Analysis of heart rate in developing bird embryos: effects of developmental mode and mass

Bird embryos may be regarded as developing in their thermo-neutral zone, at rest, and stay in the egg for a fixed period of time until hatching. It is therefore interesting to investigate if they follow the same 'rule' set fo r adult homeotherms, which states that, within a taxonomically or functiona lly defined category such as mammals or birds, the number of heart beats th roughout the life span (s(L)) is more or less constant. This rule stems fro m the allometric relationships between heart rate (f(H)) and body mass (m(B )) and between s(L) and m(B). As a step towards understanding the general a llometric nature of avian embryonic physiology we analyzed the f(H) values of avian embryos in relation to their incubation span (s(I)). Data from 30 species were selected from the scientific literature for the analyses. Valu es obtained from invasive methods which were judged to grossly alter natura l incubation conditions, or from undefined or unmatched temperature conditi ons were not used. These include most values obtained below the first 30% o f the incubation. Also, data obtained after internal pipping were discarded since hatching activity influences them. Values for s(I) and egg mass (m(E )) as representatives of embryonic mass were also collected. Embryonic f(H) was normalized to 70.1-80% s(I). At 20.1-30% s(I) it was only 85% of the v alue at 70.1-80% s(I) and increased to a plateau at about 50.1-60% s(I). It was almost constant among species between 50.1 and 60% s(I) and pre-intern al pipping (PIP) time and thus, the mean f(H) value between 50.1 and 60% s( I) and between 90.1 and 100% excluding pipped eggs ((f) over bar(H)) was ta ken as a representative value for each given species. The (f) over bar(H) ( min(-1)) and the corresponding s(I) (days) values for the 30 species, scale d with m(E) (g) as follows: (f) over bar(H) = 371.1.m(E)(-0.112) and: s(I) = 12.29.m(E)(+0.209). Both powers were significantly different from 0. The product of (f) over bar(H) and s(I) ((f) over bar(H).s(I)), representing th e total number of heartbeats throughout the incubation, scaled with m(E) fo r the entire data set as follows: (f) over bar(H).s(I) = 6.565.10(+6).m(E)( +0.096), where the +0.096 power is significantly different from 0. Values f or (f) over bar(H).s(I) from embryos of altricial birds tended to concentra te at the low rn, end of the plot while those of the precocial ones tended towards the high end. Separate analyses showed that the m(E) power for the combined altricial and semi-altricial species (ASA), and the combined preco cial and semi precocial species (PSP), of log (f) over bar(H).s(I) against log m(E) regressions, were both insignificantly different from 0. Thus, mea ns (f) over bar(H).s(I) for ASA and PSP were calculated. The mean ASA value of 7.27.10(+6) heartbeats for (f) over bar(H).s(I), was significantly diff erent from the mean PSP value of 10.93.10(+6). The difference of 3.66.10(+6 ) (33.5%) heartbeats can be attributed to either the more advanced stage of the PSP hatchlings at hatch, to the larger m(E) values of these hatchlings , to the difference in water fraction of the hatchlings or all. The result of a linear regression of (f) over bar(H). s(I) against the rate of s(I) completion (the inverse of incubation span, f (I); day(-1)) was: (f) over bar(H).10(-6) = 0.205 +/- 3.940.s(I)(-1). Thus, the faster is the average rate of development accomplished per day (shorte r incubation) the higher is daily heart rate. Data tended to cluster such t hat large eggs, mostly of the PSP type with relatively low (f) over bar(H), complete 2-4% of their incubation per day, while small, ASA type eggs with relatively high (f) over bar(H), complete 6-8% of their incubation time pe r day. We conclude that, at this stage of knowledge, the data is insufficie nt to resolve whether the different modes of hatch stage alone can explain differences in the total number of heartbeats throughout embryonic life amo ng all bird species, or egg mass and water content differences contribute v ariability. This should be investigated on a larger sample of species in mo re depth. (C) 1999 Elsevier Science Inc. All rights reserved.