摘要:

Summary1. The high percentage of fat, about 26–28% in the mature human female, may influence reproductive ability directly through two mechanisms: (a) fat converts androgens to oestrogens; (b) relative fatness influences the direction of metabolism of oestrogen to the most potent or least potent forms. The relative degree of fatness thus is directly related to both the quantity of circulating oestrogen and the biological effectiveness of the oestrogen. This is a neat mechanism for relating rates of growth, nutrition and energy outputs to the energy requirements for reproduction. Fat is the most labile body tissue; it therefore reflects environmental changes more rapidly than other tissues of the body.2. The slow maturation of the hypothalamus and pituitary up to menarche, or first oestrus, is accompanied by a slow maturation of the body, which changes not only in size but in the relative proportion of bone, muscle, and fat. Evidence is presented that a particular threshold ratio of fat to lean mass is normally necessary for puberty and the maintenance of female reproductive ability in the human and in the rat. The synchronizing mechanisms may be metabolic, relating food intake to core temperature and/or fat storage.3. Undernutrition and weight loss in the range of 10–15 % of normal weight for height delays menarche and causes amenorrhoea. Ballet dancers and athletes also have delayed menarche and amenorrhoea. The cessation of reproduction in both groups can be considered adaptive. Refeeding and/or cessation of intense activity results in the initiation or resumption of menstrual cycles after varying periods of time. A necessary threshold weight for height can be predicted from a fatness index.4. Differences in the natural fertility of historical and contemporary populations may be explained by a direct effect of food intake and energy outputs on fecundity.5. The late maturing fat ‐ ‘sex fat’ ‐ may have a special role in r

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00406.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

SummaryIt is proposed that the incorporation of a unique parasitic stage in the life‐cycle of unionaceans which involves an obligate relationship between a vertebrate host, usually a fish, and a highly modified larval stage, the glochidium, has had far‐reaching consequences with respect to overall morphology, extent of species' geographic ranges, and rate of speciation in the group.Glochidia are separable into three main types with respect to overall shape and attachment features, and are retained in variously modified brood pouches. When mature, glochidia are released in several different ways which reflect various adaptations involved in either attracting the fish host and/or increasing the probability of attachment. Glochidia do not seem capable of host selection, and the reaction of the host to the parasite seems to be the main factor in determining specificity. Release of glochidia is synchronized to correspond to periods of predictable host availability, such as during spawing migrations and nesting behaviour. Other adaptations include modifications of glochidial conglutinates to mimic host food items, and modifications of the unionacean mantle edges to attract hosts. In all cases, a good correlation exists between the type of lure used and host food preferences, but, despite these adaptations, host specificity among unionaceans seems low.Parasitism among unionaceans is postulated to be mainly advantageous in terms of predictability of dispersal by habitat‐specific hosts, but parasitism is hypothesized to entail constraints in terms of the degree to which shell shape and life‐habit can be diversified among unionaceans. The type of host parasitized is considered to affect the rate of diversification among populations and speciation among unionaceans: those that parasitize strictly freshwater hosts are more likely to exhibit highly individualistic populations in different drainages with respect to molecular genetic and soft‐part characters, while those that parasitize anadromous or saltwater‐tolerant hosts show little differentiation among widely distributed

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00407.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

Summary1. Cockroaches are ubiquitous in most habitats where insects occur. Although most reports on cockroaches are physiological in nature, sufficient information is available to indicate that forest, desert, and cave‐dwelling cockroaches select microhabitats on the basis of finely resolved environmental preferences. This is particularly true for oviparous females which select specific substrates for oviposition and embryogenesis. Selection and diel movements between microhabitats are related to diel changes in micrometeorological profiles and predation, feeding, and enhancement of sexual communication.2. With some exceptions oviparous species live in wooded habitats; ovoviviparous species tend to occur in protected environments such as caves and logs. Oviparous species are exposed to greater predation, parasitism, and environmental pressures during embryogenesis than are ovoviviparous species, where internal incubation and some parental care reduce these risks. Most ovoviviparous species produce larger clutches, but the interval between broods is significantly longer than in oviparous species. Long gestation, clumping of food resources, and relatively little movement probably selected for male control of resources as a mate‐attraction tactic in ovoviviparous species; agonistic interactions, and in some cases morphological specializations for fighting, and highly ritualized behaviours are common. In most oviparous species, volatile pheromone communication and resource‐based aggregations are common. Rapid ovarian cycles and patchily distributed nutritional resources result in the need for greater mobility, and hence adults encounter greater

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00408.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

摘要:

Summary(1) Catch connective tissue is defined as the collagenous connective tissue whose mechanical properties can be changed rapidly (in seconds or minutes) under nervous control.(2) Catch connective tissues are found in all five classes of Echinodermata. They function in tone control of the tissues and in autotomy.(3) The change in mechanical properties occurs in viscosity.(4) Muscle cells are not responsible for the viscosity change.(5) The viscosity change is controlled by nervous activities. Neurosecretory‐like cells with large electron‐dense granules are found in all the catch connective tissues so far studied.(6) The viscosity change is quite likely caused by the change in the ionic environment in the connective tissues, which alters the weak (non‐covalent) interactions between extracellular macromolecules in the t

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00409.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1984.tb00410.x

出版商:Blackwell Publishing Ltd    

年代:1984

数据来源: WILEY