Theodore H. Fleming

Anolis Cupreus: The Response of a Lizard to Tropical Seasonality

Seasonal changes in the population ecology of Anolis cupreus, a sexually dimorphic lizard, were studied over a 2—yr period in a deciduous and riparian forest ecosystem in western Costa Rica. This tropical environment undergoes strong seasonal changes, including a 6—mo yr dry season during which maximum air temperatures, hours of daily sunshine, and heavy winds occur. Many aspects of the ecology of A. cupreus undergo striking seasonal changes in response to this climatic seasonality. Both sexes were sexually active for about 6 mo of the year, with the ♀ cycle coinciding with the wet season. Fat body cycles in both sexes were inversely related to the reproductive cycles. Males and ♀ ♀ attain sexual maturity at 1 yr or less, and the population apparently undergoes an annual turnover. Population density was high (> 1,200 anoles/ha) in the dry season, but in part because of changes in the structural habitat of ♂ ♂ and territorial behavior, density was much lower in the wet season. In the dry season social interactions were rarely seen, both sexes perched at or near ground level, and ♀ ♀. In contrast, in the wet season social interactions occupied a majority of the ♂ ♂ time as they defended territories and courted ♀ ♀. At this time of the year ♂ ♂ perched much higher than ♀ ♀ fed primarily on the ground; ♂ ♂ obtained at least half their food up to 6 m aboveground in the wet season. Because of seasonal changes in perch heights and size of prey items eaten, intrasexual (seasonal) values of place x food niche overlap were lower than intersexual values. The response of this anole to stressful climatic conditions is analogous to that of its Temperate Zone congener, Anolis carolinensis. Whereas A. carolinensis hibernates to avoid low winter temperatures, A. cupreus becomes inactive during periods of high winds and air temperatures.

ON THE EVOLUTION OF LITTER SIZE IN PEROMYSCUS LEUCOPUS

Litter or clutch size is an important component of an organisms total reproductive effort. Because of its importance, the evolution of litter or clutch size has received considerable theoretical attention (e.g., Brockelman, 1975; Cody, 1966; Lack, 1948; Mountford, 1968; Ricklefs, 1970; and Smith and Fretwell, 1974). Most authors point out that optimal clutch sizes are the products of diverse selective pressures, including levels of resource abundance, the intensity of intraand interspecific competition, predation pressure, and other sources of mortality. In the face of these diverse pressures, organisms can produce clutches containing a relatively large number of offspring each of small size or clutches containing a smaller number of relatively large offspring. The balance that is struck between offspring size and number often depends on the relative importance of the selective pressures determining both parental and offspring fitness. In highly competitive environments, for example, selection will favor increased parental investment per offspring and decreased litter or clutch size whereas in environments in which predation and/or climatic changes cause heavy mortality, reduced parental investment per offspring and increased litter or clutch size will be selectively favored (Brockelman, 1975). Behavioral complexity can also influence the evolution of clutch or litter size. In vertebrates, large clutches of small hatchlings often occur in fish, amphibians, and reptiles that lack parental care whereas small clutches or litters of larger young occur in birds and mammals in which parental care is usually much more highly developed. In this paper we examine experimentally some of the factors that may determine optimal litter size in the common North American forest-dwelling cricetid rodent Peromyscus leucopus. Results of a number of studies (summarized in Fleming (1970) and Hill (1972)) indicate that average litter size in this species, whose adults weigh 22-25 g, ranges from 4.1 to 5.5. Part of this variation is geographic (Smith and McGinnis, 1968) and part is age (parity)-related (Drickamer and Vestal, 1973). In laboratory colonies of P. leucopus noveboracensis, the subspecies with which we worked, litter size averages 4.7 (Drickamer and Vestal, 1973; Hill, 1972). The basic question we set out to answer was: Is a litter size of 4.7 (or its closest integer value, 5) most fit? Does it produce the greatest number of surviving and reproducing offspring, as Lack (1948), but not Mountford (1968), would predict? In attempting to answer this question, we have examined the effect of litter size and female parity on the following parameters: (1) birth weight, (2) survivorship and growth rates of young during their first four weeks of life, (3) weight at weaning, and (4) the ability of weanlings to survive exposure to acute stress.

Artibeus jamaicensis: Delayed Embryonic Development in a Neotropical Bat

In Panama the phyllostomid bat Artibeus jamaicensis is seasonally polyestrous, and young are born in March or April and July or August. Blastocysts conceived after the second birth implant in the uterus but are dormant from September to mid-November, when normal development again resumes.

The reproductive cycles of three species of opossums and other mammals in the Panama Canal Zone.

