NOTE Chaloupka and Zug: A polyphasic growth function for Lepidochelys kempii 
855 
Meanwhile, it is common practice to use somatic 
growth functions to estimate mean age at sexual 
maturity. The difficulty in using growth functions for 
this purpose is that there are no conclusive growth 
criteria to indicate onset of sexual maturity. Mini- 
mum or mean female nesting size, or an arbitrary 
size set slightly below mean nesting size, is a com- 
monly used criterion. Using an arbitrary size crite- 
rion based on reasonable biological considerations, 
Caillouet et al., (1995b) proposed that head-started 
Kemp’s ridley sea turtles irrespective of sex, took 10 
years to reach sexual maturity at ca. 60 cm SCL. 
Zug et al. (1997) estimated 11-16 years for age-at- 
maturity for wild stock Kemp’s ridley sea turtles on 
the basis of mean female nesting size. The upper 
asymptote of a parametric growth function is the 
correct estimate of mean adult size, if the correct 
growth function was used (see Ricker, 1979). By us- 
ing the upper asymptote metric, it is then apparent 
that sexual maturity could be reached at >20 years 
of age for the current study (see Fig. 2B; Table 2) 
compared with 30 years of age for the Caillouet.et al. 
(1995b) growth function (see Table 2). 
But as Caillouet et al., (1995b) point out, mean 
adult size (nesting females) is a questionable crite- 
rion for estimating age at sexual maturity. That is 
why Caillouet et al. (1995b) defined an arbitrary size 
criterion to estimate age-at-maturity. However, the 
correct function for estimating mean age at matu- 
rity is an age-specific maturity-rate function condi- 
tioned on time-varying age, year, and cohort effects 
derived from a mixed longitudinal sampling study 
(see Chaloupka and Musick, 1997, for time-depen- 
dent demographic sampling issues). In the absence 
of such a complex function, a useful growth criterion 
for estimating age-at-maturity might be negligible 
growth derived from the age-specific growth-rate 
function indicating the onset of maturity. It is increas- 
ingly apparent that growth for sea turtles becomes 
negligible approaching the onset of sexual maturity 
(see Chaloupka and Limpus, 1997). Although this is 
a study-dependent and subjective metric, it is clear 
that negligible growth in the current study occurs > 
15 years of age or < 0.25 cm SCL/year (see Fig. 2B 
and Table 2). Growth was imperceptible by 21 years 
of age (Table 2); thus the age range of 15-20 years 
appears to be a reasonable interval estimate of ex- 
pected age at sexual maturity for the Kemp’s ridley 
sea turtle. 
It is therefore noteworthy that the Kemp’s ridley 
sea turtle tagged in Chesapeake Bay as a juvenile 
(ca. 49 cm SCL) and discovered nesting seven years 
later at Rancho Nuevo (see “Discussion” above) was 
estimated by reference to Figure 2 to be about 9 years 
old when tagged and therefore 16 years old at the 
first recorded nesting. The nesting turtle was ca. 67 
cm SCL, which is larger than the estimated upper 
asymptote of ca. 63 cm SCL (Table 1: 2(a 1 +a 2 ) cm 
SCL). Recall, however, that the upper asymptote here 
represents mean adult size (or mean nesting size, as- 
suming growth is not sex-specific); therefore 50% of a 
random sample of adult or nesting Kemp’s ridley sea 
turtles would be >63 cm SCL , whereas 50% of the 
sample would be smaller. 
Despite sampling design constraints, cautions 
about skeletochronological methods, small sample 
size, and perhaps nonequivalent geographic sub- 
samples, the data set presented in Zug et al. (1997) 
is of considerable importance for helping to improve 
our understanding of the growth dynamics of the en- 
dangered Kemp’s ridley sea turtle. The re-analysis 
of these data with exploratory nonparametric 
smoothing suggested that expected age-specific 
growth for the Kemp’s ridley sea turtle was 
polyphasic and could be modeled with a sequence of 
parametric curves. A parametric model comprising 
a summation of logistic functions fitted the data well, 
implying growth spurts at >1 year of age (mean 
size=13 cm SCL) and ca. 8 years of age (mean size=46 
cm SCL) followed by negligible growth approaching 
the onset of maturity ca. 15-20 years of age (mean 
size=63 cm SCL). Polyphasic growth behavior is 
therefore one of many reasons why a monophasic 
growth function cannot fit the entire postnatal de- 
velopmental phase of the Kemp’s ridley sea turtle, 
let alone for any other sea turtle species (see 
Chaloupka and Musick, 1997). 
Acknowledgments 
We thank members of the USA sea turtle stranding 
network for collection of skeletal material and 
Heather Kalb and Stephen Luzar for processing 
skeletochronological data. We are grateful for the 
Smithsonian Research Opportunities Fund that pro- 
vided support of the skeletochronological work be- 
ing undertaken by George Zug. We thank Charles 
Caillouet, Col Limpus, Jack Musick, and the anony- 
mous referees for helpful comments on the manuscript. 
Literature cited 
Becker, R. A., J. M. Chambers, and A. R. Wilks. 
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Burke, V. J., S. J. Morreale, and E. A. Standora. 
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