Abstract
Intra-specific competition occurs in all animal species and can lead to escalated conflict. Overt fighting entails the risk of injury or death, and is usually avoided through the use of conventions or pre-fight assessments. However, overt fighting can be expected when value of the contest outweighs the value of the future, as contestants have little or nothing to lose. In these situations, respect for conventions and asymmetries between contestants can break down, and overt fighting becomes more likely (the desperado effect). Such conditions can arise in contests between queens over colony ownership in social insects, because the value of inheriting a colony of potentially thousands of helpers is huge and queens may have very limited alternative reproductive options. However, in social species the balance of possible outcomes may be influenced by inclusive fitness, as contestants are often relatives. Here we present a simple model based on social insects, which demonstrates that not fighting can be selectively advantageous when there is a risk posed by fighting to inclusive fitness, even when not fighting is likely to result in death. If contestants are related, a loser can still gain indirect fitness through the winner, whereas fighting introduces a risk that both queens will die and thereby obtain zero inclusive fitness. When relatedness is high and fighting poses a risk of all contestants dying, it can be advantageous to cede the contest and be killed, rather than risk everything by fighting.
Highlights
Intra-specific contests, primarily over resources and reproductive opportunities, are a ubiquitous aspect of animal behaviour
Our model was developed to explain behaviour observed in social insect colonies, and we initially focus on these systems
The model We define our contest arena as a nest containing two queens and associated workers. This matches the experimental data on A. senilis, where the majority of contests during queen replacement are between two queens [20]
Summary
Intra-specific contests, primarily over resources and reproductive opportunities, are a ubiquitous aspect of animal behaviour Such contests are highly significant as they can determine a large proportion (or all) of an animal’s lifetime fitness, and pose the risk of injury or death when they escalate to fights. Related contestants in ‘desperado’ contests, such as late-born A. senilis queens, may secure more indirect fitness by not fighting than direct fitness from victory when the latter is devalued by the risk of both queens dying [21] We formalise this reasoning by developing a simple model to explain how not fighting can be selectively advantageous in a ‘desperado’ contest between relatives. Obligate monogyny in social insects is widespread, despite the fact that polygyny can be favoured for a variety of reasons including increased colony productivity, increased colony longevity, and high mortality of dispersing queens [[27], pp 126-134]
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