Problem 13 - Animal Behavior: An Evolutionary Approach

Question

Mate guarding should be common in species in which females retain their receptivity after mating and are likely to use the sperm of their last mating partners when fertilizing their eggs. But there are species, including some crab spiders, in which males remain with immature, unreceptive females for long periods and fight with other males that approach these females. \(^{59}\) How can "guarding" behavior of this sort be adaptive? Produce sexual selectionist hypotheses and some predictions derived from them.

Step-by-Step Solution

Answer

Guarding unreceptive females ensures males are first to mate, providing reproductive advantages in competitive environments.

Step 1: Understanding the Context

In some species, males guard females who are not yet ready to mate to ensure that they are the first and sometimes only mating partner. This behavior might seem less intuitive when females are not reproductively receptive since it includes long investments of time and energy without immediate mating benefits.

Step 2: Develop Hypotheses

Develop the hypothesis that mate guarding of unreceptive females is adaptive because it increases the probability that the guarding male will mate with the female when she becomes receptive, thereby passing on his genes.

Step 3: Elaborate Hypotheses with Sexual Selection Theory

Consider sexual selection theories, namely the operational sex ratio and mate competition. In scenarios with more males than receptive females, securing a mate even before she is ready ensures exclusivity and prevents other potential male competitors from mating with her.

Step 4: Make Predictions

Predict that males who guard unreceptive females will have higher reproductive success compared to those who don't, due to reduced male competition upon the female's maturation. Another prediction is that guarding behavior increases in environments where females are fewer or mate competition is intense.

Step 5: Consider Evolutionary Stability

Evaluate that over time, guarding behavior can be stable if the benefits (like assured mating) outweigh the costs (time and energy spent) given the competitive context that might otherwise limit mating opportunities.

Key Concepts

Sexual Selection

In the fascinating world of sexual selection, different strategies have evolved to increase the odds of reproductive success. Mate guarding is one of these strategies, where males stay close to females to increase their chances of mating.
This tactic is part of the broader realm of sexual selection, which encompasses all traits that help an individual to mate and produce offspring. These traits could involve physical attributes, such as a peacock’s colorful feathers, or behavioral strategies, like mate guarding observed in some crab spiders.

• Mate guarding involves behaviors that aim to increase mating opportunities by ensuring a male is the sole or first partner.
• This increases the chances that a male's genetic material will be used in fertilizing the female's eggs.

Understanding sexual selection helps us see why certain seemingly counterintuitive behaviors have evolved and why they might persist across generations.

Reproductive Success

Reproductive success is the ultimate goal of any mate guarding behavior. This refers to the ability of an organism to pass on its genes to the next generation. In the context of the exercise, males engage in mate guarding to boost their reproductive success by ensuring they can mate when the female becomes receptive.
Mate guarding is an investment. Although it requires time and energy to protect a female, once she is ready to mate, the guarding male has a higher likelihood of being her partner.

• Passing genes on to the next generation is essential for evolutionary fitness.
• Mate guarding can increase reproductive success, leading to a stable strategy where benefits outweigh the costs.

By focusing on the long-term benefits, organisms employing these strategies can boost their chances of survival and the propagation of their genes.

Operational Sex Ratio

The concept of the operational sex ratio (OSR) plays a critical role in mate selection and competition. OSR is the ratio of sexually receptive males to sexually receptive females at any given time.
Mate guarding behaviors are particularly influenced by the OSR when males outnumber receptive females. In such scenarios, the competition increases, and strategies like mate guarding become more common.

• If there are more males than receptive females, fierce competition dictates that males use strategies to stand out.
• Securing a female early can be advantageous, reducing the likelihood of losing mating opportunities to rivals.

The OSR provides insight into why certain behaviors, which might seem inefficient, actually provide a selective advantage in a crowded mating market.

Mate Competition

Mate competition is another driving force behind behaviors like mate guarding. When many males vie for the attention of few females, strategies that secure mating opportunities become valuable.
Mate competition can drive males to invest in tactics that assure they are present when a female becomes receptive, thus increasing the chance of reproductive success.

• In environments with high mate competition, males may even engage in fights to access potential mates.
• Guarding an unreceptive female acts as a preemptive move against competitors, ensuring a mating opportunity when she is ready.

By understanding mate competition, we can better comprehend the investments made by males in securing mates, even when the immediate benefits are not apparent.