Mammalogy Lecture 15 - Social Behavior II: Evolution

Transcription

1 Mammalogy Lecture 15 - Social Behavior II: Evolution I. Evolution of Social Behavior In order to understand the evolution & maintenance of social behavior, we need to examine costs & benefits of group living. A. Costs of group living. 1. Increased conspicuousness - crypsis is not an available means of predator avoidance. 2. Increased intraspecific competition for food and mates, especially true in cases with dominance hierarchies (Papio example). 3. Increased risk of parasitism/diseases (e.g., spread of WNS through bat colonies). 4. Increased potential for inbreeding and potential inbreeding depression. B. Benefits of group living. 1. Selfish herd phenomenon. The risk of predation incurred by any single individual may be smaller in a herd than for solitary prey species. 2. Cooperative predator defense possible - Musk ox - Ovibos moschatus, where adults form a ring around young. 3. Cooperative hunting - Canis lupus - Panthera leo 4. Information sharing possible and cultural inheritance. In some primates, learned behavior may be passed between generations. 5. Division of labor possible. The cost/benefit ratio will determine whether or not group living will evolve. This will vary among species obviously, and we ll discuss some examples. This also may vary between genders within a species. Male gametes are cheap relative to the costly nature of female gametes, and this has implications for behavior. This explains why polygyny is so much more common than polyandry. Mate choice is usually driven by females. Manifestations of sexual selection are usually seen in males (e.g., antlers). It also may explain the predominant pattern of female philopatry and male dispersal in mammals. Females are typically philopatric. They often reproduce in the same population where they were born; is; it s a site that s proven to be adequate. Males tend to disperse between populations and seek new suitable habitat. For example, Hamilton et al. (2005) demonstrated that nearly all migrants in Microtus arvalis are males.

2 C. Social behavior often appears altruistic, and to benefit others at one's own expense. Reciprocal altruism (e.g., Desmodus rotundus, Carter & Wilkinson 2013) is rare (Silk 2013). Kin selection is a much more common cause of apparently altruistic behavior. A series of studies by Paul Sherman on alarm calls in Urocitellus (Spermophilus) beldingi addressed apparently altruistic behavior, behavior that enhances the fitness of other individuals at the expense of your own fitness. - These squirrels occur in grasslands in the inter mountain west and they form breeding colonies. - As predators approach, they ll give alarm calls and the rest of the colony will flee into burrows. - This really does appear to be altruistic behavior because Sherman has been able to show that there is a higher probability of predation on call givers. That is, the call attracts attention to the giver, who then suffers a higher risk of predation. This should be maladaptive, but Sherman hypothesized that kin selection could explain it In order to understand this, we need to understand a little about kin selection and inclusive fitness (Hamilton J. Theoretical Biology, 7:1-6.). Our own offspring share on average 50% of our genes. Therefore, we share 25% of our genes with our grandchildren. On average we share 50% of our genes with our siblings. Their offspring share 50% of their genes; thus, our nieces and nephews share 25% of our genes, the same as our grandchildren. I ll illustrate this on the board with two pedigrees.

3 So one way to optimize our genetic contribution to subsequent generations is to act in a manner that increases the success of our relatives. So we need to focus on inclusive fitness, not just direct fitness. This is the sum of direct fitness accrued through direct descendants plus indirect fitness accrued through relatives: Inclusive Fitness = Direct Fitness + Indirect Fitness. Another way to formalize this is through an equation known as Hamilton's rule. Social behavior will evolve under the following inequality: rb > c, where r is the measure of relatedness, b and c are benefits and costs, respectively. Thus, behaviors that increase the chance of success of relatives may be favored by selection, even if those behaviors reduce the direct fitness of the individual. This is Kin Selection, and Sherman hypothesized that this drove the evolution and maintenance of alarm calling in U. beldingi. In this case, females are philopatric. That is, females don t disperse and, therefore, a female is likely to reproduce with the same colony as she was born in (like M. arvalis). Males on the other hand do disperse among colonies and are therefore not likely to be related to colony members. A recent study indicated that all males eventually disperse. Kin selection theory would predict that only females give alarm calls, because only females will increase their inclusive fitness by doing so. This is exactly what the squirrels do; call rates are much higher in females than males. Furthermore, kin selection theory predicts that in colonies where females are more closely related genetically, there is a greater tendency to give alarm calls. That is, females must be able to discern relatives from non-relatives. This was actually born out by experiments. Females call less frequently when there are fewer close relatives in the colony. Females can ascertain relatedness of colony mates. D. Carnivores provide an example of the diverse array of benefits accrued by social behavior. Most Carnivora are solitary or asocial. This is probably the ancestral condition for the order (although this has been questioned recently). All ursids are asocial. Many other families are primarily asocial. Within each of these families, sociality has evolved, perhaps independently.

4 - Canids - Mustelids - Herpestids - mongooses - Felids - Hyaenids Canids - situation is very flexible in coyotes Canis latrans are social when there is much carrion and asocial when feeding on rodents. Canis lupus - family groups ---> cooperative hunting Vulpes vulpes - Cooperative hunting pairs Lyacon pictus (African hunting dogs) Cooperative hunting & communal feeding pups. Felids - Panthera leo - up to 4 breeding females, several non-breeding females, and - 2 males may or may not be related - females hunt cooperatively and are related - some males will forego breeding if they are related to the breeding males. - satellite males - belong to no pride and attempt sneak copulations Hyaenids - Crocuta crocuta - rather large groups of up to 80, both males and females. - These are clans, where females establish dominance hierarchy and males disperse. - They tend to forage alone (or in very small groups) - Societies probably form to defend kills from lions Mustelids - Meles meles - They forage alone and are rather omnivorous. - Live in communal dens, called setts. - A clan (family group) of up to two dozen animals occupy the setts - Setts are used for several generations - Clans defend a territory and share in burrowing work. Herpestids - Helogale Dwarf mongoose - Forage independently - Very complex social system - There are non-reproductive males and females, with only a single breeding pair. - There is coopoerative predator defense. - Give alarm calls and there s some division of labor A series of hypotheses have been erected to explain this repeated evolution of social behavior in carnivorans. These focus on cooperative hunting, predator defense, or defense of kills, really depending on the expertise of the researcher.

5 However, as we just illustrated, the benefits vary widely across species, and no hypothesis that emphasizes only a single aspect of sociality can provide a general explanation. Nevertheless, there may be a general initial catalyst: D. W. MacDonald (1983. Nature, 301:379) -- Resource Dispersion Hypothesis Reviewed by Johnson et al. (2002. TREE. 17:563); re-evaluated by Elbroch et al. (2016. J. Animal Ecol., 85: ) in Puma concolor. Common set of starting conditions: - Basic territoriality - Individuals defend the smallest area that will support them in a bad year - Size of the territory depends on dispersion of resources. - Key is that it s usually not a bad year and resources are readily available -- each resource patch may support more than one individual - Most of the time there is a very low cost to group formation --> benefits outweigh the costs. Type of social system that will evolve then depends on the biology of the particular organism.