Monogamous Pair Bonds Research Articles

Social Monogamy in the Big‐Clawed Snapping Shrimp, Alpheus heterochelis

AbstractSocial monogamy has evolved independently in many taxa, and often involves biparental care of the young. Where it does not, mate guarding and shared territoriality have been invoked as causal factors. We evaluated mate guarding and shared resource defence (a common shelter) as factors that could have led to social monogamy in the snapping shrimp, Alpheus heterochelis. This species is found in male–female pairs that defend a common shelter together. Female receptivity lasts only for a few hours immediately after her periodic moult. Their monogamous pair bond may represent mate guarding or joint defence of a territory. Monogamy in A. heterochelis seems most importantly driven by the cryptic nature of the female's moult cycle. We found that males did not discriminate among females at different intermoult stages for pairing, nor did they modulate their defence of mate and shelter (vs. the risk in finding a new shelter and mate) according to female moult stage. This, together with the short period of female receptivity before her single copulation per cycle, make extended mate guarding the most efficient method for a male to secure a mating opportunity. Comparing eviction rates of paired and unpaired shelter residents by conspecific intruders provided no evidence of enhanced resource defence that would confer a selective advantage to a pair. Male presence during the moult is beneficial for the female, as searching for a male during her soft‐bodied receptive phase would put her at mortal risk. Our results show empirically for the first time that guarding may be beneficial, even if males are not able to assess the female's reproductive stage. This extends the theoretical framework for understanding the evolution of social monogamy in taxa without biparental care of young.

Behavioral and adrenocortical responses to mate separation and reunion in the zebra finch

The adaptive value of social affiliation has been well established. It is unclear, however, what endogenous mechanisms may mediate affiliative behavior. The Australian zebra finch ( Taeniopygia guttata) breeds colonially and adults maintain lifelong pair bonds that may be disrupted in the wild due to high mortality rates. Many of its natural, social behaviors are maintained in laboratory conditions, making this species well suited for studying the mechanisms of affiliation. This study examines the behavioral and neuroendocrine responses to pair mate separation and reunion in zebra finches. We measured plasma corticosterone (CORT) and behavioral changes following separation from a pair bonded mate, and again upon reintroducing the mate or an opposite-sex cagemate. Plasma CORT concentrations were: (1) elevated during pair mate separation, even in the presence of other same-sex individuals, and (2) reduced to baseline upon reunion with the pair mate but not upon re-pairing with a new opposite-sex partner. These findings show that zebra finches exhibit hormonal responses to separation and reunion specifically with a bonded pair mate and not with other familiar conspecifics. In addition, alterations in behavior during separation and reunion are consistent with monogamous pair bond maintenance. This study presents evidence for adrenocortical involvement in avian pair bonding, and for hypothalamic–pituitary–adrenal activation in response to an ecologically relevant social stressor.

Reproductive Ecology of Tibetan Partridge Perdix hodgsoniae in Lhasa Mountains, Tibet

Data on the breeding ecology of Tibetan Partridges (Perdix hodgsoniae) were collected in shrub environments near Lhasa, Tibet, during 1999-2001. Partridge flocks broke up in mid-March and individuals typically formed socially monogamous pair bonds. During daily activities the both members of the pair were close to each other, with 73.5% of observed individual distances less than 2m. The egg-laying period extended from late May to late June. Average egg size was 16.1 (±SE=0.2) g in mass and 39.2 (±0.1)×28.1(±0.1) mm in dimension. Average clutch size was 8.3 (±0.8, range 5-12) The partridge produced larger eggs and smaller clutches than its two congener species, Grey Partridge (P. perdix) and Daurian Partridge (P. dauuricae). Incubation lasted about 23 days, and 44.4% of clutches successfully hatched. Vegetation characteristics were the most important determinants of partridge nest-site choice, with patches of low, sparse, and secondary cover being preferred. Nest-sites were also significantly closer to paths than random sites.

