 (soil). Therefore in D.giganteum and D. purpureum, chimaeric social groups seem to be the norm. This suggests that other species of CSMs form intra-species chimaeric social groups in wild, though clonal fruiting bodies occur too. The next objective of this thesis was to test whether genetic heterogeneity had functional consequences. That is, when different strains come together in an aggregate, do they contribute equally to the reproductive (spore) and non-reproductive (stalk) pathways? Amoebae of different clones (strains) of D. giganteum or D. purpureum were mixed and developed together and the number of spores formed by each strain was counted. These experiments confirmed that strains of D. giganteum or D. purpureum can aggregate together and form chimaeric fruiting bodies. The ability to mix (measured as the frequency of chimaerism) depended on the strains used and varied from one mix to another. One strain was often found to 'exploit' the other during sporulation, that is, it formed more spores than its expected share. Despite this, strains are found in very close proximity in the soil, which raises an important question: when one strain is more efficient at sporulating than other, how can the two co-exist stably? To investigate what might lie behind the stable co-existence of strains, I studied various fitness-related traits in the life cycle of D. giganteum. They included the rate of cell division, the time taken to go through multicellular development, the efficiency of slug migration through various depths of soil and the probability of differentiation into a spore. Measurements were carried out on strains taken separately and on their pair wise mixes. Five different D. giganteum wild strains (46a3, 46d2, 48.1a1, F5 and F16) were used. All were isolated from the Mudumalai forest (India). 46a3 and 46d2 came from soil within 10 cm of each other, 48.1a1 from soil about 200m away from 46a3; and F5 and F16 from the same fruiting body (Kaushik et al., 2006; Sathe et al., 2010). Members of a pair differed significantly in the measured fitness-related traits. For example, in the case of 48.1a1 and 46d2, 48.a1 grew faster than 46d2 both individually and in a mix. After starvation, 48.1a1 formed fruiting bodies faster than 46d2; a mix of the two developed at the rate of the faster member, implying that the slower one (46d2) gained from the association with 48.1a1. During slug migration, slugs formed by 48.1a1 came up through a higher depth of soil than 46d2 slugs and did so earlier. Chimaeric slugs were like the more efficient member, 48.1a1, in terms of the maximum depth of soil that was covered, but like the less efficient member, 46d2, in terms of the time taken for slugs to be seen on the soil surface. 48.1a1 seems to have an advantage over 46d2 in all these respects. However, during sporulation in chimaeras, 48.1a1 formed relatively fewer spores than 46d2. Similar trade-offs were seen in all mixes. F5 and F16 displayed an unexpected feature during sporulation; the spore-forming efficiency of either strain depended on its proportion in the initial mix in a frequency-dependent manner that was consistent with a stable equilibrium. Thus, trade-offs between different fitness-related traits contribute to the co-existence of strains. Next, I studied interactions between members of different CSM species. Several species of CSMs were isolated from the same environment (Sathe et al., 2010); a question of interest was to see if amoebae of different species came together to form a chimaeric multicellular body. Five strains (two D. purpureum and three D. giganteum) were used in this study. Amoebae of D. giganteum and D. purpureum co-aggregated. However, there were factors that caused amoebae of the two species to sort out thereafter. The extent of segregation differed between strains, a characteristic that inter-species mixes share with intra-species mixes. In conclusion, the ability of cellular slime moulds to form multiclonal social groups in the wild suggests that one should look to factors in addition to close relatedness to understand the evolution of CSM social behaviour. The existence of fitness-related trade-offs between different traits indicates that individual-level selection can also contribute to the maintenance of chimaeric social groups.