Alternative Mating Strategies of Animals

Animal Behaviour Essay

Reproductive success can be defined as a mechanism which allows the passing of genes from one generation to the next in such a way that the offspring will too successfully pass on these genes. When male reproductive success depends on male-male competition and aggression, as is usually the case in polygamous species, individuals who are at a competitive disadvantage sometimes adopt an entirely different constellation of reproductive behaviours. In most cases, individuals practice only a single reproduction option throughout their lifetime (Kelly R. Zamudio, 2000). However, when such alternative mating patterns are practiced as part of a developmental sequence, they can be considered parts of a single lifetime reproductive strategy (Kelly R. Zamudio, 2000). Alternative Mating strategies have long fascinated behavioural biologists. A system in which not all males are equal in physical attributes (phenotypes) and have therefore developed an alternative strategy in order to pass on individual genes by producing offspring (Kelly R. Zamudio, 2000). We hear of sneakers, satellites, mate guarding and more. Alternative mating strategies shed light on fundamental evolutionary processes (Kelly R. Zamudio, 2000). How can sexual selection overcome the combined forces of natural selection on males and female who oppose it? Highly modified male phenotypes are well known to impose great survival costs upon the males that possess them (Kelly R. Zamudio, 2000). How is it that these extreme male variants, as well as the females that mate with or produce them, are not simply eliminated by natural selection outright (Kelly R. Zamudio, 2000). Alternative mating strategies invade a population when relatively few conventional individuals secure mates. There are two types of alternative strategies exist. First, phenotype differences through polymorphism and second, genetically determined alternative strategies (Kelly R. Zamudio, 2000).

The side-blotched lizard, (Uta stansburianathe), is a small common territorial lizard that is widely distributed in North America. Males of this species are highly territorial and their mating system has been described as resource-defence polygyny, with large territory holders gaining access to females whose home ranges are included within their territory (Stanley F. Fox, 2003). Some populations of this species in the coastal range of California exhibit a curious combination of alternative states that has been described as a rock paper scissors game (Stanley F. Fox, 2003). Three alternative strategies interact in a system that has no single winner; instead each male has strengths that allow it to out- compete one other strategy, but weaknesses that leave it vulnerable to tactics of the third (Stanley F. Fox, 2003). Orange-throated males are aggressive, have higher levels of testosterone, and vigorously defend large territories, which presumably affords them access to larger number of females (Stanley F. Fox, 2003). Blue-throated males are also territorial but mate guarders and stay with their females after copulating. Mate guarding may prevent their female from copulating with other males, however this behaviour interferes with territorial defence, and potentially limiting access by blue males to additional mates (Stanley F. Fox, 2003). Yellow-throated males are non- territorial of other mates and copulate with their females. In order to do this sneakers not only behave surreptitiously to avoid detection, but also rely on female mimicry, their throat and dorsal coloration are most similar to patterns found in females (Stanley F. Fox, 2003). Each male has specific behavioural attributes that allow it to out compete only one of the other males orange-throated males are able to outcompete the blue-throated mate guarders through aggression (Stanley F. Fox, 2003). On the other hand mate-guarding of blue males allows them to out-compete yellow sneakers, effectively deter sneakers from copulating. Yellow throated sneakers have been most successful at mating with the orange throated females in that territory (Stanley F. Fox, 2003).

This is a genetically based system requires very specific evolutionary conditions (Stanley F. Fox, 2003). Previous behavioural estimates of fitness found that the three males exist in an evolutionary stable state. Negative frequency-dependent selection maintains each phenotype in the population and all three males may have equal fitness (Stanley F. Fox, 2003). Fitness of sneakers will be highest when orange males are present in large numbers, because this should offer sneakers ample opportunity to sneak copulation from within territories of these males (Stanley F. Fox, 2003). Long term fitness of strategies must average the frequency-dependent fitness of each morph at all stages of cycle and across the entire population (Stanley F. Fox, 2003). This requires long term paternity data sets. Analysis for local frequency-dependent processes that underlie these patterns provides a more rapid assessment of the focus maintaining alternative strategies in this population (Stanley F. Fox, 2003). Demonstrating frequency-dependent selection provides a better estimate of the global stability of the system is maintained by negative frequency dependence in which rare morphs have a fitness advantage (Stanley F. Fox, 2003). The scale at which these behavioural interactions occur is the level of competing groups of neighbouring males, in that the exact composition of males within a neighbourhood is expected to determine the fitness of all males within that group. For example the success of any one male should depend on the number of the other two males that come in direct competition with him (Stanley F. Fox, 2003).

Another species that exhibits alternative mating strategies within species is the fallow deer. Males of this species may adopt alternative mating strategies within single populations (Thirgood, 1990). There are several explanations for this; Thirgood declared that first, a particular strategy may be optimal under certain environmental or social conditions (Thirgood, 1990). Secondly the best strategy for an individual to adopt may depend upon the strategies adopted by other males in the population (Thirgood, 1990). Third, males may simply be making the best of a bad situation, because they are incapable of competing with other males to gain maximum access to females (Thirgood, 1990). Fallow deer follow three types of mating strategies the first of which is the pursuit of a non-territorial strategy. The second strategy is to defend one single territory which may or may not contain resources desired. The last strategy is to defend multiple or what are known as lek territories (Thirgood, 1990).

