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INTRODUCTION – LIFE HISTORY
The term ‘life history’ describes an organism’s strategy of allocating time and energy between growth, reproduction and survival (Levin 2009; Bergon, Townsend and Harper 2008). Life history traits make up an organism’s life history and include growth patterns, size at birth, size and age at sexual maturity, the number, size and sex ratio of offspring, parental care and length of life (Levin 2009; Bergon, Townsend and Harper 2008). The effect of finite resource availability on organisms for potential investment into growth, reproduction, and survival, is to set limits on life history traits, often referred to as trade-offs (Levin 2009; Bergon, Townsend and Harper 2008; Cotgreave and Forseth 2008). If an organism commits more time and energy into one specific life history trait, it will come at a cost, a reduction of time and energy that may have been available to one or more other life history traits (Levin 2009; Bergon, Townsend and Harper 2008). An example of this often observed in relation to reproductive strategies (Forbes and Mock 2000). Many reproductive strategies have evolved but are always subject to tradeoffs; it is a case of quantity versus quality (Forbes and Mock 2000). Life history traits respond like any other phenotypic trait to natural selection and therefore represent adaptations that have been made by organisms to their environments (Levin 2009; Cotgreave and Forseth 2008).
The question; Offspring are difficult to produce and are a critical determinate of fitness. So how does siblicide and infanticide evolve as a life history strategy and are they adaptive?
The demise of an individual as a result of another directly related individual’s actions is referred to as siblicide (Anderson 1995; Godfray and Harper 1990; Anderson 1989). Siblicide may transpire among siblings or be brought about by the actions of parents (Hausfater and Hrdy 2008; Anderson 1995). Siblicide has been documented occurring in plants, fish, insects and mammals but has been best observed in avians (Holcomb 2001; Mock and Parker 1997). There are many hypotheses surrounding siblicide and why it occurs (Godfray and Harper 1990). Siblicide may take place as a direct form of assault on another sibling through physical attacks or expulsion from the nest, or may transpire indirectly in the form of exclusion from food by competition which results in starvation (Hausfater and Hrdy 2008). There are two forms of siblicide known as obligate and facultative siblicide (Hausfater and Hrdy 2008; Anderson 1995; Anderson 1988).
The Brood Reduction hypothesis best supports facultative siblicide as an adaptive reproductive strategy that benefits both parents and surviving offspring in response to the event of unpredictable resource shortages (Forbes and Mock 2000; Mock and Parker 1997). The strategy is to hatch as many offspring as would normally be expected if conditions were optimal (Hausfater and Hrdy 2008; Forbes and Mock 2000; Anderson 1995). In the event that environmental conditions are severe and the resources available are unable to meet the demands for all of the offspring to survive, sibling rivalry ensues and is anticipated to become fatal resulting in ‘brood reduction’ (Hausfater and Hrdy 2008; Forbes and Mock 2000; Mock and Parker 1997). The reduction in brood size will then allow the parents to successfully raise the remaining nestlings (Hausfater and Hrdy 2008; Forbes and Mock 2000; Godfray and Harper 1990). Blue-footed boobies are well known for facultative siblicide (Anderson 1995; Anderson 1988). They are capable of laying between one and four eggs, the average clutch size though is usually two (Anderson 1995). When resources are plentiful all of the chicks are hatched, and fledge, however, should resources be scarce, the first hatched chick will dispatch the younger siblings (Anderson 1995).
Obligate siblicide is the occurrence of siblicide regardless of resource abundance (Anderson 1989). Parents regularly produce more offspring than they can successfully fledge and it is the case that one the first born nestling will eliminate the second nestling soon after hatching (Hausfater and Hrdy 2008; Forbes and Mock 2000; Anderson 1989). The hypothesis that best supports obligate siblicide is the Insurance Egg hypothesis (Hausfater and Hrdy 2008; Forbes and Mock 2000; Anderson 1989). The Insurance Egg hypothesis is the theory that parents actually produce a second egg specifically as a backup, in the event that the first egg fails or succumbs to predation (Hausfater and Hrdy 2008; Forbes and Mock 2000; Anderson 1989). The cost of the second egg is essentially less to the parents than the benefit of producing that insurance egg should the first egg fail (Forbes and Mock 2000; Anderson 1989). This overproduction is an adaptive response to the insecurity of offspring survival or viability (Forbes and Mock 2000; Anderson 1989. In general, very few species of bird commit obligate siblicide (Anderson 1989). The masked boobies and brown boobies are two obligately siblicidal species (Anderson 1995; Anderson 1989).
Blue-footed boobies, masked boobies and brown boobies all utilize the same life history trait of asynchronous hatching (Anderson 1995; Anderson 1988). There are differences however, between the asynchronous hatching times as a result of facultative or obligative siblicide (Anderson 1995; Anderson 1988). The length of asynchronous hatching is shorter in the blue-footed boobies than the masked or brown boobies, mainly due to the form of siblicide (Anderson 1988). The effect of asynchronous hatching on the nestlings is conferred in age, size and hierarchy of the first hatched nestling over the second or third nestling and therefore, the offspring hatched first has the competitive advantage over the later hatched offspring as a result (Anderson 1988). In regard to facultative siblicide, this manipulation by the parents in staggering hatching times can be viewed as a strategy to counteract the uncertainty of resource availability by bestowing the competitive advantage on the first hatched nestling should brood reduction become necessary in the event of a shortage of resources (Hausfater and Hrdy 2008; Anderson 1988). In regard to obligative siblicide, the first hatched chick will inevitably commit siblicide and therefore the advantages of being first born come into play (Hausfater and Hrdy 2008; Anderson 1989).
