Brain/body relations among myomorph rodents

The observed increase in brain size (E) with body size (P) 'from mouse to elephant' may be described by a power relation E = kPb, where b is near 2/3 or 3/4. That this reflects a single, strong interaction between brain and body evolution is challenged by two observations: (1) different species with the same body size may have markedly different brain sizes, and (2) the value of b at the species level is usually nearer 1/3 than 2/3. Furthermore, the idea that a bigger brain means greater versatility on the part of its owner makes a strong statement about such animals. We examined these notions by measuring cranial volumes of 1,480 skulls from 62 subspecies of cricetid and murid rodents. Values of k and b were obtained by computing a reduced major axis on E and P across all specimens (b = 0.693), and when specimens were partitioned by genus (b = 0.456) and subspecies level (b = 0.338). Thus, the overall slope of 7/10 is not a simple extension of the developmental rules at the subspecies level (b near 1/3) nor even at the genus level (b near 4/9). Rather, it may reflect the most likely path for an interbreeding population subjected to varying selective pressures on one or more correlated traits. Furthermore, among the rodents studied, folivorous subspecies averaged about 2/3 as much brain as granivorous, insectivorous or generalist subspecies of the same body weight. Also, Old World rats, which may be more versatile than wood rats, gained their competitive advantage despite having relatively smaller brains.

Androstenedione may organize or activate sex-reversed traits in female spotted hyenas

Spotted hyenas (Crocuta crocuta Erxleben) present a unique syndrome of reversal in behavioral and anatomical distinction between the sexes: females are heavier and more aggressive than males and dominant over them. The female's external genitalia include a false scrotum and a fully erectile pseudopenis through which mating and birth take place. Results of studies of circulating testosterone levels in wild spotted hyenas do not account for the "male-like" characteristics of the female. Androstenedione, however, is consistently higher in females than in males, particularly during early infancy. Experiments on rodents show that androstenedione can be a potent organizer of anatomical and behavioral differentiation. This study suggests that it may also produce the profound virilization of female spotted hyenas.

Seasonal aggression independent of seasonal testosterone in wood rats

Levels of inter-male aggression, both in laboratory encounters and in the field, rise dramatically during the breeding season, closely paralleling the seasonal rise in testosterone. However, post-pubertally castrated males also show the dramatic seasonal rise in aggression in laboratory encounters with castrated opponents and show no decrement in fighting ability when paired with intact opponents, clearly demonstrating the independence of seasonal aggression from the proximate modulating effects of testosterone in wood rats.

Chemical stimulation of the midbrain reticular system

An intracerebral cannula system was surgically implanted in each of 21 rats. The responses of each S were observed following the temporary deposition of acetylcholine chloride, sodium pentothal, 1-epinephrine, or the placement of a blank cannula. Acetylcholine produced drinking in the hypothalamus and stereotyped standing postures in the reticular system Ss. Sodium pentothal produced an increment in feeding responses in the ventrolateral-reticular-system Ss and an increment in grooming in the dorsomedial-reticular-system Ss.

Shredding of nesting material in the Mongolian gerbil

Experiments were conducted to determine whether the stereotyped shredding of nesting material by the Mongolian gerbil ( Meriones unguiculatus) is influenced by sex of S, accumulation of shredded material, or the thickness of paper provided for shredding. The first two variables did not significantly affect shredding behavior in our tests. However, in an acceptance-rejection situation, Ss showed an orderly preference for papers of increasing thickness.

Curiosity in zoo animals

Abstract

The reactions of more than 200 zoo animals to a standardized set of novel objects were recorded and quantified. Our results indicated significant differences among various taxonomic groups, both in the quantity and form of object manipulation. Our major quantitative findings were as follows: A. Primates and Carnivores exhibited more investigatory behavior than Rodents or a group of "primitive" mammals. B. A sample of 20 reptiles showed very little response, with the exception of a single Orinoco crocodile. C. There were no significant overall sex differences among 53 mature pairs of mammals drawn from different orders. D. A sample of subadult mammals tended to be more reactive than adult members of the same species. E. Within the Old World monkeys, there was considerable variation between the two subfamilies: The Cercopithecinae indulged in more investigatory behavior than the Colobinae, while the latter exhibited a greater percentage of purely visual "orienting" responses. F. Among the Cercopithecinae, the baboons and macaques demonstrated more investigatory behavior than the guenons and patas monkeys, although the latter genera displayed a larger percentage of visual orienting responses. G. The limited sample of Prosimians did not differ significantly from the highly evolved Colobinae in total reactivity. H. There were no significant differences among the families of carnivores within our sample, although the smaller cats comprising the genus Felis were less reactive as a group than the larger cats of the genus Panthera. In addition, there was some hint that the meerkats, which constituted the only representatives of the Vivveridae, were less immediately reactive than the remaining carnivorous species. I. Among a relatively small sample of Rodents, the members of the suborder Hystricomorpha exhibited more reactivity than those in the suborder Myomorpha, with the Sciuromorpha in an intermediate position. In terms of the forms (motor sequences) of reaction, chewing constituted the primary contact response within all groups. However, the following additional reactions were noted: A. Primates were characterized by grasping with the forepaws, visual inspection, and manipulation of objects. The latter was particularly evident in the baboons and macaques where many "individualistic" patterns appeared. These, in turn, contrasted with the Prosimian group where such diverse manipulatory behavior was rarely, if ever, recorded. Indications of fear were observed in 19/100 adults. B. Carnivores showed a vigorous, relatively fearless, approach to the objects which approximated the patterns that might normally be used in the capture and consumption of prey: swatting with the paws, stalking, chasing, biting, "worrying," and tugging against the paws. C. Rodent reactions were generally limited to gnawing, although there appeared to be differences in the aggressiveness of approach among the three suborders. There were also some hoarding responses, (depositing objects in a particular part of the cage, or burying them in the sawdust). D. Some possibly species-characteristic techniques of object manipulation were noted, e.g., the use of the tongue and snout by the giant anteater, the tail by the spider monkey, and the lips by the guanaco. A preliminary attempt was made to interpret our results in relation to feeding patterns and danger from predators in the natural habitats of the species that we tested. The possible significance of species differences in reactivity for brain-behavior correlations, and contemporary behavioristic theory, was also briefly considered.