Ontogenic differences in sexual size dimorphism across four plover populations

Abstract

Sexual size dimorphism (SSD) among adults is commonly observed in animals and is considered to be adaptive. However, the ontogenic emergence of SSD, i.e. the timing of divergence in body size between males and females, has only recently received attention. It is widely acknowledged that the ontogeny of SSD may differ between species, but it remains unclear how variable the ontogeny of SSD is within species. Kentish Plovers Charadrius alexandrinus and Snowy Plovers C. nivosus are closely related wader species that exhibit similar, moderate (c. 4%), male‐biased adult SSD. To assess when SSD emerges we recorded tarsus length variation among 759 offspring in four populations of these species. Tarsus length of chicks was measured on the day of hatching and up to three times on recapture before fledging. In one population (Mexico, Snowy Plovers), males and females differed in size from the day of hatching, whereas growth rates differed between the sexes in two populations (Turkey and United Arab Emirates, both Kentish Plovers). In contrast, a fourth population (Cape Verde, Kentish Plovers) showed no significant SSD in juveniles. Our results suggest that adult SSD can emerge at different stages of development (prenatal, postnatal and post‐juvenile) in different populations of the same species. We discuss the proximate mechanisms that may underlie these developmental differences.

Demographic causes of adult sex ratio variation and their consequences for parental cooperation

“…An alternative but not necessarily mutually exclusive explanation is that males and females may differ in their premature investment into reproductive traits, which could inflict survival costs for the larger or more ornamented sex 33 . Although sexual dimorphism among adults is negligible in the species we studied 34,35 , sex-specific ontogeny does appear to vary in three of these populations. In male-biased populations of C. nivosus and C. alexandrinus, female hatchlings are smaller and grow more slowly than their brothers during the first weeks of life, whereas juveniles of the unbiased C. alexandrinus population exhibit no such sex-specific differences during early development 34 .…”

Demographic causes of adult sex ratio variation and their consequences for parental cooperation

“…An alternative but not necessarily mutually exclusive explanation is that males and females may differ in their premature investment into reproductive traits, which could inflict survival costs for the larger or more ornamented sex 34 . Although sexual dimorphism among adults is negligible in the species we studied 35 , 36 , sex-specific ontogeny does appear to vary in three of these populations. In male-biased populations of C. nivosus and C. alexandrinus , female hatchlings are smaller and grow more slowly than their brothers during the first weeks of life, whereas juveniles of the unbiased C. alexandrinus population exhibit no such sex-specific differences during early development 35 .…”

High fidelity: extra‐pair fertilisations in eight Charadrius plover species are not associated with parental relatedness or social mating system

“…Females have been observed to resist extra‐pair copulations and act aggressively to males other than their social mate (Lanctot et al ). Plovers of our study populations have only moderate sexual size dimorphism (dos Remedios et al ), which would probably allow females to resist enforced copulations by males.…”