PERRIN ET AL.: GROWTH AND REPRODUCTION OF SPOTTED PORPOISE 



16- 





X 10 

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2 4 6 8 10 12 14 16 18 



LAYERS (no) 



Figure 22. — Relationship between weight of ovaries and 

 number of postnatal dentinal layers in Stenella attenuata. 

 Open dots are females with a corpus luteum. 



average total length of about 170 to 180 cm and 

 weight of 50 to 60 kg (Figure 23). 



Analysis of lengths of females with and without 

 ovarian scars yields a more precise estimate of 

 length at attainment of sexual maturity. The 

 smallest of 1,410 specimens (160 cm long or 

 longer) that possessed scars were two that were 

 167 cm long (one with a corpus luteum only and 

 one wdth a corpus luteum and four corpora al- 

 bicantia). The largest female with no scars was 

 193 cm long. The length-maturity curve is 

 slightly asymmetrical, but a linear regression 

 line through the nearly linear central portion (M 

 = 5.76L - 960.95) estimates that average length 

 at which scarring is first evident is 175.4 cm. This 

 analysis probably underestimates length at at- 

 tainment of maturity, because some of the small 

 adults (170 to 180 cm) with many scars are those 

 that have stopped growing at a shorter-than- 

 average length. In other words, the left-hand por- 

 tion of the frequency distribution of physically 

 mature adults to an unknown extent artificially 

 elevates the central portion of the length- 

 maturity curve, making it asymmetrical. 



An estimate of age and length at attainment of 

 sexual maturity can also be derived directly from 

 the smaller sample of females for which the 

 number of tooth layers was determined. The 

 youngest specimen exhibiting ovarian scarring 



had 7.5 layers. The oldest with no scarring had 12 

 layers. The estimated age at which 50% have 

 scars is 9.14 layers (M = 19.5^ - 128.25). Pre- 

 dicted length at this age is 181.6 cm (based on 

 growth equation above). This estimate is less 

 biased than the others above but based on much 

 smaller samples, especially at the lower end of 

 the layer-maturity curve. 



Another estimate of length and age at first ovu- 

 lation can be made by back extrapolation of a 

 relationship between body length and number of 

 corpora (including corpora lutea) in the ovaries 

 (Figure 24). Length increases with corpora count 

 until at least six to eight corpora have been ac- 

 cumulated, at about 183 to 190 cm. A fit of the 

 data to the Laird growth model (above) yields the 

 equation 



L = 180.17 cm exp{0.0541[l - exp(-0.2815C)]}, 



where L = length in centimeters 

 C = number of corpora. 



Back extrapolation of the curve to zero corpora 

 yields an estimate of 180.2 cm. Predicted age from 

 the growth equation is 8.74 layers. 



An estimate of length at first conception can be 

 made by calculating the average length of preg- 

 nant females with a corpus luteum only (indicat- 

 ing first pregnancy) and subtracting the growth 

 that they can be assumed to have undergone dur- 

 ing pregnancy. Fifty-four primiparous females 

 averaged 181.7 cm in length (range 167 to 193 cm). 

 Predicted age at that length is 9.17 layers. The 

 average length of their fetuses was 372 mm. This 

 length is attained by about the beginning of the 

 sixth month of gestation. Using the growth equa- 

 tions above to predict growth during 6 mo for the 

 various tooth-layering models and substracting 

 the growth increment from 181.7 cm yields esti- 

 mates of length at first conception ranging from 

 177.7 to 180.0 cm (number 4 in Table 2). The 

 primiparous females in this sample, however, are 

 only those that became pregnant at the first ovu- 

 lation. This may cause the estimate to be an 

 underestimate, because many females ovulate 

 several times, and presumably continue to grow, 

 before becoming pregnant the first time (see 

 Ovarian Changes below). 



The various methods of estimating age and 

 length at attainment of sexual maturity yield es- 

 timates of varying accuracy (Table 2). The esti- 

 mates based on tooth layers and length at first 



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