Reproduction of Cnemidophorus inornatus and Cnemidophorus neomexicanus (Sauria, Teiidae) in northern New Mexico. American Museum novitates ; no. 2442

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New York, N.Y. : American Museum of Natural History
"Cnemidophorus inornatus and C. neomexicanus are teiid lizard species adapted for a dry terrestrial existence. The two species have distinct ecological preferences that favor the dissemination of one species at the expense of the other. Cnemidophorus inornatus is found primarily in undisturbed desert grassland but C. neomexicanus is found chiefly in disturbed areas, often those disturbed by man and his animals, and is able to survive under nearly metropolitan conditions. Reproductive isolation results from the specific habitat preferences of the two species. This isolation is reinforced by an offensive attitude by the larger species C. neomexicanus toward the smaller species C. inornatus. This was seen only once in the field but many instances were seen in the laboratory. The two species have similar cycles of activity and hibernation. Adults cease surface activity by the third week of September, but a few juveniles remain active until the last week of September or the first week of October. Both species emerge from hibernation in mid-April. Juveniles are more active than adults during the first weeks in spring following hibernation and are also more active than adults before hibernation begins in the fall. The reproductive cycle of the male of C. inornatus is initiated with the resumption of spermatogenesis in late August or September. Coincidentally, there is an increase in testicular size and weight so that immediately before hibernation these organs weigh about 9 mg., approximately three times the mean August weight. At this time they contain numerous spermatogonia and early spermatocytes. Specimens removed from hibernation in March show greater testicular weights (around 19 mg. heavier) and maximum diameters (about 2 mm. longer) indicating that testicular growth takes place during winter. This growth primarily involves production of spermatogonia and spermatocytes. Maximum testicular weights and lengths are attained in April, the first month of activity. Mean testicular weight in April is approximately 33 mg. and mean testicular length is 5.5 mm. A peak in sperm production is probably reached in May or the first week of June, probably the period of most active mating. Size and weight of testes decrease through June and July as the spermatogenic cycle is completed and the last spermatozoa mature and are eliminated from the testis. Minimum size and weight are attained in late July and early August. Mean length and weight for August testes are the smallest obtained for any month (2.5 mm. and 3 mg. respectively). Cyclic changes in the thickness of the epididymides and vasa deferentia occur parallel to the testicular cycle. These changes appear to result primarily from changes in height of the columnar epithelium lining the lumina and produce an increase in opacity and thickness as the organs prepare to receive the spermatozoa. The fat bodies of males undergo a cycle that is approximately the reverse of that of the testes. The reproductive cycles of females of the two species are nearly identical. These cycles begin in spring (April or May) with the enlargement of up to three ovarian follicles per lizard and follicular enlargement continues until ovulation. The first ovulations occur during the last week of May or the first week of June, therefore permitting approximately 23 days for follicular enlargement. Corpora lutea form shortly after ovulation and remain for at least a day after eggs are laid. It seems likely that these corpora lutea have some secretory function and may be involved with egg retention. Eggs are laid from the first week of June until the third week of July, although the period of most active egg laying occurs in mid-June. Ova are retained in the oviducts for about one week. Approximately 25 per cent of the reproductive females of each species probably produce a second clutch. Many of the remaining lizards begin enlargement of follicles for a second clutch but these follicles become atretic and are never ovulated. Trans-coelomic migration of ova is detectable in about 50 per cent of the lizards that possess oviducal eggs. It seems that this phenomenon helps to keep eggs as equally distributed as possible in the oviducts and may be of selective advantage for reasons of weight distribution. Cyclic changes of oviducal morphology take place parallel to the phases of the ovarian cycle. These changes involve primarily hypertrophy and recession of epithelial layers surrounding the lumina and are associated with great increase in secretory activity in advance of receiving eggs. Follicular enlargement and oviduct hypertrophy correspond with decrease in fat-body size. This association may reflect the large energy drain on the organism at this time to produce oviduct hypertrophy, as well as the high demand for lipids by the enlarging follicles. The first hatchlings appear in the last two weeks of July, and newly hatched lizards continue to be found in the population until the first week in September. The incubation period ranges from 34 to 46 days. Mean clutch size is the same in the two species, approximately 2.13 eggs per female, but ovarian follicles and oviducal eggs are considerably larger in C. neomexicanus than in C. inornatus. It appears that approximately 25 per cent of the reproductive females of both species may lay a second clutch of eggs each year. Assuming that this is true, 266 eggs would be produced by each 100 fertile females each year. A logical estimate of longevity for a typical specimen of either species is approximately four calendar years; possibly five for 25 per cent of the specimens (individuals do not attain reproductive maturity until their third calendar year). Assuming that these estimates are correct, the mean number of eggs produced by each reproductive female would be approximately 5.98 (598 for each 100 fertile females). Because no males exist in populations of C. neomexicanus, the number of eggs produced relative to the total population is approximately double that in populations of C. inornatus. This factor, however, may not benefit the former species directly because viability of eggs of C. neomexicanus may be only about half that of C. inornatus. The reproductive potential of the two species may therefore be effectively the same. The aggressive behavior of C. neomexicanus, its preference for forms of disturbed habitat, which are becoming more and more widespread, and its ability to reproduce in areas where the food supply is too short to sustain two lizards, are probably the major factors responsible for the displacement of C. inornatus populations by this species"--P. 42-45.
48 p. : ill., ; 24 cm.
Includes bibliographical references (p. 45-48).