The frequency and sequencing of aggressive behaviors by naive female hamsters change during series of brief encounters, probably because of the lack of stable dominance relations. Such initial encounters seem most representative of interactions likely in free-ranging hamsters and have been emphasized in studies of the hormonal mediation of female aggression. Nonestrous females exhibit intense aggression toward conspecifics of either sex. Estrous females are not aggressive and spend much time in lordosis, indicative of sexual receptivity. The inhibition of fighting on estrous day depends on estrogen and progesterone. Whereas oil-injected adrenalectomized-ovariectomized females fight at high levels, comparable with intact nonestrous females, the combination of 17/3-estradiol benzoate and progesterone suppresses fighting completely. In contrast, replacement of estradiol, progesterone, or testosterone propionate individually has no consistent effect. Hypophysectomized females also fight at high levels, indicating that pituitary hormones are not required for vigorous aggression. Further, individual anterior pituitary hormones did not produce marked changes in fighting. These results emphasize the roles of estrogen and progesterone in synchronizing aggression with current reproductive state.

Basal rates of high-frequency vocalization by estrous female hamsters exceeded those typical of nonestrous females. Even higher rates of calling by estrous females were provoked by male odors (male shavings or anesthetized males). This suggests that cues which normally indicate a male's proximity can increase the rate of high-frequency calls by an estrous female. These findings are consistent with a view of female "ultrasounds" as sexual advertisements which indicate the locations, sexual receptivity, and relative passivity of estrous females to nearby male conspecifics. Male hamsters also produced ultrasounds at rates that seemed to depend on the availability of potential mates. Brief exposure to an awake female stimulated male calling; however, estrous females provoked higher call rates than did nonestrous females. Exposure to anesthetized females also increased the rate of male calling, which suggests that the stimulation of male calling by awake females depends in part on female odors. These results suggest that both male and female ultrasounds are parts of a communication system that facilitates reproduction by helping to coordinate social behavior with endocrine state.

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Female hamsters emitted high-frequency sounds at average intensities of 53 dB (SPL). These calls tended to be 80-100 msec long and to emphasize frequencies of 34-42 kHz. However, female "ultrasounds" typically included rapid fluctuations in frequency and amplitude. Male hamsters also emitted high-frequency vocalizations, with dominant frequencies of 32-38 kHz and average durations of 70-150 msec. Although male calls generally included fewer rapid changes in amplitude and frequency than did female calls, male call structures depended on contextual factors. Calls produced by males in the presence of estrous females tended to have lower minimum frequencies, higher maximum frequencies, longer durations, and fewer rapid frequency changes than calls by solitary males. These results show that both sex differences and situational factors influence the structures of hamster ultrasounds. The frequency and amplitude changes typical of calls by females and solitary males should facilitate the localization of a calling individual over moderate distances. Calls by males in the presence of females should be more difficult to localize and might operate over shorter distances to serve a different social function.Virtually all species of small mammals examined produce, and apparently hear, sounds with frequencies well above the range of human hearing (e.g., Brown, 1973; Sales, 1972a Sales, , 1972b. These "ultrasounds" seem to serve as social communications in maternal, sexual, and aggressive behaviors (e.g., Noirot, 1972; Sales, 1972a Sales, , 1972b. For example, infant rodents vocalize in response to thermal and tactile stress (Okon, 1972). Noirot suggested that This article is based on a dissertation submitted by Owen R. Floody to the Rockefeller University in partial fulfillment of the requirements for the PhD degree.