Question

Suppose you were studying several species of monkeys. In one, males never helped females raise offspring. In another, males provided just as much parental care as females except for actually carrying the fetus during pregnancy. How does the fundamental asymmetry of sex compare in the two species? How would you expect sexual dimorphism to compare between the two species?

Short answer

The fundamental asymmetry of sex, which refers to the difference in parental investment between males and females, is greater in the first species of monkeys as males do not contribute to raising offspring, while it is smaller in the second species where males provide significant parental care. As a result, we would expect higher sexual dimorphism in the first species due to stronger competition among males for access to females, leading to larger differences in appearance and physical traits between males and females. Conversely, the second species would likely exhibit lesser sexual dimorphism, as the smaller asymmetry in sex and male parental investment reduces overall competition for mates.

Step-by-step solution

Understand the fundamental asymmetry of sex

The fundamental asymmetry of sex refers to the inherent difference in parental investment between males and females. In most species, females have a higher parental investment because they invest more time and resources in producing and caring for offspring (e.g., through pregnancy, egg-laying, and lactation). Males, on the other hand, typically invest less in individual offspring (e.g., through sperm production) and can potentially have more offspring through mating with multiple females. This asymmetry in parental investment shapes many aspects of mating behavior and sexual selection. #Step 2: Understand sexual dimorphism#

Understand sexual dimorphism

Sexual dimorphism refers to the differences in appearance and physical traits between males and females of the same species. It results from sexual selection and can lead to various features that differ between the sexes, such as body size, ornamentation, and coloration. In species where there is stronger competition for mates, sexual dimorphism tends to be greater because males and females have different strategies to maximize their reproductive success. #Step 3: Examine the parental care in the two species#

Examine the parental care in the two species

In the first species of monkeys, males do not contribute to raising offspring at all, while in the second species, males provide significant parental care (except for carrying the fetus during pregnancy). This means that the fundamental asymmetry of sex will be greater in the first species, where there is a larger difference in parental investment between males and females. In the second species, the asymmetry will be smaller since males contribute more to raising offspring. #Step 4: Compare the expected sexual dimorphism#

Compare the expected sexual dimorphism

Based on the differences in parental care between the two species, we can make predictions about the levels of sexual dimorphism. In the first species, where there is a greater fundamental asymmetry of sex, there is likely to be stronger competition among males for access to females since they have lower parental investment. This could result in a greater degree of sexual dimorphism with larger differences in appearance and physical traits between males and females (e.g., males may be larger, more brightly colored, and/or more ornamented). In the second species, with a smaller asymmetry in sex, the competition for mates might be less intense since males also invest in parental care. This potentially could lead to a lesser degree of sexual dimorphism, with more similar appearances between the sexes as the male's investment reduces the overall competition for mates.

Key concepts

Parental Investment

Parental investment is the amount of time and energy a parent gives to ensure the survival and well-being of their offspring. This can vary greatly across species and leads to many interesting questions in evolutionary biology. In many species, females are the primary caregivers, investing more due to factors like pregnancy and nursing. In contrast, males often invest less due to their ability to produce many offspring through multiple mating opportunities.

Interestingly, parental investment is not always the same in every species. In one example of monkey species discussed, males do not help at all with offspring raising, highlighting a significant difference in parental roles. In another example, males contribute equally to parental care, except during pregnancy. This sharing of responsibilities can lead to different evolutionary strategies and behaviors.

Typically, the level of parental investment is closely tied to reproductive success, shaping complex interactions and strategies for survival across different species.

Sexual Dimorphism

Sexual dimorphism refers to the visual and physical differences between males and females of the same species. These differences arise because of sexual selection, where traits evolve to increase chances of mating success. For instance, in some species, males might evolve to be larger or more colorful if those traits help with attracting mates or winning fights against rivals.

In species where males do not invest much in offspring, like the first monkey species mentioned, there tends to be a high level of sexual dimorphism. Males might be larger or more ornamented because they compete more aggressively for mates. Females typically select these dominant traits, promoting their prevalence in the population.

On the other hand, when males share parental responsibilities as seen in the second monkey species, sexual dimorphism usually reduces. Since both sexes contribute to offspring survival, the competition for mates is less intense, which results in males that do not need exaggerated features to secure a mate.

Asymmetry of Sex

The asymmetry of sex highlights the unequal parental contributions between males and females, heavily influencing behaviors and characteristics across species. This concept explains why different animals adopt various reproductive strategies. In most species, females usually have higher parental investment, spending more energy and resources in producing and nurturing offspring.

In the case of monkeys where males do not help raise offspring, the asymmetry of sex is pronounced. Females carry nearly all the burden of ensuring the survival of their young. This often results in intense male competition for mating opportunities because their involvement does not extend to parental care.

Conversely, in species where males contribute to parenting, this asymmetry decreases. Here, the difference in investment between the genders reduces, leading to less competition among males and consequently, less pronounced differences in physical traits between the sexes. This balance fosters shared efforts towards offspring care and can result in a more equitable distribution of parental roles.