MEAD ET AL.: OBSERVATIONS ON MASS STRANDING OF SPINNER DOLPHIN 



sured near the surface of the testis and it was 

 noted that tubule diameter averaged about 10% 

 less at that level in the mature males. The process 

 of selection of the tubules for measurement may 

 have introduced a slight bias in favor of smaller 

 tubules, as these are possibly less likely to have 

 been affected by decomposition artifacts. Much of 

 the variation in tubule diameter within an indi- 

 vidual slide may have been the result of autolytic 

 distortion, which would tend to increase the 

 diameter of the tubules. 



There is a sharp increase in the size of the testes 

 of animals with length of 188 or 189 cm (Table 2), 

 which apparently is the size range at which mat- 

 uration of the testes begins. Spermatogenesis was 

 taking place in the testes of the 189 cm individual, 

 but the testes weights were still low relative to 

 those of fully mature animals and no sperm was 

 present in the epididymis. In the next largest ani- 

 mal (190 cm), the testes were slightly larger and 

 sperm was present in the epididymis, indicating 

 that this animal was functionally sexually ma- 

 ture. All of the animals above 190 cm had large, 

 active testes and were sexually mature, with the 

 exception of a single 203 cm individual (504452), 

 whose testes were markedly small, though there 

 was a slight indication of spermatogenesis. The 

 body weight of this animal was also low for its 

 length, and it is probable that it was an abnormal 

 individual. 



Although the sample of males was too small to 

 statistically define sexual maturity, it seems 

 likely that maturation begins around a body 

 length of about 190 cm and a weight of about 60 kg, 

 and maturity is reached at a length of about 192 

 cm and a weight of about 65 kg. Animals with a 

 seminiferous tubule diameter of less than about 

 150 fjLm were immature or maturing, and those 

 with a diameter in excess of this were sexually 

 mature. The corresponding figures for testis 

 weight and length were about 300 g and 24 cm. 

 The sample of males with the pulp cavity open in 

 the teeth consists of only four specimens. One of 

 these did not have well-defined growth layers, 

 leaving only three usable individuals. These are 

 an immature animal with 7 growth layer groups 

 and two mature animals with 10 groups. 



Perrin et al. ( 1977) found a mean length at sex- 

 ual maturity of about 175-180 cm (the middle of 

 several estimates based on different criteria, and 

 the estimate which is most comparable with that 

 applied to the present sample) and a mean age at 

 sexual maturity of about 10-12 groups in the east- 



ern spinner dolphin. As was seen when comparing 

 the sexual maturity figures for females from the 

 two populations, the eastern spinner reaches 

 maturity at a shorter length than our sample from 

 the Gulf of Mexico. In the case of males, the ages at 

 attainment of sexual maturity are more similar 

 and the length difference is probably due to popu- 

 lation differences in mean size of individuals. 



PRODUCTIVE SEASONALITY 



Of the six mature females in this sample, one 

 (504456) was lactating and one had a large corpus 

 luteum with no visible conceptus. Both of these 

 had probably given birth recently. None of the six 

 were pregnant. Six of the seven mature males 

 were examined for presence of sperm in the 

 epididymis. Sperm was present in all six and was 

 judged to be copious in three. Admittedly, this is a 

 very small sample, but it is indicative of recent 

 calving and breeding activity. 



Perhaps the most convincing evidence for recent 

 reproductive activity in this sample are the three 

 calves which were present, with lengths of 90, 91, 

 and 97 cm. Perrin et al. ( 1977) estimated length at 

 birth in the eastern spinner to be 75.5 cm. Since 

 the mean lengths of mature animals and the mean 

 lengths at attainment of sexual maturity in the 

 Florida sample are uniformly about 14*7^^ greater 

 than the corresponding figures for the eastern 

 spinner dolphin, it is logical to assume, for an 

 initial approximation, that length at birth would 

 also be about 14% greater, or about 86 cm. Perrin 

 et al. ( 1977) estimated the postnatal growth rate 

 in the first 10 or 11 mo after birth to be 4.77 

 cm/mo. Again, allowing a difference of 14% for the 

 larger mean size in the Florida sample, a usable 

 estimate of the growth rate during this period 

 would be 5.4 cm/mo. This provides projected ages 

 for the two smaller calves of about 1 mo old, and for 

 the larger of about 2 mo, with birth dates of mid- 

 June and mid-May. 



The only other data available for spinner dol- 

 phins in the Gulf of Mexico are the 8.1 cm fetus 

 which Layne (1965) found in an animal which 

 stranded in mid-September and the 61 cm fetus 

 which Schmidley and Shane (see footnote 5) found 

 in early March. Using the fetal growth curve for 

 the eastern spinner (Perrin et al. 1977), and as- 

 suming that the mean size difference between the 

 populations would not be significant for small 

 fetuses, the approximate date of conception for the 



357 



