F. H. Bronson

Aggression in Adult Mice: Modification by Neonatal Injections of Gonadal Hormones

Incidence of spontaneous aggression in adult male mice given a single injection of estradiol benzoate (0.4 milligram) when they were 3 days old was less than that of controls injected with oil. Aggressiveness was increased among adult females injected with either estradiol or testosterone propionate (1 milligram) at the same age. The increased aggressiveness noted among females given androgen was further documented during subsequent mating tests, when these females often attacked, wounded, and, in one case, killed naive males.

Are Humans Seasonally Photoperiodic

Humans exhibit seasonal variation in a wide variety of behavioral and physiological processes, and numerous investigators have suggested that this might be because we are sensitive to seasonal variation in day length. The evidence supporting this hypothesis is inconsistent. A new hypothesis is offered here—namely, that some humans indeed are seasonally photoresponsive, but others are not, and that individual variation may be the cause of the inconsistencies that have plagued the study of responsiveness to photoperiod in the past. This hypothesis is examined in relation to seasonal changes in the reproductive activity of humans, and it is developed by reviewing and combining five bodies of knowledge: correlations of human birthrates with photoperiod; seasonal changes in the activity of the neuroendocrine pathway that could link photoperiod to gonadal steroid secretion in humans; what is known about photoperiod, latitude, and reproduction of nonhuman primates; documentation of individual variation in photoresponsiveness in rodents and humans; and what is known about the evolutionary ecology of humans.

Behavioral and physiological responses of female house mice to foraging variation

Peripubertal female house mice were required to work for their food at either 23 degrees C or 9 degrees C. We used a special caging system in which animals had to emerge from a thermally-buffered burrow and run a programmable number of running wheel revolutions to obtain a pellet of food. Of concern here were the behavioral and physiological adjustments necessary to accommodate growth and reproductive development when faced with the need to forage for different lengths of time at different temperatures. When female house mice are confronted with poor foraging at cool temperatures they allot their highest priorities to maintaining energy balance; body growth is next, and reproductive development and nonforaging activity have the lowest priorities. Our results also demonstrate that the time spent foraging while exposed to low ambient temperature is critical for this species. This relationship probably determines whether or not house mice will breed continuously or seasonally in a particular habitat.

Effect of anabolic steroids on behavior and physiological characteristics of female mice

Adult female mice were exposed to a combination of four anabolic-androgenic steroids for 9 weeks at doses that were either one or five times the androgenic maintenance level for male mice. Relative to control females, steroid treatment depressed gonadotropin secretion and increased both dry body weight and fat content but without an increase in food intake. Steroid treatment depressed spontaneous use of a running wheel and open-field activity, and it increased aggressiveness. It also eliminated a behavior related to encounters between the sexes--the rejection of genital inspection. There was no effect of steroid treatment on the time required to recover from 10 h of enforced running on a treadmill. Overall, regardless of the test or measure, there was little or no difference in the effect of the high and low dose of steroids. This indicates a threshold of response below the low dose used in these studies, which itself is probably well below that used by many female athletes and body builders.

Social Rank in Male Mice and Adrenocortical Response to Open Field Exposure

Summary Plasma corticosterone levels were examined in socially dominant, subordinate, or unranked male mice taken directly from an undisturbed cage or following 10 min of a nonsocial neurogenic stress of exposure to an open field. There were no significant differences in plasma corticosterone levels among the social rank categories in undisturbed animals. Following open field exposure, subordinate males had the greatest levels of plasma corticosterone while dominant males had greater levels than unranked males. These data indicate that the process of social organization leads to an increase in responsiveness of male mice to a strange environment and under these circumstances the socially subordinate males are more responsive.

Patterns of sexual receptivity in the female musk shrew (Suncus murinus)

Female musk shrews (Suncus murinus) were tested daily to examine patterns of sexual receptivity. When only mounting was used as a criterion (to avoid pregnancy), nonpregnant females remained sexually receptive to males every day for 14 consecutive days. When insemination was allowed, most females continued to copulate for the first 5 days of pregnancy. Receptivity declined markedly around Day 10 of gestation, but a few females were receptive even into late pregnancy. Lactating females copulated with males 5 and 10 days after parturition. In general, unlike most mammals studied in the laboratory, the nonpregnant female musk shrew has no behavioral estrous cycle. Musk shrews are ready to mate anytime except in mild to late pregnancy, and even then occasionally mating is found.

Sensitivity of Syrian hamsters (Mesocricetus auratus) to amplitudes and rates of photoperiodic change typical of the tropics.

