Problem 17 - Animal Behavior: An Evolutionary Approach

Question

Birds are not the only animals in which intense, and sometimes fatal, sibling conflicts are known to occur. For example, spotted hyena females (see page 68 ) often give birth to twin pups, which compete aggressively, even lethally, for their mother's milk. Develop one or more adaptationist hypotheses about siblicide, and then make use of the following four findings: (1) the total amount of milk given to pairs of offspring in which siblicide eventually occurs is lower than from mothers with surviving twins, (2) females do not reduce the amount of milk they provide after siblicide has occurred, (3) siblicide is more common when females have to travel great distances in search of prey, and (4) females sometimes separate fighting twins and may preferentially nurse the subordinate cub. 35,110

Step-by-Step Solution

Answer

Siblicide in hyenas may optimize offspring survival by focusing resources on the most viable cub, especially when resources are scarce.

Step 1: Identify the Problem

The exercise asks us to develop adaptationist hypotheses about siblicide, a behavior where siblings compete aggressively, sometimes leading to the death of one sibling, specifically in spotted hyenas.

Step 2: Examine the Findings

The exercise provides four critical findings: (1) less total milk is given to pairs with siblicide, (2) no reduction in milk after siblicide, (3) siblicide is more common when mothers travel far for prey, and (4) mothers sometimes intervene and prefer the weaker cub.

Step 3: Hypothesize Based on Energy Scarcity

Given that siblicide is more frequent when milk supplies are limited and when the mother travels distances, an adaptationist hypothesis could be that siblicide is a strategy to ensure that the surviving sibling gets enough resources for survival in resource-scarce environments.

Step 4: Consider Parental Investment

Another hypothesis can be that by allowing siblicide, mothers optimize their parental investment by ensuring that the offspring with the best survival chance receives the full benefit of their resources. This aligns with the observed behavior of not reducing milk post-siblicide.

Step 5: Evaluate Maternal Intervention

The fact that mothers sometimes intervene to protect the weaker sibling suggests a selective investment strategy, possibly to maintain genetic diversity or in case the dominant sibling later becomes nonviable.

Step 6: Synthesize Hypotheses

Combining these, the adaptationist hypothesis could be that siblicide serves as a natural selection mechanism to allocate scarce resources to the most viable offspring, but mothers intervene when there's a chance of optimizing combined fitness outcomes.

Key Concepts

Animal Behavior

In the animal kingdom, behavior can be seen as a series of complex responses to environmental and social stimuli. These behaviors have evolved to maximize survival and reproductive success. Siblicide, or the act of a sibling killing another, is a particularly interesting behavior observed in numerous animal species, including birds and mammals like spotted hyenas. This behavior can occur due to intense competition between siblings for limited resources such as food and parental care.

Siblicide is often regarded as a mechanism to increase the survival likelihood of the stronger offspring, especially under resource-scarce conditions. This behavior appears cruel from a human perspective, but in nature, it serves to enhance the reproductive success of the parents by ensuring that at least one offspring survives to maturity.

• Siblicide reduces competition for resources like milk or food.
• It ensures the survival of the stronger, often more viable offspring.
• This can be advantageous in harsh environments where resources are limited.

In spotted hyenas, this behavior aligns with how mothers adjust their care, sometimes intervening to balance the scales when necessary.

Evolutionary Biology

Evolutionary biology seeks to understand how organisms have adapted over time through natural selection and other mechanisms. Siblicide can be viewed through this lens as a strategy that has evolved to enhance reproductive success. The aggressive competition among siblings, leading to siblicide, may have developed to counter environment-induced resource limitations, aiding in the adaptation of species like the spotted hyena.

This behavior reflects the reality of survival of the fittest in nature. Those individuals with advantageous traits (e.g., strength, aggression) tend to reproduce and pass those traits to their offspring. In the context of siblicide, it might be seen as an evolutionarily advantageous trait because it ensures that the siblings who survive are those who are capable of thriving in challenging conditions.

• It is a natural selection mechanism where stronger genes are passed on.
• Siblicide could be viewed as a trigger for evolutionary change, ensuring species adapt well to their environment.
• The behaviors determining survival are reinforced through successive generations.

Parental Investment

Parental investment refers to any parental expenditure (time, energy, resources) that benefits the offspring. The choices animals make about parental investment often determine the reproductive success of both parents and offspring. In cases of siblicide, parental investment reflects an optimization strategy. Spotted hyena mothers, for example, may allow siblicide to occur to allocate their limited resources efficiently.

The key is to ensure that their investment leads to at least one offspring reaching maturity. If resources are scarce, it's more beneficial for the parents to concentrate their investment on one offspring rather than stretching their resources too thin. This not only increases the survival odds for the chosen offspring but maximizes the parent's reproductive success in terms of genetic legacy. Mothers' interventions indicate strategic decisions in complicated scenarios to optimize resource distribution.

• Concentration of resources on fewer offspring can be more successful under scarcity.
• Ensuring survival of at least one offspring helps perpetuate genetic inheritance.
• Mothers adjust their investment based on situational needs and offspring viability.

Resource Competition

Resource competition is a significant driving force in the natural world, frequently shaping animal behaviors like siblicide. When resources such as food or territory aren't abundant, competition becomes fierce, often leading to siblicide. The findings about spotted hyenas illustrate how resource scarcity can intensify competition between siblings.

The concept of resource competition involves direct conflict for access to limited resources essential for survival. In environments where a mother must travel extensive distances for food, the energy and resources available become even more precious, possibly leading to siblicide as an extreme form of competition. Understanding resource competition helps explain why such drastic behaviors are not just possible but sometimes necessary for survival.

• Resource scarcity leads to intensified competition among siblings.
• The environment's harshness can dictate the level of competition.
• Siblicide as a behavior arises from the pressure to outcompete for resources.

These behaviors highlight the intricate balance of life and survival strategies that have evolved over time.