The cardiovascular hemodynamic responses to various levels of orthostatic stress in children

The ability of the cardiovascular system to quickly and efficiently adapt to an orthostatic stress is vital for the human body to function on earth. The way in which the various aspects of the cardiovascular system work together to counteract an orthostatic stress has been previously quantified in the adult population. However, there are still many unknowns surrounding the topic of how the cardiovascular system functions to cope with this same stress in children. The purpose of this study was to describe the cardiovascular hemodynamic adaptations to various levels of orthostatic stress induced using a lower body negative pressure (LBNP) chamber in pre-pubertal boys. A secondary purpose was to determine indices of baroreceptor sensitivity (BRS) at both rest and during low levels of LBNP in this same pediatric sample. Finally, this study aimed to compare the relative responses to LBNP between the children and adults. To complete the study 20 healthy pre-pubertal boys and adult males (9.3 ± 1.1 and 23 ± 1.8 years of age respectively) were recruited and randomly exposed to three levels of LBNP (15, 20 and 25 mmHg). At rest and during the application of the LBNP heart rate (HR), manual and bcat-by-beat systolic (SBP), diastolic (DBP) and mean arterial blood pressure (MAP) were monitored continuously. Aortic diameter was measured at rest and peak aortic blood velocity (PV) was recorded continuously for at least I minute during each baseline and LBNP condition. From the raw data HR, stroke volume (SV), cardiac output (Q), total peripheral resistance (TPR), low frequency baroreceptor sensitivity (LF BRS), high frequency baroreceptor sensitivity (HF BRS) and LFIIIF ratio were calculated. At rest, llR wa'i higher and SBP, SV, Q and LF/HF ratio were lower in the children compared to the adult males (pgJ.05). In response to the increasing LEN!> IIR and TPR increased, and LF BRS. SV and Q decreased in the adult group (pSf).05). while the same levels of LBNP caused an increase in TPR and a decrease in SBP, SV and Q in the children (pSf).05). Although not significant, the LF/HF ratio in the adult group showed an increasing trend in response to increased negative pressure (p=O.088). As for resting BRS, there were no significant differences in LF or HF BRS between the children and the adults despite a tendency for both measures to be 18% lower in the children. Also the LF/HF ratio was almost significantly greater in the adults compared to the children (p=O.057). In addition, a comparison between the relative adult and child responses to LBNP yielded no significant group by level interactions. This result should be taken with caution though, as the low sample size and high measurement variability generated very low statistical power for this analysis. In conclusion, the results of this study suggest that the hemodynamic adaptations to an orthostatic stress were less pronounced in the prepubertal males, most likely due to an underdeveloped autonomic system. These results need to be strengthened by further research before any implications can be derived for health care purposes.

Mating behaviours of the field cricket (Gryllus integer) at different levels of male density

The reproductive behaviour of the field cricket, Gryllus integer, was systematically observed in indoor arenas to determine the extent of female Choice and male-male competition at different sex ratios representing two male densities (12:6 and 6:6). The costs and benefits to males and females in those two densities were analyzed according to the theory of the evolution o£ leks. Observations were conducted during the dark hours when most calling occurred since hourly rates of courtship song and mating did not fluctuate significantly over a 24 h period. Female mating rates were not significantly different between densities, therefore males at high densities were not advantaged because of increased female tendencies to mate when social stimulation was increased. Mean rates of acoustical signalling (calling and courtin"g) did not differ significantly between densities. Mean rates of fighting by males at the high density were significantly greater than those of males at the low density. Mating benefits associated with callin~courting and fighting were measured. Mating rates did not vary with rates of calling at either density. Calling was not a prerequisite to mating. Courtship song preceded all matings. There was a significant power fit between male mating and courting rates, and male mating and fighting rates at the low, but not at the high, density. Density differences in the benefits associated with increased courting and fighting may relate, in part, to greater economic defensibility and monopoly of females due to reduced male competition at the low density. Dominant males may be preferentially chosen by females or better able to monopolize mating opportunities than subordinate males. Three criteria were used to determine whether dominant males were preferentially chosen by females. The number of matings by males who won fights (within 30 min of mating) was significantly greater than the number of matings by males who were defeated in such fights. Mating rates did not vary significantly with rates of winning at either density. There was a significant power fit between male mating rates and the percentage of fights a male won (irrespective of his fighting-frequency) at the low density. The mean duration a male guarded the female after mating did not vary significantly between densities. There was a significant linear relationship between the duration a spermatophore was retained and the duration a male guarded the female after mating. Courtship song apparently stimulated spermatophore removal. Male guarding involved inter-male aggression and reduced courtship attempts by other males. Males at the high density received no apparent reproductive benefits associated with increased social stimulation. Conclusive evidence for preferential choice of males by females, using the criteria examined here, is lacking. Males at the lower density had fewer competitors and could monopolize females more effectively.