Abstract: The shy-bold continuum is a fundamental axis of behavioral variation in humans and at least some other species, but its taxonomic distribution and evolutionary implications are unknown. Models of optimal risk, density- or frequency-dependent selection, and phenotypic plasticity can provide a theoretical framework for understanding shyness and boldness as a product of natural selection. We sketch this framework and review the few empirical studies of shyness and boldness in natural populations. The study of shyness and boldness adds an interesting new dimension to behavioral ecology by focusing on the nature of continuous behavioral variation that exists within the familiar categories of age, sex and size.

Abstract: Right hemisphere advantage in individual recognition (as shown by differences between response to strangers and companions) is clear in the domestic chick. Chicks using the left eye (and so, thanks to the complete optic decussation, predominantly the right hemisphere) discriminate between stranger and companion. Chicks using the right eye discriminate less clearly or not at all. The ability of left eyed chicks to respond to differences between strangers and companions stimuli is associated with a more general ability to detect and respond to novelty: this difference between left and right eyed chicks also holds for stimuli which are not social partners. The right hemisphere also shows advantage in tasks with a spatial component (topographical learning; response to change in the spatial context of a stimulus) in the chick, as in humans. Similar specialisations of the two hemispheres are also revealed in tests which involve olfactory cues presented by social partners. The special properties of the left hemisphere are less well established in the chick. Evidence reviewed here suggests that it tends to respond to selected properties of a stimulus and to use them to assign it to a category; such assignment then allows an appropriate response. When exposed to an imprinting stimulus (visual or auditory) a chick begins by using right eye or ear (suggesting left hemisphere control), and then shifts to the left eye or ear (suggesting right hemisphere control), as exposure continues. The left hemisphere here is thus involved whilst behaviour is dominated by vigorous response to releasing stimuli presented by an object. Subsequent learning about the full detailed properties of the stimulus, which is crucial for individual recognition, may explain the shift to right hemisphere control after prolonged exposure to the social stimulus. There is a marked sex difference in choice tests: females tend to choose companions in tests where males choose strangers. It is possible that this difference is specifically caused by stronger motivation to sustain social contact in female chicks, for which there is extensive evidence. However, sex differences in response to change in familiar stimuli are also marked in tests which do not involve social partners. Finally, in both sexes there are two periods during development in which there age-dependent shifts in bias to use one or other hemisphere. These periods (days 3-5 and 8-11) coincide with two major changes in the social behaviour of chicks reared by a hen in a normal brood. It is argued that one function of these periods is to bring fully into play the hemisphere most appropriate to the type of response to, and learning about, social partners which is needed at particular points in development. Parallels are discussed between the involvement of lateralised processes in the recognition of social partners in chicks and humans.

Abstract: In horses, a circadian rhythm in plasma cortisol concentrations has been reported in some but not all studies. When a rhythm occurred, horses were accustomed to a management routine, comprising stabling, feeding and sometimes exercise, which may entrain a circadian pattern. In this work, we monitored plasma cortisol by collecting jugular blood through indwelling cannulae from four groups: 1): 10 untrained, unperturbed mares grazing excess pasture, bled hourly for 26 hr; 2) 4 mares housed in a barn for 48 hr before sampling every 15 min for 20–24 hr; 3) 5 mares placed in an outdoor yard for sampling every 30 min from 0930–2100 hr; and 4) 4 stabled racehorses in training, bled every 30 min from 0730–2000 hr and once the following morning at 0830 hr. Plasma cortisol showed a similarly-timed circadian rhythm (P<0.0001) in all Group 1 horses, with a peak at 0600–0900 hr, and a nadir at 1800–2100 hr. By contrast, cortisol concentrations did not vary with time in either Group 2 or 3. Neither daily mean nor peak cortisol values differed in Group 1 and 2 (i.e. bled for >= 20 hr); however nadir values were higher (P<0.05) in Group 2. In Group 4, cortisol declined (P=0.004) during the sampling period but had returned to initial concentrations the next morning. Values did not differ from those for Group 1, except between 1000 and 1300 hr when cortisol in Group 4 was lower (P<0.05). We conclude that a circadian cortisol rhythm exists in horses in the absence of any known cues imposed by humans. However, this rhythm can be obliterated by the minor perturbation of removing the horse from its accustomed environment. By contrast, the rhythm occurs in trained racehorses, suggesting either that they have adapted to their environment thereby allowing an endogenous rhythm to emerge, or that the rhythm is entrained by their daily routine. These observations highlight the difficulties in determining the cortisol status of a horse, since measurements will be affected by time of day, the occurrence of short-term fluctuations, and how accustomed the horse is to its environment.

Abstract: Synopsis
The family Equidae have an extensive fossil record spanning the last 58 million years, and the evolution of the horse has frequently been used as a classic example of long-term evolution. In recent years, however, there have been many important discoveries of fossil horses, and these, in conjunction with such new methods as cladistics, and techniques like precise geochronology, have allowed us to achieve a much greater understanding of the evolution and biology of this important group. This book synthesizes the large body of data and research relevant to an understanding of fossil horses from several disciplines including biology, geology and palaeontology. Using horses as the central theme, the author weaves together in the text such topics as modern geochronology, palaeobiogeography, climate change, evolution and extinction, functional morphology, and population biology during the Cenozoic period.