THE GENESIS OF SOCIAL GENES II. THE EVIDENCE – MALES
There is no single form of sociality. Different species, with different needs, have found a variety of solutions for increasing their group size and range. In addition, the rules of behavior of each may be different. In this blog, I will describe some general forms of association that are appropriate for the needs of the species. Seasonal Breeding: Males Disperse This is the simplest mode of animal cohesion which operates for a wide range of mammals, and birds that breed at the same time each year. The advantage of seasonal breeding is that it allows the birth of the next generation, at the most benign season of the year, the best chance for the vulnerable infants to thrive. Vegetarian mammals and many birds depend on an abundance of grasses, leaves, and seeds in the springtime for their fodder. Autumn mating is the ideal time to allow the mother to gestate when she has already built up her reserves. Many social species break up at the end of the mating season to form separate bachelor herds of males that disperse from their partners, while nursery herds of females remain in their territory. Any hypotheses of animal sociality need to accommodate the change from intersexual attraction in the mating season to single-sex herds during the rest of the year. Experimental studies by scientists have shown that various sheep spontaneously segregate into two separate herds, even when their pasture is devoid of other features.1,2 These studies also show their awareness of their neighbors by synchronizing their periods of rest and grazing. The scientists have interpreted their results in terms of a social preference for association with their own gender. Assays of many different hormones in this class of species have revealed a specific candidate for explaining both the herd formations and the breakup of cohesion among males at the approach of the breeding season. Seasonal Breeding: Alpha Males form Mixed-sex Group with a Harem of Females; Unattached Males Disperse The members of this group are large vegetarian mammals including horses, camels, impala, and peccaries that resemble those described above. The breeding parameters are similar, but they differ from them in a particular way. One or two dominant males guard a small group of females in the off-season known as a harem group, while the residual males form a bachelor group similar to that above. Although the difference between the two forms of sociality may appear slight, the motivations for this society are quite different. The bachelor males have not dispersed voluntarily, but have been exiled from the females. Nevertheless, they still form a herd that is presumably controlled by a similar proximity reward. They maintain a distant contact with the harem, and individuals may replace any dominant male who is lost from the herd.3 On the other hand, the dominant males chose to live with the pregnant females and adjust to the foraging patterns of the female colony. We may expect that the appropriate hormones which are responsible for sociality in each herd will retain a degree of opposite-sex attraction and, therefore, the seasonal changes are less pronounced than in the previous category. The data will show that this is indeed the case. In the book, I have identified one specific hormone that fulfills the role of controlling sociality for both categories of sociality described above. It responds to the needs of the animal by synchronizing precisely both in magnitude and in time with the mating period and with the separate social one. In the next blog, I will discuss the role of hormones in the sociality of females. 1Perez-Barberia, F. J., Robertson, E., & Gordon, I. J. (2005). Are social factors sufficient to explain sexual segregation in ungulates?. Animal Behavior, 69, 827-834. 2Michelena, P., Gautrais, J., Gerard, J. F., Bon, R., & Deneubourg, J. L.(2008). Social cohesion in groups of sheep: effect of activity level, sex composition and group size. Applied Animal Behavior Science, 112, 81-93. 3McDonnell, S. M., & Murray, S. C. (1995). Bachelor and harem stallion behavior and endocrinology. Biol Reprod Mono, 1, 577-590. Read the full article