Empirical data suggest that reproductive photoresponsiveness occurs in some populations of mammals above 13° of latitude, but may be absent in populations from 0° to 10° of latitude. The present experiments examined the degree to which the low amplitude of change in photoperiod in the tropics constrains mammals from using daylength as a seasonal cue. The Syrian hamster, a temperate-zone species, was studied because of its well-documented ability to respond to small changes in photoperiod, and because of the absence of an alternative robustly responding species from the tropics. We subjected adult male hamsters to photoperiods that mimicked the amplitude and rate of photoperiod change of 30°, 20°, 10°, and 5° of latitude, but centered around an estimate of their critical daylength. For comparison, a fifth group was subjected to an abrupt change in daylength of a magnitude equal to the total annual variation occurring at 30°. The two groups experiencing the gradually changing daylengths of 30° and 20° showed less within-group synchrony during testicular regression; in other dimensions of the annual testis cycle, including the degree of synchrony exhibited during recrudescence, they reacted similarly to the hamsters given the abrupt change in daylength. Some of the hamsters exposed to the gradually changing daylengths of 10° responded to this challenge, as did a few in the 5° treatment-in both cases, with poor within-group synchrony and a submaximal decrease in testis size. In an abbreviated second experiment, hamsters given abrupt decreases in daylength of magnitudes equal to those of the 10° and 5° groups responded slightly more frequently, and with maximal decreases in testis size. This suggests that mammals may not be constrained absolutely by an inability to respond to changes in photoperiod at 5° to 10° latitude. Seasonally breeding populations of mammals in the deep tropics that do not use photoperiod to regulate reproduction may use nonphotoperiodic cues because they offer a higher signal-to-noise ratio than do tropical changes in photoperiod.

Sex differences in the energy allocation strategies of house mice

SummarySex differences in energy allocation were studied in wild stock house mice (Mus domesticus). Peripubertal animals of both sexes, either intact or gonadectomized, were subjected for 8 weeks to one of two feeding requirements by using a caging system in which a pellet dispenser was controlled by activity on a running wheel. Recently weaned animals were required to run 200 vs 300 or more wheel revolutions to obtain a pellet of food. The 200 revolution requirement allowed normal body growth and reproductive development; the 300+ requirement was adjusted weekly to maintain food intake at a level that allowed survival but did not permit normal body growth. Reproductive development was completely inhibited in intact females at 300+ revolutions whereas intact males at 300+ revolutions, despite stunted growth, all experienced normal sexual development. At both feeding requirements, however, females exhibited more total locomotor activity and consumed more food than males, regardless of gonadectomy. Furthermore, at the 200 revolution requirement we often observed extensive running activity beyond that needed to generate the amount of food actually eaten, especially among females. This “extra” locomotor activity was gonad-dependent in males, but not so in females. These results suggest that male and female house mice employ different strategies when relating their behavior and reproductive development to existing foraging conditions. Females appear more resource-dependent than males.

Relationships between Scent Marking by Male Mice and the Pheromone-Induced Secretion of the Gonadotropic and Ovarian Hormones that Accompany Puberty in Female Mice

Many mammals use specific olfactory cues (pheromones) as an integral part of their intrapopulation communication. These compounds can be conveniently categorized according to function as:(a) signalling pheromones, which elicit a more or less immediate change in behavior (if indeed a response does occur), and (b) priming pheromones, which trigger neuroendocrine and endocrine activity. Sex attractants are an example of the former:the regulatory effect of urinary odors on the mouse estrous cycle is an example of the latter. Much of the recent interest in mammalian pheromones stems from the discovery in the 1950’s of a series of dramatic primer effects in female mice (e.g., Whitten, 1956:Bruce, 1959). Many contributions have since been made by ethologists, psychologists, and reproductive physiologists:but work by chemists and neuro-physiologists is conspicuously absent. Recent reviews include Bronson (1968, 1971, 1974), Bruce (1966, 1967, 1970), Cheal and Sprott (1971), Eisenberg and Kleiman (1974), Gleason and Reynierse (1969), Mykytowycz (1970), Ralls (1971), Schultz and Tapp (1974), Whitten (1966), and Whitten and Bronson (1970).

LABILITY OF FAT STORES IN PERIPUBERTAL WILD HOUSE MICE

SummaryTraditionally, the adaptive value of mammalian white fat stores is considered in relation to longterm needs such as providing protection against the vagaries of winter or signalling the reproductive system when energy reserves are sufficient to risk pregnancy. As shown here, the fat stores of young house mice could not serve such needs. Despite prolonged acclimation and excess nesting material, food deprivation at 10°C significantly lowered the fat stores of peripubertal female house mice in only 12 h, and would exhaust them in 30 h. Even close to thermoneutrality (24°C) the calculated time to exhaustion was only 70 h. The fat stores of a young house mouse are obviously too meager to offer any meaningful protection over a winter of several months duration, or even over a 5–6-week cycle of pregnancy and lactation. Furthermore, in a wild habitat where food availability and ambient temperature can vary rapidly and greatly, such fat stores would be too labile to effectively coordinate puberty with somatic development.