RT10: Variation of the rate of postembryonic development

¹Institute of Biology, University of Bialystok. 15-950 Bialystok, PO Box 109, Poland. e-mail marekk@cksr.ac.bialystok.pl; ²Institute of Zoology and Evolutionary Biology, Friedrich-Schiller-University, Jena, Germany, e-mail starck@pan.zoo.uni-jena.de

To date, postembryonic growth rate, typically measured as changes in chick’s body mass/size has attracted a considerable attention. In contrast, little is known about the significance of variation of the rate of development quantified as changes in the form and function of internal organs, or changes at the tissue level. During our discussion, Clas Lilja (Division of Biology and Chemistry Växjö University, Sweden) presented data showing that the main change, which characterized early stages of postembryonic development in quails selected for high growth rates consisted of an increased relative size of the digestive organs and a lowered early growth rate of pectorals and feathers. This suggests that the digestive organs in general function at an almost maximum rate and that a useful increase in digestive capacity can only be accomplished by enlargement of the digestive tract. However, Lilja admitted that a major obstacle to the precise physiological interpretation of these results is the structural and functional inhomogeneity of the tissues concerned. He presented morphometric data on growth of quails selected for high growth rate. The results show that the need for higher food uptake is met by a relatively shorter intestines with a larger contribution of mucosal volume per unit of intestinal length and serosal area, and, during early stages, also in relation to the volume of muscularis. In conclusion, these findings suggest that an increase in gut function is accomplished by enlargement of the relative size of the mucosal compartment, rather than by a change in the net efficiency per unit of mucosal tissue.

Another constraint on growth rates is a basic trade-off determined by a mutual exclusion of cell division and cell differentiation. J.M. Starck and R.E. Ricklefs (Department of Biology, University of Missouri-St. Louis) argued that avian altricial-precocial spectrum is a tempting 'system' to test such hypothesis of a growth-rate - functional maturity trade-off; with (on average) fast growing but immature altricial birds and slow growing, but rather independent and mature precocial chicks. Evidences suggest that there is indeed such a trade-off, but some tissues and organs circumvent the trade-off by specific localisations of growth zones and functional zones in tissues (e.g., intestine, brain, liver). Tissue partitioning and specific localisations of tissue functions seem to be important mechanisms that allow growing birds to escape the trade-off between cell division and cell function. Starck has presented the data on the intestinal and skeletal growth supporting the idea that tissue partitioning might be an effective mechanism to escape the 'growth trap'.

However, changes of the internal organs does not need to be correlated with the variation in metabolic rate. Claus Bech (Department of Zoology, Norwegian University of Science and Technology, Trodhiem, Norway) has presented data on the relationship between the development of both the resting (RMR) and cold-induced peak metabolic rates and body composition in two species: altricial European Shag Phalacrocorax aristotelis and precocial Pekin Duck Anas platyrhynchos. In both species he found a typical biphasic development of RMR, which maximum values exceeded those, expected for a similar sized adult bird. At the ages when RMR was highest (15 day in the shags and 5 days in the ducks) he analysed the relationships between body composition and the resting metabolic rate. In both species, the RMR of the chicks was highly correlated with liver mass, but not with the masses of kidney and heart.