Reunion vigour: an experimental test of the mate guarding hypothesis in the monogamous sleepy lizard (Tiliqua rugosa)

AbstractThe Australian sleepy lizard Tiliqua rugosa forms socially monogamous pair bonds for 6 to 8 weeks each spring before mating occurs. This behaviour could not be explained by any hypothesis involving males helping to rear their young, because pairs split up soon after mating, and males seem to have no contact with the offspring of their partners. Short monogamous associations in other lizard species are often interpreted as mate guarding by the male to assure paternity. The mate guarding hypothesis was tested by experimental separation of pairs at various times during the pairing season. Data from 73 trials indicated that males were more likely to return to females than vice versa, and that the tendency of males to return to females increased as the time of mating approached. Also, reunion was more likely in pairs that had a greater tendency to remain together during the season. These results are consistent with predictions that the monogamous relationship is a form of mate guarding by males. However, the incidence of females re‐uniting with males, and the long duration of pair bonds before mating occurs are not predicted by a mate guarding hypothesis. There may be additional explanations for monogamous pairing in the sleepy lizard.

The annual testosterone profile in cooperatively breeding superb fairy-wrens, Malurus cyaneus , reflects their extreme infidelity

Superb fairy-wrens are cooperatively breeding birds that combine stable, socially monogamous pair bonds and high levels of paternal care, with extreme levels of extra-pair mating and high levels of sexual competition. Our aim was to determine which testosterone correlates would prevail in such a life history that combines features that are conventionally associated with divergent hormone profiles. Unlike the situation in other species with monogamous pair bonds and high levels of paternal care, testosterone was elevated for a very long period of several months. During breeding there was a broad peak in testosterone followed by a gradual decline: this resembles the profile found in polygynous and promiscuous species. We found that three factors correlated with testosterone: development of the sexually selected nuptial plumage, social status and extra-group mating opportunities. Testosterone started increasing months prior to breeding, when the males that are later preferred as extra-group sires develop their nuptial plumage. Although these males did not have higher testosterone levels during breeding, they sustained high testosterone for much longer, and this might lend reliability to this sexual signal. Dominant males in groups had higher testosterone than pair-dwelling males and subordinate helpers. This was not due to differences in age, reproductive capability or mating opportunities, but was presumably associated with the assertion of dominance. In contrast to findings in other species, male testosterone level was not correlated with whether the resident female was fertile or had dependent nestlings. However, testosterone was strongly correlated with the total number of fertile females in the population, and hence with the opportunities for extra-group mating.

Monogamous pair bonds and mate switching in the Western Australian seahorse Hippocampus subelongatus

Abstract Apparently monogamous animals often prove, upon genetic inspection, to mate polygamously. Seahorse males provide care in a brood pouch. An earlier genetic study of the Western Australian seahorse demonstrated that males mate with only one female for each particular brood. Here we investigate whether males remain monogamous in sequential pregnancies during a breeding season. In a natural population we tagged males and sampled young from two successive broods of 14 males. Microsatellite analyses of parentage revealed that eight males re-mated with the same female, and six with a new female. Thus, in this first study to document long-term genetic monogamy in a seahorse, we show that switches of mates still occur. Polygynous males moved greater distances between broods, and tended to have longer interbrood intervals, than monogamous males, suggesting substantial costs associated with the breaking of pair bonds which may explain the high degree of social monogamy in this fish genus.

Differences in potential reproductive rates of male and female seahorses related to courtship roles

Dwarf seahorses, Hippocampus zosterae (Syngnathidae), are distinguished by extreme morphological specialization for paternal care, the formation of monogamous pair bonds and mating repeatedly over the course of a breeding season. To determine the potential reproductive rates of male and female dwarf seahorses, we measured (1) the maximum number of offspring produced per breeding cycle when sexually receptive mates were unlimited, and (2) the relative time each sex was unavailable for mating (‘time out’). We paired sexually isolated males and females with sexually receptive partners and observed them from the day of introduction through to copulation, to determine the length of time it takes each sex to prepare to mate. We conducted additional experiments to determine the length of gestation, which when added to the time needed to prepare to mate and copulate gives an estimate of total reproductive cycle duration, T. We estimated potential reproductive rate by dividing the mean number of offspring produced per breeding cycle by the duration of the breeding cycle (T). We estimated reproductive ‘time out’ by identifying the period of time males and females were physiologically capable of mating (‘time in’, S) and subtracting time S from time T. When provided with sexually receptive partners, females took 2 days longer than males to complete courtship and copulation, but neither males nor females remated during gestation. Therefore, males could potentially produce 17% more offspring than females over the course of one breeding season. Females had reproductive ‘times out’ 1.2 times longer than did males, as they were only capable of mating during the 4h directly preceding copulation. Thus, H.zosterae males have higher potential reproductive rates and shorter reproductive ‘times out’ compared with H.zosterae females. These results and previous work indicating that seahorses display traditional courtship roles support the prediction that the sex having the higher potential reproductive rate, or equivalently, the shorter ‘time out’, will compete more intensely for access to the opposite sex.