The term infanticide can be described as the killing of dependent offspring by individuals belonging to the same species (Hausfater and Hrdy 2008; Hiraiwa-Hasegawa 1988). Infanticide is not limited to the killing of unweaned offspring, it can occur during the reproductive cycle, for example re-absorption of the embryo or abortion, and can be committed by females and males as well as offspring or other members within the social group (Hausfater and Hrdy 2008). Infanticide has been observed in mammals, including several primate species and lions (Hausfater and Hrdy 2008; Hiraiwa-Hasegawa 1988; Packer and Pusey 1983). The act of infanticide is an adaptive behaviour strategy to enhance individual fitness (Hausfater and Hrdy 2008; Agrell and Wolff 1998; Hiraiwa-Hasegawa 1988; Packer and Pusey 1983). Infanticide of unrelated infants committed by males due to reproductive competition is supported by the sexual selection hypothesis (Kappeler and van Schaik 2004; Borries et al. 1999; Agrell and Wolff 1998; Hiraiwa-Hasegawa 1988). Under this hypothesis, infanticidal males secure mating opportunities and increase their chance of siring infants and therefore gain a reproductive advantage and increase fitness (Borries et al. 1999).
African lions (Panthera leo) and hanuman langurs (Presbytis entellus) both live in groups consisting of one dominant male and a number of females (Hiraiwa-Hasegawa 1988). The residence time of the dominant male is usually short, approximately two years (Hiraiwa-Hasegawa 1988). Infanticide occurs as a consequence of a group takeover, one male gaining control of another’s group (Hausfater and Hrdy 2008; Kappeler and van Schaik 2004). As a result of short term dominance status of males in these instances, it is to the advantage of the usurping male to dispatch of infants within the group so he can take full advantage of the females reproductive career (Kappeler and van Schaik 2004; Borries et al. 1999). Both female langurs and lions share a common life history trait, they are almost always ready to resume sexual activity and begin reproducing after the loss of their unweaned infant; much earlier than they otherwise would if they still had care of their dependant offspring (Hausfater and Hrdy 2008; Kappeler and van Schaik 2004). There is evidence to show that female primates and lions swiftly revert to estrus after the loss of their unweaned infant (Hausfater and Hrdy 2008; Kappeler and van Schaik 2004; Borries et al. 1999; Packer and Pusey 1983). Whilst females are lactating, they are effectively unresponsive to further reproduction (Hausfater and Hrdy 2008; Packer and Pusey 1983). Therefore, the act of infanticide and the quick return of estrus as a result, ensures the females bear infants to the usurper much sooner than if the females had surviving infants (Hausfater and Hrdy 2008; Kappeler and van Schaik 2004; Borries et al. 1999; Agrell and Wolff 1998).
Infanticide is undoubtedly a major disadvantage for female reproductive success even though it may well be an adaptive behavioural strategy for male reproductive success (Hausfater and Hrdy 2008; Kappeler and van Schaik 2004; Packer and Pusey 1983). As such females have developed counter strategies in an attempt to decrease their reproductive losses as a result of infanticidal males after a takeover has occurred (Hausfater and Hrdy 2008; Kappeler and van Schaik 2004; Agrell and Wolff 1998; Packer and Pusey 1983). Research into these behavioural strategies has revealed a range of different tactics employed by females in an effort to rescue their infants from almost certain death (Hausfater and Hrdy 2008; Kappeler and van Schaik 2004; Agrell and Wolff 1998; Packer and Pusey 1983). Some of the strategies employed may include leaving the group, sometimes in the company of the deposed male, aggressively defending the infant from attacks by the new male, repeatedly mating with the new male to confuse paternity of already pregnant females at the time of takeover, pseudo-estrus of already pregnant females or in some cases, the termination of an early pregnancy to avoid the inevitable (Agrell and Wolff 1998; Packer and Pusey 1983).
How and why siblicide and infanticide life history strategies have evolved has been the subject of great debate for many years. The results of studies conducted provide quite a lot of evidence in support of the different theories for both siblicide and infanticide being adaptive life history strategies (Kappeler and van Schaik 2004; Forbes and Mock 2000; Borries et al. 1999; Mock and Parker 1997; Agrell and Wolff 1998; Hiraiwa-Hasegawa 1988). In consideration of siblicide, studies conducted provide evidence in support of theories that facultative siblicide has evolved in response to the unreliability of resources and, obligate siblicide has evolved due to the uncertainty of survival or viability of offspring. Where infanticide is concerned, evidence favours the theory that evolution of this life history strategy is predominately an adaptive male behavioural strategy to increase reproductive success in response to short term dominant male status within groups. The act of infanticide increases the males’ chance of successfully siring offspring while they hold the ‘dominant male’ status. In response, females have evolved counter-strategies to reduce the impact of infanticide on their reproductive success.
In conclusion and in consideration of the evidence available, both siblicide and infanticide life history strategies are adaptive even though they seem to be contradictory to the success of reproduction.
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