Males and females play complementary yet distinctly different evolutionary roles. They may differ in size and/or shape, behaviour and response to environmental factors. As a result, the abundance of males and females in a population may be unequal. This inequality in numbers may have important ecological and evolutionary consequences. This chapter starts with the description of differences in size, behaviour and physiology between male and female fleas. I attempt to understand how these differences are reflected in flea ecology. Then the effect of various gender differences on sex ratio in flea populations is considered.

Sexual dimorphism

Size dimorphism and Rensch's rule

Similar to many arthropods, fleas demonstrate strong female-biased sexual size dimorphism. This is true not only for species with neosomic females that are tens and hundreds of times larger than males (see Chapter 5) but also for species where gender size differences are less pronounced. For example, in six of seven flea species from the Negev Desert studied by Krasnov et al. (2003a), females were significantly larger than males (Fig. 7.1a).

Despite sexual size dimorphism occurring in a great number of animal species, male and female sizes usually covary within a lineage. However, in many cases variation in size is greater in males than in females (e.g. Fairbairn, 1997, 2005; Colwell, 2000). This generates an allometric pattern of sexual size dimorphism known as Rensch's rule.