The Cost of Mate Guarding in the Black-browed Reed Warbler Acrocephalus bistrigiceps. When Do Males Stop Guarding Their Mates?

To clarify the cost of mate guarding, when males attract additional mates, I studied the mate guarding behaviour of the Black-browed Reed Warbler Acrocephalus bistrigiceps in south-central Honshu, Japan in 1997. The percentage of time spent by males close (<5m) to their mates was significantly higher, and the distance between mates was significantly closer during the fertile than during the post-fertile period. Males followed their mates, whereas females did not follow the males. Territorial intrusions, by conspecific males, occurred more frequently in territories with nesting females than in those of solitary males, and an intruding male was found to court a fertile female when her mate was absent. These observations indicate that there is a real risk of extra-pair fertilization. Three males switched their behaviour from mate guarding to singing and attracted secondary females by resuming singing. Two of them stopped mate guarding on the day the first egg was laid, so their mates were unguarded during the remainder of their fertile period. In contrast, three other males, which maintained monogamous pair-bonds, continued to guard their mates throughout the fertile period. Male warblers who attract secondary mates appear to incur an additional cost in mate guarding.

Endocrine and Behavioural Responses of Male Greylag Geese (Anser anser) to Pairbond Challenges during the Reproductive Season

Although greylag geeseAnser anserestablish long‐term monogamous pairbonds, some of the existing pairs do split up (divorce) and new pairs are formed during the annual spring mating period. In this study, male greylag geese which were involved in the challenge of an existing pairbond (challenged males and challengers) were regarded as ‘natural experimental’ groups and compared with males in stable pairbonds (unchallenged males and male‐paired males, a common male strategy when the availability of females is low). In total, 37 males were investigated. The analysis included a description of the seasonal patterns of hormone levels, aggression and courtship. We tested whether hormone levels correlated with aggressive and courtship behaviours. Finally, we compared hormonal and behavioural patterns amongst the four groups. Immunoreactive testosterone (T) and corticosterone (B) equivalents were measured in faecal samples. Individual hormone levels were correlated with frequencies of agonistic male‐male interactions and with frequencies of male‐female courtship. During early mating and pre‐laying phases, T was at its seasonal maximum, which may have masked hormone‐behaviour correlations. During egg‐laying, at the onset of seasonally decreased T, agonistic male‐male interactions and the frequencies of courtship behaviour were significantly correlated with T. Unchallenged males had higher rates of agonistic interactions than any other males. However, unchallenged and challenged males tended to excrete T at higher levels than challengers. The high rates of being attacked and elevated levels of faecal B were indicative of the social conflict experienced by challengers. No hormonal differences were observed between heterosexually paired males and male‐paired males. In summary, pairbond status and situations of social conflict had a modulating effect on T and B; however, in this study, the two hormones seem to be affected independently of one another.

Behavioral rules and tail-up display in extra- and intra-pair interactions of the butterflyfish,Chaetodon lunulatus

The butterflyfish,Chaetodon lunulatus, forms monogamous pair bonds. Each pair defends a feeding territory against conspecifics. The tail-up or lateral display of the butterflyfish is performed not only towards non-partners in territorial disputes (extra-pair interaction), but also towards partners (intra-pair interaction). In order to explain this phenomenon, I investigated the two interaction types, and found that a simple model explains both interactions very well. The model consists of four behavioral rules and two provisos for applying the rules. Rule 1: Perform the tail-up display when approached by another individual. Rule 2: Perform the tail-up display when another individual performs the tail-up display. Rule 3: Attack another individual that neither approaches nor performs the tail-up display. Rule 4: Swim with your partner. Proviso 1: Apply Rules 1 and 2 when you do not intend to run away. Proviso 2: Apply Rules 1, 2, and 3 when you do not recognize another individual as your partner. When Rules 1–4 and Provisos 1–2 are applied, the display can prevent improper attacks between partners caused by imperfect partner recognition.

