Sexual size dimorphism (SSD) describes the consistent difference in body size between adult males and females of a single species. While the male is typically the larger sex in many familiar animals, such as most mammals and numerous bird species, this is not a universal rule. A significant number of species exhibit the opposite pattern, known as female-biased sexual size dimorphism (FBSD), where the female is notably larger than the male. This phenomenon represents a widespread evolutionary outcome driven by unique selective pressures that favor larger females.
Where Female-Biased Dimorphism Flourishes
Female-biased sexual size dimorphism (FBSD) is the standard pattern across numerous major animal groups, particularly invertebrates and ectotherms. This pattern is overwhelmingly common in insects, present in over 70% of species across several orders, including moths, butterflies, and beetles. FBSD is also the norm among arachnids, such as spiders, where the size difference is often immediately noticeable.
Among vertebrates, FBSD is widely observed in ectothermic groups—animals that rely on external sources to regulate their body temperature. These include fish, amphibians, and reptiles, where a larger female offers distinct advantages. This contrasts sharply with endotherms, such as birds and mammals, where FBSD is generally rare.
Even in groups where males are typically larger, like birds, FBSD appears prominently in specific ecological niches, such as in birds of prey (raptors). These include falcons, eagles, and hawks. The size difference in a Sharp-shinned Hawk, for instance, can be extreme, with the female weighing over 80% more than the male. The prevalence of FBSD in cold-blooded animals and select predatory birds shows that the evolutionary push for a larger female is a successful strategy.
Fecundity Selection and Reproductive Strategies
The primary evolutionary mechanism driving the larger size of females is fecundity selection. This concept posits that natural selection directly favors traits that increase an organism’s reproductive output. For females that produce numerous, small gametes, a larger body size translates directly to a greater capacity for egg production.
A physically larger female possesses a greater internal body volume, allowing her to accommodate a higher number of developing eggs or embryos. This capacity leads to a larger clutch size or brood size, which directly increases the female’s lifetime reproductive success. This relationship between size and offspring quantity is especially pronounced in invertebrates and ectotherms that have indeterminate growth, meaning they continue to grow throughout their lives.
Larger females can also accumulate and store more energy reserves. These energy stores are invested directly into reproduction, either by producing higher-quality eggs or by being able to reproduce more frequently. In species with short or infrequent reproductive opportunities, maximizing the number of offspring in a single event is highly advantageous. Fecundity selection acts as a strong selective force, pushing female size upward.
Ecological Drivers of Female Size Advantage
While reproductive capacity is a major factor, other ecological pressures also favor a size advantage for females. One such driver is niche separation, where the size difference allows the sexes to exploit different resources, reducing competition within the species. For instance, in many raptors, the smaller, more agile male is better suited for hunting smaller, faster-moving prey, while the larger female can target larger or slower animals.
This partitioning of the food supply ensures that both sexes can secure enough energy to fulfill their distinct reproductive roles without competing directly. In certain crab spider species, the evolution of energy-rich foraging methods, such as luring prey, correlates with a more pronounced FBSD, providing the greater energy intake necessary to sustain the female’s large body size.
A larger body size can also confer a direct survival advantage. In some birds of prey, the larger female size is strongly linked to nest defense. Since the female often assumes the primary role of guarding the eggs and young, a heavier body provides a more effective deterrent against predators threatening the nest. A larger size also provides better resistance to starvation and can improve thermal regulation in cold-blooded animals, enhancing survival during periods of environmental stress.
Extreme Examples and the Spectrum of Size Difference
The spectrum of female-biased dimorphism ranges from subtle to dramatically extreme, with some species exhibiting size ratios that defy common biological expectations. Among the most extreme examples are the deep-sea anglerfish (suborder Ceratioidei). The female can be more than 60 times longer and half a million times heavier than the male.
This disparity is a consequence of their deep-sea habitat, where finding a mate is a rare event. The tiny male has evolved to become a permanent sexual parasite, biting onto the female and fusing his circulatory system to hers. The male loses his independence, transforming into a permanent source of sperm, which ensures the female is ready to reproduce whenever she encounters suitable conditions.
Other notable examples of pronounced FBSD include certain species of praying mantises, where the female is significantly larger and often engages in sexual cannibalism. This extreme disparity is driven by a combination of fecundity needs and the female’s aggressive predatory behavior. FBSD is predominantly a product of natural selection favoring female survival and maximum reproductive output.