Energetics of Early Development
Electronic supplementary material: Methods and Statistics
Time to First Heartbeat (TFH)
To the best of our knowledge, we have included all species for which there were data available on
TFH, along with sizes and temperatures at this stage. In a few cases, embryo mass was estimated
based on length-weight regressions, or based on linear dimensions assuming that biomass density is
equal to the density of water. Wet-mass values were converted to dry mass by assuming 90% body
water at TFH (references 20-21 below).
Time to hatching (TH) and time to maturity (TM)
TH data were taken from the same studies as the TFH data, when available, so the TH data
represent a subset of the species used for TFH. We used TH because it was a late stage in
development having the most available data for this group of species. However, TH is not an ideal
late-stage measure because larger species tend to hatch at earlier stages of development (see
reference 19 of Online Supplement 2). The TM data include all the data compiled by Savage et al.
(2003) for heterotrophic eukaryotes (excluding endotherms). These data represent a diverse
assortment of ectothermic invertebrates and vertebrates, and do not include species listed in Table 1.
For the TH and TM data, wet masses of hatchlings and adults were converted to dry masses, when
necessary, by assuming 75% body-water content (Gillooly et al. 2002).
Statistical Analyses
We chose Model I regression, rather than Model II regression, in evaluating the relationship of time
to first heartbeat with size and temperature for three reasons. First, Model I assumes a causal
relationship between the independent and dependent variables such that size and temperature
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Energetics of Early Development
control developmental times (as assumed by our model), and not vice versa. Second, since all of the
body size and temperature values were direct measures taken at the time to first heartbeat, we
assume the error in measuring the independent variables of these plots (i.e. size and temperature) is
negligible relative to variability among taxa in the time to first heartbeat, Third, we therefore
assume that most of the variability about the fitted lines is due to biological factors that operate
through their effects on Em and bo. Model I (and not Model II) regression yields the corrected sizeand temperature-corrected slopes regardless of variability about the fitted lines attributable to
variation among taxa in Em and bo.
With respect to the issue of possible phylogenetic effects, note that our analyses include a
diverse set of taxonomic groups (birds, amphibians, fish, invertebrates), and that species are
generally distantly related even within taxonomic groups. Thus, it is unlikely that a phylogenetic
correction would change the slopes or intercepts of the relationships in Table 1. Still, to address this
issue, we performed ANCOVA analyses on the TFH and TH data, assuming differences in
intercepts among taxonomic groups. For all four models (the two size-corrected models and the two
temperature-corrected models), the slope remains significant (P < 0.05), but the differences in
intercepts among groups are not statistically significant. These findings are consistent with other
recent work indicating that size- and temperature-corrected rates of growth (i.e. bo/Em) are similar
across varied taxonomic groups (Gillooly et al. 2002; Ernest et al. 2003; Savage et al. 2004). Thus,
the chance that these relationships are a spurious result due to differences among groups is very
small. Unfortunately, we had insufficient statistical power to simultaneously test for differences in
slopes and intercepts among groups using ANCOVA.
Ernest, S. K. M., Enquist, B. J., Brown, J. H., Charnov, E. L., Gillooly, J. F., Savage, V. M., White,
E. P., Smith, F. A., Hadly, E. A., Haskell, J. P., Lyons, S. K., Maurer, B. A., Niklas, K. J. &
Tiffney, B. 2003. Thermodynamic and metabolic effects on the scaling of production and
population energy use. Ecology Letters 6, 990-995.
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Energetics of Early Development
Gillooly, J. F., Charnov, E. L., West, G. B., Savage, V. M. & Brown, J. H. 2002 Effects of size and
temperature on developmental time. Nature 417, 70-73.
Savage, V. M., Gillooly, J. F., Brown, J. H., West, G. B. & Charnov, E. L. 2004 Effects of body
size and temperature on population growth. The American Naturalist 163, 429-441.
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Energetics of Early Development
Data and sources from Figure 1
Table 2. Data compiled on times to first heartbeat (TFH) and times to hatching (TH) for diverse taxa.
The term “NA” is used to indicate no data available.
Taxon
fish
fish
fish
fish
fish
fish
amphibian
amphibian
amphibian
amphibian
amphibian
invertebrate
invertebrate
invertebrate
bird
bird
bird
bird
Species
Clupea harengus
Phoxinus phoxinus
Raja erinacea
Salmo gairdneri
Salmo gairdneri
Salmo gairdneri
Ascaphus truei
Bryobatrachus nimbus
Bryobatrachus nimbus
Eleutherodactylus coqui
Heleioporus eyrei
Artemia franciscana
Daphnia magna
Procambarus clarkia
Coturnix chinensis
Gallus gallus
Larus schistisagus
Tyto alba
Temperature (°C)
12
25
17
10
7
13
15
10
15
25
21
20
20
25
38
38
38
37
Time to first Heartbeat
mass at TFH (g dry)
time (h)
1.00x10-4
97
1.70x10-4
48
-3
2.00x10
648
1.21x10-3
360
1.17x10-3
535
9.62x10-4
240
2.34x10-2
276
1.20x10-3
969
1.20x10-3
569
3.00x10-4
151
1.40x10-3
172
1.34x10-5
63
4.74x10-6
70
-4
4.40x10
312
1.00x10-4
48
2.00x10-3
48
7.00x10-3
58
4.20x10-1
168
4
Time to Hatching
mass at TH (g dry)
1.10x10-4
5.00x10-4
1.09x100
7.50x10-3
7.50x10-3
6.80x10-3
2.13x10-2
1.92x10-3
1.92x10-3
5.73x10-3
1.90x10-2
NA
1.10x10-5
1.25x10-3
9.84x10-1
8.55x100
2.08x101
3.70x100
time (h)
168.0
52.0
3456.0
592.8
NA
520.8
432.0
1809.6
1060.8
NA
240.0
NA
76.1
528.0
372.0
504.0
648.0
480.0
Source
1,2
3
4
5,6
5,6
5,6
7
8
8
9, 10
11
12
12
13
14
15
16
17,18
Energetics of Early Development
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