Ecology and Behavior of Yellow Warblers Breeding in Rhode Island's Great Swamp

I examined breeding phenology, territoriality, social interactions, reproductive success, and survivorship in a population of yellow warblers (Dendroica petechia) breeding in the Great Swamp, Washington County, Rhode Island. In four seasons of study (1992-1995), males preceded females in arriving on the breeding grounds. Females began building nests in early May. Of 94 nests whose host plants were identified, 28 were in Rosa, 28 in Salix, 12 in Clethra alnifolia, and 26 in plants of 12 other species. Clutches were initiated between May 14 and June 14. Males defended territories and formed monogamous social pair bonds with the females nesting on these territories. The breeding density was approximately five pairs/ha of occupied habitat. Only females were observed building nests or incubating clutches. Both members of social pairs participated in provisioning nestlings, with no significant difference in provisioning rates between the sexes. The mean size of first clutches was 4.5 (SE = 0.09), and clutch size declined as the season progressed. The probability that a newly laid egg would yield a surviving fledgling was 0.46. One instance of brood parasitism by brown-headed cowbirds (Molothrus ater) was observed. The percentage of birds breeding in one year that returned to the study area the next year was 56% for males and 42% for females. These results provide an opportunity for comparison with populations in other regions and a baseline against which future environmental impacts in southern Rhode Island might be evaluated.

High Paternity without Paternity-Assurance Behavior in the Purple Sandpiper, a Species with High Paternal Investment

Among birds, males investing highly in paternal care should be under par- ticularly strong selection pressure to prevent extrapair paternity in their broods. Current literature implies that to obtain a high certainty of paternity, males should invest in pater- nity-assurance behaviors, and this makes them more likely to increase their investment in reproduction. The Purple Sandpiper (Calidris maritima) is a relatively long-lived arctic shor- ebird with a socially monogamous pair bond and strong mate and territory fidelity by both sexes. Paternal investment is high because males alone perform all of the brood care from hatching to fledging. We investigated the mating strategies of male and female Purple Sand- pipers through genetic analysis and examined the possibility that this species exhibits pa- ternity-assurance behaviors. We found no evidence that males exhibited behaviors such as frequent copulation or mate guarding during their mate's fertile period. Furthermore, high rates of copulation were not necessary for pairs to achieve complete fertility of the clutch. Multilocus DNA fingerprinting revealed little evidence for extrapair fertilizations: only 1 of 82 offspring was sired by an extrapair male. These results suggest that male and female Purple Sandpipers rarely seek extrapair copulations (EPCs). In contrast to several recent studies with similar findings, we believe that the apparent rarity of EPCs results because females have few genetic or other benefits to gain from seeking them. Thus, paternity-as- surance behaviors by male Purple Sandpipers were not necessary for achieving full paternity of the clutch, and such behaviors cannot be said to have led to the observed high level of paternal investment in reproduction. Uniparental brood care, however, is associated with a high certainty of paternity in this species. Received 3 June 1997, accepted 13 November 1997.

Neuroendocrine Correlates of Separation Stress in the Siberian Dwarf Hamster (Phodopus sungorus)

Castro, W. L. R. and K. S. Matt. Neuroendocrine correlates of separation stress in the Siberian dwarf hamster (Phodopus sungorus). Physiol Behav 61(4) 477–484, 1997.—Siberian dwarf hamsters form monogamous male-female pair bonds. Disruption of the pair bond results in increases in body mass and behavioral alterations similar to profiles seen in human atypical depression. We examined behavioral and neuroendocrine correlates associated with separation of the male from his mate. Animals were paired (n = 28 pairs) for 3 weeks, then 15 pairs were separated and 13 pairs remained as controls. Behaviors of the males were observed in a novel environment following 3 weeks of pairing and 4 weeks of separation. The 5-min behavioral test monitored exploratory, territorial behavior, and locomotor activity. Separated males showed a significant increase in body mass (p < 0.01), paralleled by an increase in food consumption (p < 0.01). Separated males had decreased seminal vesicle mass (p < 0.05) and testicular mass (p < 0.05). Behavioral analysis revealed that separated males showed no significant differences in grooming, scent-marking, alert on 2 feet, or escape behavior when compared to paired males. Separated males did show significant increases in inactivity (p < 0.05). Plasma cortisol levels were significantly increased in separated animals (p < 0.05), but there were no significant effects on testosterone. Resting levels of plasma norepinephrine and epinephrine were less in separated males, but this was significant only for norepinephrine (p < 0.05). In conclusion, separation stress was accompanied by increased hypothalamic-pituitary-adrenal axis function and decreased peripheral sympathetic nervous system activity and decreased reproductive profiles.

