This entry was written by Rowena McPhee as part of a project done in BIAN 2133 ‘Human Reproductive
Strategies’ at The Australian National University in 2019 Semester 2.
Introduction
Many discussions in evolutionary biology focus on the topic of parental care and resource allocation. In iteroparous species, families may have offspring of different ages or stages of development as a result of asynchronous reproduction. The marginal returns associated with investing resources in a younger offspring are different compared to the marginal returns of investing resources in an older offspring; therefore, resources may be allocated unequally between siblings so as to maximise fitness. Throughout this essay, I will discuss the phenomenon of age biased parental care, and the different fitness payoffs associated with offspring of different ages. I will then examine a model used to predict whether age bias will move in favour of older offspring or younger offspring.
Main Text
Resource Allocation
In an environment of limited resources, individuals must make decisions regarding how to allocate those resource among family members; often times, resource allocation responsibilities fall upon one or both of the parents. Therefore, parents must make decisions about how to allocate resources between themselves and their offspring, as well as how to allocate resources between each of their offspring, who may be of varying quality or condition. Decisions about how to allocate resources and parental care are based on information about the external and internal conditions of an offspring (Rivers, 2007). External conditions include the offspring’s age, sex, and probability of survival to reproductive age. Internal conditions include the offspring’s need for additional resources; this may be communicated via begging. Offspring will experience greatest fitness when they receive more resources, and so they often demand, or beg for, more than the parent is able to provide (Schlomer, 2011). Differences in internal and external condition may result in differential resource and parental care allocation between offspring of the same family; a pertinent example is the biased resource allocation to siblings of different ages. The phenomenon of age biased resource allocation can only occur in iteroparous or asynchronous species, and as such, it has been omitted from many models which focus on semelparous or synchronous species.
Differential Payoffs
The trend of age biased parental care occurs because the payoffs associated with allocating additional resources to younger offspring are different to those associated with allocating those resources to older offspring (Jeon, 2008). In general, younger offspring experience greater marginal returns for every additional unit of resource allocated when compared to older offspring; as such, it may be preferable to allocate resources to younger offspring in order to better ensure their survival and gain maximised marginal returns. On the other hand, older offspring have greater reproductive value as a result of being closer to reproductive age; therefore, it may be preferable to allocate resources to older offspring to better ensure that they survive to reproductive age and redeem their high reproductive value. Both of these perspectives have a strong evolutionary basis, and it is easy to argue in support of parental bias towards younger or older offspring. However, evidence suggests that older offspring tend to receive more resources than their younger siblings; that is, resource and parental care allocation is biased in favour of older siblings (Aviles et al., 2011).
Jeon's Model
In their paper on aged bias parental care, Jeon (2008) tested models of optimal resource allocation under two sets of circumstances. Firstly, circumstances in which younger offspring yield higher marginal returns for every additional resource provided, regardless of how many resources they already have. Alternatively, circumstances in which younger offspring yield higher marginal returns for additional resources provided, but only if there are no or few resources already provided. The predictions supported by this model indicate that parents should bias their resource allocation in favour of older offspring in almost all cases; that is, in both of the given circumstances, older offspring should be favoured over younger offspring. The model also suggests that the degree to which older offspring should be favoured over younger offspring increases as the age gap between the siblings increases.
Some traditional theories on the topic of age biased parental care suggest that parents should allocate resources such that the marginal returns for each offspring are equal; however, Jeon’s model also considers the impact of probability of survival to reproductive age. As such, it is predicted that resources will not be allocated so the marginal returns of each offspring will be equal, but rather, that resources will be allocated so that the marginal returns of each offspring, weighted by their probability of survival to reproductive age, will be equal. Given that younger offspring gain higher marginal returns on any unit of additional resource allocated to them, older siblings must receive more resources in order to match the marginal returns of younger siblings, even when weighted by probability of survival to reproductive age. Therefore, parental care and resource allocation must be biased in favour of older offspring and against younger offspring in order to equalise the level of weighted marginal returns between the siblings. An evolutionarily stable strategy will occur when the marginal gains to each offspring, weighted by their probability of survival to reproductive age, is equal to the marginal cost to the parents’ survival of allocating resources to their offspring over themselves.
Conclusion
Decisions of resource allocation to offspring depend heavily on the quality and condition of those offspring, including their age. Compared to older offspring, younger offspring tend to have higher marginal returns on any unit of resource allocated to them; these differential returns prompt parents to allocate resources unequally between their offspring of different ages. Jeon’s model of resource allocation between siblings of different ages shows that age biased parental care will almost always occur in favour of older siblings, with younger siblings needing fewer resources to match their older sibling’s weighted marginal returns.
Reference List
Aviles, J. (2011). Parental favouritism strategies in the asynchronously hatching European Roller (Coracias garrulus). Behavioral Ecology and Sociobiology, 65(8).
Jeon, J. (2008). Evolution of parental favouritism among different-aged offspring. Behavioral Ecology, 19(2), 344-352.
Rivers, J. (2007). Nest mate size, but not short-term need, influences begging behaviour of a generalist brood parasite. Behavioral Ecology, 18(1), 228-229.
Schlomer, G, Ellis, B, & Giudice, M. (2011). Parent-Offspring Conflict Theory: An Evolutionary Framework for Understanding Conflict Within Human Families. Psychological Review, 118(3).