Donald A. Dewsbury

Ejaculate Cost and Male Choice

According to much evolutionary thinking, males of promiscuous species are able to produce quantities of sperm that are virtually unlimited and should be selected to mate indiscriminately with all available females. However, sperm are generally delivered in batches (ejaculates or spermatophores) that may include many millions of gametes. Males are limited with respect to the number of ejaculates they can deliver and the time required to restore depleted reserves. Because ejaculates are a limited resource, prudence would be expected in the allocation of ejaculates to females. Among the forces acting to limit the number of females with which a promiscuous male mates are high pregnancy initiation requirements, sperm competition, female choice and control, and the costs and risks of searching. Optimal strategies for ejaculate allocation will vary with the operational sex ratio. Males can and do discriminate among females as potential mating partners. The problems of limited ejaculatory capacity and male choice merit greater attention in both theory and in empirical research.

Dominance Rank, Copulatory Behavior, and Differential Reproduction

The view that high social rank is associated with high levels of both copulatory behavior and the production of offspring is widespread in the study of animal social behavior. In order to demonstrate the validity of this hypothesis it is necessary first to resolve ambiguities in the concept of dominance and to assign ranks by means of valid procedures. Second, copulatory behavior must be properly sampled, measured, and related to rank. Finally, it must be demonstrated that rank and increased copulatory behavior actually lead to increased reproduction. Each step in this process entails conceptual and methodological difficulties. There have been many studies of rank and copulatory behavior, fewer of rank and differential reproduction, and very few of rank, copulatory behavior, and differential reproduction. The consistency of results obtained varies with taxon; results of particular consistency appear in studies of carnivores and ungulates. Both the concept of dominance and the validity of the hypothesis relating it to copulatory behavior and to differential reproduction appear viable for at least some species, although the body of data relating rank to both copulation and differential reproduction remains minimal.

Patterns of copulatory behavior in male mammals.

A schema for systematic inquiry into different patterns of copulatory behavior in male mammals i proposed. Three stages in the development of a complete analysis are isolated: classification, elaboration, and experimental analysis. A classification schema based on four attributes is proposed. Species may or may not display a lock, thrusting during intromissions, multiple intromissions, and multiple ejaculations. Literature on species-typical patterns is reviewed in terms of these attributes.

Diversity and adaptation in rodent copulatory behavior

Copulatory patterns of muroid rodents provide an ideal locus for comparative behavioral research. Such patterns are highly stereotyped within and between the individuals of a given species, variable across species, readily elicited in the laboratory, and of great biological significance. Detailed behavioral comparisons of a broad range of muroid species have revealed extensive behavioral diversity that was not anticipated from research confined to laboratory rats. Various muroid species display postejaculatory compulations without sperm transfer, locking, thrusting, and other behavioral patterns. This behavioral diversity appears not to be the result of a simple linear pattern of evolutionary history. Rather, patterns appear to have evolved repeatedly in response to particular selective pressures acting on particular species. While understanding of the adaptive significance of these behavioral patterns remains rudimentary, important beginnings have been made.

Communal nesting and communal nursing in house mice, Mus musculus domesticus

Abstract The functional significance of communal nesting and nursing is poorly understood. Female house mice often communally nest, and within these communal nests females appear to indiscriminately nurse all pups, a rare trait for any mammal. In this study, the hypothesis that communal nesting provides protection from conspecific infanticide was tested and supported in semi-natural populations of house mice. Conspecific infanticide in single-mother nests (69%, N = 412) was twice that in communal nests (33%, N = 508). Because this major benefit of communal nesting does not require communal nursing, direct benefits to communal nursing itself were tested. Most proposed benefits should result in heavier weaning weights, but no differences were found between communal and single nests in the semi-natural populations. If communal nursing is to be avoided in communal nests, dams must recognize their own pups. Retrieval tests conducted in the laboratory produced equivocal results. Dams discriminated between pups that differed in age, but not between their own and other age-matched pups. The major survival advantage of communal nesting, coupled with the failure to find nutritional advantages for communally nursed pups, supports a recent suggestion that communal nursing is an unavoidable consequence of communal nesting. This hypothesis is further strengthened by data indicating that communal nesting partners tend to be kin, thereby providing inclusive fitness benefits to communal nursing. Although costs of communal nursing were proposed and tested, no such costs were found. We also show from 15 observations of infanticide that all classes of adults (territorial and non-territorial males, pregnant and non-pregnant females) are infanticidal. These observations are in conflict with previous laboratory studies.