Lek breeding is an uncommon mating system that has only been described in five ungulate species, including the Uganda and white-eared kob and the fallow deer (Thirgood, 1990). In lek breeding species males conjugate on small clustered mating territories, which females visit solely for the purpose of copulation (Thirgood, 1990). Males do not provide paternal investment other than gametes and their territories do not contain resources required by females other than the males themselves (Thirgood, 1990). Lekking rarely appears as a pure strategy within populations, but usually as an alternative to single territory defence. Most studies of lek breeding ungulates have suggested that males holding territories on the lek have much higher mating rates than those holding single territories away from the lek, and that these latter males are simply poor competitors (Thirgood, 1990). Fallow deer in the Blackensford region of the New Forest form part of a managed population of approximately 2000 animals inhabiting an area of mixed deciduous and coniferous woodland, heartland, bogs and grasslands (Thirgood, 1990). Mating is highly seasonal, taking place largely in the half of October, although occasional copulations have been recorded before and after that time. In the New Forest population, adult male and female fallow deer are spatially segregated for much of the year (Thirgood, 1990). A small lek of three to seven mature males would be situated on the boundary of two distinct woodland habitats. A lek was observed and copulations were recorded (Thirgood, 1990). Permanently defended single territories were established in areas close to the lek. These territories occurred in a single variety of woodland habitats ranging in resources from plentiful to absent, and are representative of the great variability of single territory defence in fallow deer (Thirgood, 1990). Non-territorial reproductive behaviour has been described previously from a number of wild and enclosed fallow populations (Thirgood, 1990). At Blackensford this consisted of males simply following groups of females, or behaving as satellites by intercepting female movement (Thirgood, 1990). The estimated mating success on a single territory was higher than that reported for other lekking populations of ungulates (Thirgood, 1990). Data suggests that within a given year successful lek males attain higher mating success than single territory males, who in turn get more mating than unsuccessful lek males (Thirgood, 1990). Males are not limited to one strategy, and are capable of switching strategies as mating opportunities dictate. It was concluded that because fighting is more common on the lek, lower costs of single territory defence may result in a longer reproductive life (Thirgood, 1990).

Another species that displays alternative mating strategies is a type of Bee known as Centris Pallida. Dimorphism is common in the male mating behaviour, of a large anthophorid bee (John Alcock, 1977). Bees of this genus have been relatively little studied, although previous studies have found males of various species established territories primarily by orchids or around flowering trees in tropical, central and south America and in Jamaica males of C. Pallida dig up buried females and males, mating with the former (John Alcock, 1977). Mate-location, techniques, size variation, and the fitness of mates as well as the parental investments of their mothers are intimately related to one another (John Alcock, 1977). Males of this Bee fall into one of two classes when searching for females. They are either patrollers, cruising rapidly near the ground in areas in which females are emerging, or they are however, poised at aerial stations around shrubs and trees (John Alcock, 1977). Patrollers search for sites at which a buried virgin female is about to emerge, upon finding such a spot, the male is capable of digging through 1-2cm of soil to the hidden female, which is usually mated by the male that uncovers her (John Alcock, 1977). Patrollers often attempt to appropriate a digging spot that another male has discovered or to separate a male from a freshly captured female (John Alcock, 1977). The second group of males, known as the hoverers, wait at sites peripheral to open emergence areas, generally by plants, whether these are flowering or not (John Alcock, 1977). Alternatively they wait at flowering shrubs or trees located well away from major emergence sites. These bees hover in the air with their hind legs dangling and held higher than the abdomen (John Alcock, 1977). They dart off rapidly in pursuit of all passing insects about their size and then almost always quickly return to their hovering station, although they may drift rather slowly over an area 1-2m in diameter (John Alcock, 1977). These individuals do chase neighbouring and intruding however commonly but they almost never make physical contact. As a general rule, 1m or so separates hovering males (John Alcock, 1977). It is difficult to say whether this spacing stems from aggressive interactions among males avoiding one another, seeking unoccupied scanning locations (John Alcock, 1977). Hoverers will pursue females that are collecting pollen but rarely grasp these individuals, which implies that they seek to secure only virgin females and can discriminate these from already mated bees. Hovering bees apparently are waiting for virgin females that have avoided capture by patroller-diggers (John Alcock, 1977).

It has long been known that males in a wide variety of animal populations practice alternative mating strategies in order to maximize their reproductive fitness. This is especially common when there is male-male competition for access to mates. In cases where such alternative strategies are as successful at obtaining mates as the predominant strategy, a coexistence of different mating strategies will evolve. The importance of alternative mating strategies is widely underestimated and often misunderstood. Polymorphic mating phenotypes provide quantifiable examples of intense frequency dependant sexual selection and its rapid evolutionary consequences. in the case of the blotched- lizard it has been shown that frequency-dependent selection arising from local competition can promote conditions that favour each individual male, and thus preserve all three strategies of the rock-paper-scissors cycle in the long term. Condition-dependent behaviour in the context of mating may result from changes in resource availability and intra-sexual competition for mates. When competition decreases, the expression of alternative behaviours also decreases. Changes in mating behaviours, especially among alternative males, have been documented in insects, fish, and amphibians upon removal of dominant males. Additionally, the availability of mates and resources also affects the expression of alternative strategies within a sex. The gain or loss of territory has been shown to affect mating approaches among insect species, while the receptivity and spatial distribution of mate’s impacts tactics used among insects, fish, and mammals. Mating behaviours are also affected by an individual’s size and age, as smaller or younger individuals are more likely to attempt reproduction through alternative means, including mimicry or sneak tactics. As a result, the ability to choose a behaviour that maximizes fitness under certain circumstances evolves.

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