Honest advertisement of pairing status: evidence from a tropical resident wood-warbler

This paper reports the first investigation of the mate attraction function of song in a tropical resident wood-warbler. Male Adelaide’s warblers, Dendroica adelaidae(Emberizidae: Parulinae), which sing two categories of song (A and B), maintain territories and monogamous pair bonds year-round. Males that were naturally or experimentally unpaired delivered A songs at significantly higher rates than did paired males during both breeding and non-breeding periods. In contrast, use of B songs and singing at dawn did not vary with pairing status. The dawn chorus, characterized by intense and versatile displays of the B song category by neighbouring males, was absent from the non-breeding season. Dawn singing and the B song category thus appear relatively unimportant in mate attraction in this species. Results are consistent with the hypothesis that the A song category functions in mate attraction in wood-warblers. Furthermore, data from the non-breeding season show that the mate attraction function of song is not narrowly tied to the breeding season in a resident species. These data provide evidence for honest signalling of male pairing status in a socially monogamous species. Throughout the year, male Adelaide’s warblers reveal their pairing status honestly, with a comparatively higher rate of A song delivery signalling that a male is unpaired. Paired and unpaired males were distinguishable based on their singing behaviour. Honest advertisement of pairing status may have been favoured in this species by female defence of mates and a habitat saturated with territories, which would limit opportunities for polygyny or polyterritoriality.

Sex differences in aggressive behavior in zebra finches (Poephila guttata).

Aggressive behavior between pairs of female or male zebra finches (Poephila guttata) was measured before, during, and after presentation of either a female or a male finch (the stimulus) in an adjacent cage. Presentation of the stimuli led to marked increases in aggressive behavior for both sexes of subjects, such that presentation of a female stimulus caused an elevation in aggression in male pairs and presentation of a male stimulus caused an elevation in female pairs. These results are interpreted in the context of competition for mates in species, such as zebra finches, that have long-term monogamous pair bonds and biparental care

Benefits of Courtship-Feeding for Rifleman (Acanthisitta Chloris) Parents

Abstract1. Courtship-feeding was studied in riflemen (Acanthisitta chloris) in a population at Kaikoura, South Island, New Zealand between 1982 and 1984. The proportion of the food males collected which was donated to his mate was calculated and what proportion of the female's diet this represented. This information revealed that males made a significant early season contribution to parental care. 2. Allied information collected during the pre-lay and egg-laying periods included the time to form eggs, laying interval and clutch size. 3. Courtship-feeding in riflemen involved no ceremony. 4. Copulation attempts did not correspond with bouts courtship-feeding or the peak of courtship-feeding. 5. Pairs spent 91.2% of daylight hours in each other's company, which facilitated coursthip-feeding. 6. Food items delivered in courtship-feeding were significantly larger than those eaten by males or females while foraging for themselves, and larger food items were consistently offered throughout the pre-laying and egg-laying periods. 7. The peak in the volume of food delivered to the female occurred about 3.5 days after the first egg was laid. 8. Overall the male contributed 42% of the food he gathered to the female and this comprised 35% of her total food intake. 9. Females fed themselves enough food to meet maintenance requirements and the extra required for oogenesis was received from the male through courtship-feeding and any reserves stored by the female. 10. Riflemen laid eggs every 48 hours which probably reduced peak energy demands during oogenesis. 11. Courtship-feeding was not associated with second clutches which were significantly smaller than the first clutch laid and reared in a season. 12. Incubation occurred after the last egg was laid but was sometimes delayed. During the delay courtship-feeding continued until incubation started. 13. Courtship-feeding represents a significant early season investment by male riflemen in their offspring which probably allows time to rear two broods thereby improving both parents' productivity. Such early season investment might influence sexual selection towards stable, monogamous pair bonds.