Paternal behavior in six species of muroid rodents

In Experiment I parental behavior was studied in 10 pairs of each of six species of muroid rodents: Microtus californicus, M. ochrogaster, M. californicus, M. montanus, Peromyscus maniculatus, and P. leucopus. Appreciable amounts of sitting on the nest, licking pups, and manipulating nesting materials were observed for all species, and manipulation and retrieval of pups for all Microtus species. Sex differences in parental behavior were relatively infrequent, reaching statistical significance in 9 of 60 comparisons. Species differences were significant in 58 of 150 comparisons for males and 46 of 150 for females. In Experiment II it was found that parity had little effect on parental behavior in the two Peromyscus species. These data both expand the catalog of species for which paternal behavior has been described and extend previous observations in several species. However, they provide little support for hypotheses that such behavior in the laboratory either is indicative of paternal behavior in the field or is a correlate of monogamous breeding systems, certainty of paternity, or adoption of a K-strategy in nature.

Studies of sperm competition in two species of muroid rodents

Summary1.In three experiments on the effects of mating order, wild-type male deer mice, Peromyscus maniculatus bairdi were tested in competition with brown recessive, wide-band agouti, and blonde mutant males. Females of the homozygous recessive genotype received two ejaculatory series from a male of each genotype; mating order was varied. There were no significant effects of mating order on litter composition. In a parallel experiment wild-type and albino male prairie voles, Microtus ochrogaster, mated for one ejaculation each with albino females. There was a significant advantage with respect to litter composition for the male ejaculating last. Together with data from other species, these results suggest that effects of mating order are highly species-specific and must be evaluated for each species before speculations are made regarding the effects of multiple matings.2.Males of various genotypes differed with respect to differential fertilizing capacity. Wild-type male deer mice outcompeted both brown recessive and wideband agouti males. The combinations of blonde and wild-type deer mice and albino and wild-type prairie voles yielded strain differences in litter composition in one mating order but not in the other. Counts of the numbers of sperm from males of different genotypes in the uteri of females revealed no sigificant strain differences. Thus, strain differences are not simply a function of sperm numbers. 3. Female deer mice mated or exposed to more than one male had a lower probability of pregnancy than those mated for comparable numbers of ejaculations with one male in previous studies. Additional data on pregnancy initiation in females of the brown-recessive genotype mated with one male revealed them to be only slightly less likely to become pregnant than wild-type females. Strain of male had little effect. 4. No significant effects on litter composition were attributable to the length of the delay between matings by successive males. This was established both via correlational analyses of the data from the mating order studies and in an additional experiment in deer mice in which delays of 2 h were imposed between the last ejaculation by the first male and introduction of a second male. 5. Litter composition in deer mice reflects the number of ejaculations attained by each of two males. In one experiment, female deer mice mated first with a blonde male and then with a wild-type male. The blonde male delivered three ejaculations and the wild-type male one ejaculation in one condition and the wild-type male three ejaculations and the blonde male one in the other. Litter composition was significantly affected.

Open-field behavior in muroid rodents.

Open-field behavior was observed in 10-min tests of 10 adult males of each of 12 species of muroid rodents and of the two reciprocal crosses of Peromyscus leucopus and P. gossypinus. Considerable variation was found in number of squares entered, number of fecal boli deposited, and patterns of behavior displayed in the open field. Animals generally showed decreasing ambulation as tests progressed and tended to seek walls. Most measures varied significantly as a function of the general ecology of these species.

Copulatory behaviour of rats (Rattus norvegicus) as a function of prior copulatory experience.

Abstract Two experiments were reported which were designed further to elaborate the contributions of prior copulatory experience to copulatory behaviour in rats. In experiment 1—test 4 the behaviour of two groups of rats of equal age with 0 or 3 prior tests were compared. In experiment 1—test 7, performance of rats with 0, 3, or 6 prior tests was compared. In experiment 2 performance of rats with 0 or 4 prior tests was compared. Data were related to those of Larsson (1959) . Prior copulatory experience produced small, but generally reliable, decreases in mount, intromission, and ejaculation latencies. Intromission rates were generally increased. There was some trend for decreased mount and intromission frequencies. The effect of experience on postejaculatory intervals was a weak one. The prediction of differential effects of experience on measures controlled by Beachs hypothetical Arousal Mechanism and Copulatory Mechanism was not borne out.

The Darwin-Bateman Paradigm in Historical Context

Abstract I introduce the term “Darwin-Bateman Paradigm” to include several proposals stemming from the writings of Charles Darwin and A. J. Bateman, including the notions that (a) male reproductive success is more variable than that of females, (b) males gain more in reproductive success from repeated matings than do females, and (c) males are generally eager to mate and relatively indiscriminate whereas females are more discriminating and less eager. I trace this paradigm from Darwins The Descent of Man through Batemans research and beyond. I try to clarify the terminology used in applying Batemans results and discuss both the impact and the criticisms the paradigm has engendered. I then broaden the context of the Darwin-Bateman Paradigm to show related conceptions in disparate fields that evolved in parallel with it. I conclude that gender stereotypes appear to have influenced these conceptions. The paradigm has been of great heuristic value but is in need of further empirical investigation in view of numerous exceptions to these general rules.