Reproductive Strategies: How Big Is Your Love? Florian Maderspacher  Current Biology  Volume 20, Issue 21, Pages R925-R928 (November 2010) DOI: 10.1016/j.cub.2010.10.004 Copyright © 2010 Elsevier Ltd Terms and Conditions

Figure 1 Small male, what now? Large (left) and small (right) male morphs of Xiphophorus nigrensis. The males differ in colouration and the elongated sword on the tail fin, for which females have a penchant. In the large male, the gonopodium is visible just behind the pelvic fins. Photo: Manfred Schartl. Current Biology 2010 20, R925-R928DOI: (10.1016/j.cub.2010.10.004) Copyright © 2010 Elsevier Ltd Terms and Conditions

Figure 2 Sizing up: models for Mc4r function in determining male phenotype. Top panel: Small (red) and large (blue) male morphs differ in the number of Y-chromosome-linked copies of a non-functional allele (B) of the melanocortin 4 receptor (Mc4r). High level Mc4r signalling causes early onset of puberty and hence arrested growth at small size, while lower Mc4r signalling allows for longer growth in large males. Onset of puberty is triggered via activation of the hypothalamus–pituitary–gonad (HPG) axis. Hypothetically (lower panels), different levels of Mc4r signalling might lead to attainment of a fixed threshold for HPG activation at different times (Model 1) or the HPG threshold levels themselves might differ depending on the level of Mc4r signalling (Model 2). Current Biology 2010 20, R925-R928DOI: (10.1016/j.cub.2010.10.004) Copyright © 2010 Elsevier Ltd Terms and Conditions