Most studies on the variation in avian growth rates have used broad interspecific comparisons. Due to their comparative nature, however, their results are often equivocal. Recently, experimental manipulation is called upon as a prerequisite for the conclusive analysis of variation in life-history traits, including growth rates. Artificial selection of strains of domestic fowl seems to be particularly relevant in this regard, because high incidence of metabolic diseases in modern broilers suggests that their growth rates are approaching physiological limits. To assess the effects of food limitation under standardised conditions G. Henk Visser and Carolien van der Ziel (Zoological Laboratory and Centre for Isotope Research, Nijenborgh The Netherlands) have performed an experiment in chicks of fast- and slow-growing lineages of the Japanese quail. In each line, three different groups were formed with feeding levels of 100%, 70%, and 40% of the pre-determined age-specific and strain-specific ad libitum levels. In both lines growth retardation of the structural components occurred during the first weeks of the experiment, especially with respect to the growth of the tarsus and head (40% reduction), and to a lesser extent the wing (25% reduction). In the food restriction groups growth of these components lasted longer, and the differences between the groups have almost disappeared at the end of the experiment. Chicks of the 100% groups of both strains achieved homeothermy at about eight days of age, whereas those of the 40% groups of both strains achieved homeothermy at about 15-20 days of age. RMR at thermoneutrality differed strongly between the three groups of both strains, when related to the age of the chicks, but these differences almost completely disappeared when corrected for body mass. In conclusion, chicks of both strains exhibited a remarkable developmental plasticity. It appears that selection for high postnatal growth rates has not affected the developmental plasticity of the chick. All responses to food restriction have resulted in remarkably high survival rates.

As Hubert Schwabl (Center of Reproductive Biology, Department of Zoology, Washington State University, Pullman) pointed out, variation in postembryonic development is also related to changes in hormonal levels. He measured circulating corticosterone levels in nestling and fledgling canaries reared by their parents. Levels were very low five days post-hatching. At the age of ten days they increased significantly to the levels that corresponded to adult baseline levels. Levels remained constant until 15 days of age, one to two days prior to fledging. In 23-day-old fledglings (about five days after fledging) the levels were significantly higher than those of 15 day old nestlings. While this general profile was independent of sex and brood size, the order of hatching influenced corticosterone levels. During the late nestling phase and after fledging the first-hatched chicks of a brood had higher levels than last-hatched chicks; second-hatched chicks had intermediate levels. Levels varied also among broods. Neither the differences among broods nor those among siblings within a brood were correlated with body mass. The results suggest (1) that the brain-pituitary-adrenal axis of this altricial bird becomes functional between five and ten days after hatching and (2) that environmental influences such as sibling rivalry, dominance relationships, parent offspring interactions, or maternal effects modify corticosterone levels during development. Such differences in corticosterone levels during early life may influence behavioural differentiation and may have consequences for fitness.

An infectious challenge is another, significant factor influencing the rate and composition of growth. Data discussed by Kirk C. Klasing, (Department of Avian Sciences, University of California, Davis) indicated that the immune response to pathogens results in the release of pro-inflammatory cytokines (e.g. interleukin-1, IL-1, and tumour necrosis factor, TNF). These mediators cause a stress response consisting of specific behavioural, cellular, and metabolic changes that alter the partitioning of nutrients away from growth and toward processes that support the immune and inflammatory responses. They act directly on most tissues of the body through specific receptors, and they induce distinct endocrine changes that further impair growth, including increased glucocorticoids and decreased insulin like growth factor-1. Anorexia induced by pro-inflammatory cytokines accounts for 70% of the growth reduction in young chicks with the remainder due to tissue-specific changes in metabolism. In chicken skeletal muscle, IL-1 and TNF act synergistically to increase the rate of protein degradation. In vivo, IL-1 induces the release of corticosterone that decreases the rate of skeletal muscle protein synthesis and further slows skeletal muscle accretion. Conversely, pro-inflammatory cytokines and corticosterone elevate liver protein synthesis due to increased secretion of acute phase proteins. Thus, an inflammatory response orchestrates a change in both the rate and the composition of growth in a manner that varies directly with the amount of pro-inflammatory cytokines released.

Helga Gwinner (MPIV für Verhaltenphysiologie, Andechs, Germany) showed how birds can counteract an infectious challenge using green plants as nest material, possibly to control parasite load of the nests (‘nest protection hypothesis’). She investigated this hypothesis by exchanging 148 starling nests with artificial ones, half of which contained herbs. The presence of herbs in the nest material did not influence the levels of parasitic load. However, nestlings from nests with herbs were heavier and had higher hematocrit levels and higher numbers of basophils. This suggests a direct, positive effect of herbs on the chicks' immune system.