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 for adult homeotherms, which states that, within a taxonomically or functionally defined category such as mammals or birds, the number of heart beats throughout the life span (sL) is more or less constant. This rule stems from the allometric relationships between heart rate (fH) and body mass (mB) and between sL and mB. As a step towards understanding the general allometric nature of avian embryonic physiology we analyzed the fH values of avian embryos in relation to their incubation span (sI). Data from 30 species were selected from the scientific literature for the analyses. Values obtained from invasive methods which were judged to grossly alter natural incubation conditions, or from undefined or unmatched temperature conditions were not used. These include most values obtained below the first 30% of the incubation. Also, data obtained after internal pipping were discarded since hatching activity influences them. Values for sI and egg mass (mE) as representatives of embryonic mass were also collected. Embryonic fH was normalized to 70.1-80% sI. At 20.1-30% sI it was only 85% of the value at 70.1-80% sI and increased to a plateau at about 50.1-60% sI. It was almost constant among species between 50.1 and 60% sI and pre-internal pipping (PIP) time and thus, the mean fH value between 50.1 and 60% sI and between 90.1 and 100% excluding pipped eggs (fH) was taken as a representative value for each given species. The fH (min-1) and the corresponding sI (days) values for the 30 species, scaled with mE (g) as follows: fH = 371.1.mE-0.112 and: sI = 12.29.mE+0.209. Both powers were significantly different from 0. The product of fH and sI (fH.sI), representing the total number of heartbeats throughout the incubation, scaled with mE for the entire data set as follows: fH.sI = 6.565 x 10(+6).mE+0.096, where the +0.096 power is significantly different from 0. Values for fH.sI from embryos of altricial birds tended to concentrate at the low mE end of the plot while those of the precocial ones tended towards the high end. Separate analyses showed that the mE power for the combined altricial and semi-altricial species (ASA), and the combined precocial and semi precocial species (PSP), of log fH.sI against log mE regressions, were both insignificantly different from 0. Thus, means of fH.sI for ASA and PSP were calculated. The mean ASA value of 7.27 x 10(+6) heartbeats for fH.sI, was significantly different from the mean PSP value of 10.93 x 10(+6). The difference of 3.66 x 10(-6) (33.5%) heartbeats can be attributed to either the more advanced stage of the PSP hatchlings at hatch, to the larger mE values of these hatchlings, to the difference in water fraction of the hatchlings or all. The result of a linear regression of fH.sI against the rate of sI completion (the inverse of incubation span, fI; day-1) was: fH.10(-6) = 0.205 + 3.940.sI-1. Thus, the faster is the average rate of development accomplished per day (shorter incubation) the higher is daily heart rate. Data tended to cluster such that large eggs, mostly of the PSP type with relatively low fH, complete 2-4% of their incubation per day, while small, ASA type eggs with relatively high fH, complete 6-8% of their incubation time per day. We conclude that, at this stage of knowledge, the data is insufficient to resolve whether the different modes of hatch stage alone can explain differences in the total number of heartbeats throughout embryonic life among all bird species, or egg mass and water content differences contribute variability. This should be investigated on a larger sample of species in more depth.