Data on the reproductive cycles of Didelphis marsupialis, Marmosa robinsoni , and Philander opossum in the Panama Canal Zone are presented, and the reproductive patterns of an additional 42 species of Panamanian mammals representing seven orders are reviewed. The three species of opossums breed seasonally, and females of D. marsupialis, P. opossum , and possibly M. robinsoni are polyestrous. Breeding in D. marsupialis and P. opossum began in late January and in M. robinsoni in late March in 1967. Females of each species are anestrous in October through January or longer in the case of M. robinsoni . Litter size is largest in M. robinsoni and smallest in P. opossum . Adult males of each species probably are reproductively active throughout the year. Considering all 45 species, reproductive activity, as evidenced by the presence of pregnant or lactating females, is highest in March and April (the latter part of the dry season) and lowest in August through November (middle to late wet season). Twenty-three species are known to breed seasonally, and in these species major periods of copulation occur in November and January and major birth peaks in March–April and July–August. In many species, initiation of reproductive activity appears to be stimulated by the cessation of heavy rains. Most species probably produce their young when food resources are highest. There is no evidence that closely related species stagger their breeding seasons to avoid interspecific competition for limited resources.

An Experimental Analysis of Seed Hoarding and Burrowing Behavior in Two Species of Costa Rican Heteromyid Rodents

This study compares the seed hoarding and burrowing behavior of two Costa Rican heteromyid rodents, Liomys salvini , an inhabitant of dry tropical forests, and Heteromys desmarestianus , an inhabitant of wet tropical forests. Because L. salvini lives in a more seasonal and physiologically stressful environment than H. desmarestianus , we initially predicted it would be the more intense seed hoarder and burrower. Results of two sets of five-day seed hoarding experiments indicated that both species cached seeds at a similar rate although H. desmarestianus initially reacted to a 300 gram pile of seeds faster than L. salvini. In both species females hoarded more seeds than males. L. salvini removed more seeds located in an “arboreal” beaker than H. desmarestianus. Both species probably hoard seeds to buffer seasonal fluctuations in food availability. In addition, physiological observations indicate that, unlike L. salvini , individuals of H. desmarestianus cannot tolerate weight losses of 20 percent and hence must hoard seeds to maintain a stable body weight. Results of two-day burrowing experiments in which rodents could choose between placing their nest in an artificial log or nest box or constructing a burrow system indicated that L. salvini is a more intense burrower than H. desma restianus. Eleven of 12 individuals of the former species constructed burrow systems, whereas all 12 individuals of the latter species nested in the log or nest box. Only when the length of the trial was extended to five days and the log and nest box were removed did 50 percent of the H. desmarestianus tested construct a burrow system. Field observations in Costa Rica indicate that live-trapped individuals of L. salvini are more likely to run to a burrow after being released, whereas individuals of H. desmarestianus are more likely to run into or under a log. Perhaps H. desmarestianus places its nests aboveground more often than L. salvini because the physiological disadvantages of burrow systems outweigh their advantages in wet tropical forests.

Social Organization in Two Species of Costa Rican Heteromyid Rodents

The social organization of two species of Costa Rican heteromyid rodents, Liomys salvini and Heteromys desmarestianus , was investigated using both laboratory and field techniques. Results of one-to-one encounters between strangers of the same sex in a neutral arena indicated that individuals of L. salvini were less tolerant of conspecifics than those of H. desmarestianus . In the laboratory, levels of aggression differed intraspecifically only in L. salvini , and levels of male activity, but not aggression, varied seasonally. Dominance in one-to-one encounters was positively related to size in both sexes of L. salvini but not in H. desmarestianus . Indices of home range overlap in marked field populations indicated that ranges were more likely to be clumped in H. desmarestianus than in L. salvini in which repulsion between adult ranges of the same sex was observed in the nonbreeding season. Populations of both species did not appear to contain wideranging dominant males. However, in L. salvini larger, presumably more dominant males were surrounded by more potential mates prior to the breeding season than were smaller males, and these males survived better during the breeding season. On a year-to-year basis, population levels in both species were stable in 1970 to 1972, which suggests that the populations are strongly regulated. It is suggested that behavioral interactions are important in population regulation in both species, but field experiments are needed to test this hypothesis.

Metabolic rates of small homeotherms in a waterbath.

Abstract 1. The rates of heat loss of small birds ( Junco hyemalis , Passer domesticus ) and rodents ( Rattus norvegicus albino, Spermophilus beldingi , Heteromys desmarestianus ) immersed to the neck in water were 5 to 10 times as great as those in air. 2. Maximum metabolic rates were elicited at waterbath temperatures of 28 to 30°C. 3. Minimum metabolic rates were comparable to those from the literature of the same species in air but maximum rates for R. norvegicus and P. domesticus were lower than those attained in air or in a helium-oxygen atmosphere.