On the Mechanism of Mate Selection in Black-Headed Gulls

Processes leading to the establishment of social relationships are the main topic of the present study, which is based on observations of black-headed gulls kept in experimental groups in aviaries. Among these birds various kinds of social relationships were established, such as heterosexual monogamous pair-bonds, polygynous associations, and male-male relationships. Some of the factors which give rise to the formation of such relationships could be studied, because it was possible to watch all individuals during their whole life, and during all stages of their reproductive cycle. Quantitative data were collected on the influence of sex, age, reproductive state, physical condition, and various social factors on the incidence of social relationships. On the basis of these data it could be concluded that males and females play strongly differing roles. Unmated males advertize randomly. Unmated females direct their courtship to particular individuals, belonging to the category of advertizing unmated males, or of the active mated males. The final decision for pair-formation involves an acceptance by the approached male. Unsuccessful unmated males may adopt the female strategy, and thus establish homosexual social relationships. Mated males do not initiate courtship towards other females, but they may be courted by them. Mated females sometimes display social interest for other males. The main criteria for mate choice were advanced age, early nuptial moult, small body-size, and, in particular, social experience with one another. In contrast to our expectation, body weight and matching of nuptial moult and physical condition between partners were not important in our birds. The significance of the criteria age and moult may be directly related to fitness. The preference for small males had earlier (VAN RHIJN, 1985) been interpreted as a mechanism preventing polygyny, but additional data provide some inconsistencies with that idea. Finally, the significance of social experience with each other has been related with considerable investments of prospective mates before establishing a pair-bond, and with an increase of synchronization between mates as a result of long-lasting pair-bonds. Female-female pairing among some wild larids has been interpreted as a continuation of early bonds between nest companions.

Extramarital and Pair Copulations in the Cattle Egret

Abstract The copulatory behavior of the Cattle Egret (Bubulcus ibis) was observed in a heronry in central Japan. In 38 of 147 extramarital copulation (EC) attempts with seven females, the males seemed in ejaculation. There were relatively few complete ECs prior to egg laying, because females responded aggressively to approaching males and because the mates of the females stayed in the territories for 78.8% of the time and protected the females. After egg laying, the aggressiveness of females against approaching males decreased and their mates spent more time outside the territories, resulting in an increase in the occurrence of complete ECs. Fighting males that were approaching a female exhibited a dominance hierarchy. The most dominant male stayed in the territory for the longest time and copulated most intensively with his mate and other neighboring females. Extramarital copulations are likely to occur in colonial herons, but mate-guarding by the male and aggression by the female mate enhance the probability that copulations will be performed only between members of the pair during the fertilizable period of the female. Thus, the monogamous pair bond is maintained.

Sociobiology of Bank Swallows: Reproductive Strategy of the Male

Male bank swallows pursue a mixed reproductive strategy. As previously documented, they form monogamous pair bonds with females with whom they will share parental duties of nest-building, incubation, and feeding of the young. In addition, however, they routinely seek promiscuous copulations with other females, both before and after pair-bonding.

Ecology and Evolution of Social Organization in Arctic Sandpipers

A comparative analysis of sandpiper social systems on arctic and subarctic breeding grounds (24 species in the family Scolopacidae, subfamily Calidridinae) shows four major patterns. In a majority of the species (15), populations are dispersed through a strongly developed territorial system, with strong monogamous pair bonds and only minor yearly fluctuations in numbers. The second pattern is seen in three species in which the female of a pair may lay two sets of eggs in quick succession, one for each member of the pair to incubate. This opens opportunities for facultative polygyny or polyandry (‘serial polygamy’) and for the evolutionary weakening of the strong pair bond seen in the first pattern. The third and fourth patterns are those of polygyny (three species) and promiscuity (three species). These six species show clumped dispersions; their year-to-year fluctuations tend to be strong; the males defend compressible, often small, territories; and high densities can occur locally. It is suggested that the pattern of overdispersion and monogamy represents a conservative mode of adapting to high-latitude environments, while the pattern of clumped dispersion with polygyny or promiscuity represents an opportunistic mode in that the birds are concentrated into breeding areas where and when weather, food, and/or some other environmental factors are particularly favorable. Apparently falling evolutionarily between these two basic patterns are several species conservative in their life-styles, but polygamous at least occasionally and showing some features of opportunism. There is thus a striking diversity of social systems in calidridine sandpipers, that is, in the styles of habitat exploitation they have evolved in the arctic and subarctic habitats to which their breeding is confined. A graphic model suggesting paths of evolutionary development and of interplay among factors considered critical in the evolution of these systems is proposed.