1. Chapter 9 Opener: When a bull elk bugles, other males listen

2. 9.1 The pseudopenis of the female spotted hyena can be erected

3. 9.2 Concentrations of testosterone in male and female spotted hyenas (Part 1)

4. 9.2 Concentrations of testosterone in male and female spotted hyenas (Part 2)

5. 9.3 A cost of the pseudopenis for female spotted hyenas

6. 9.4 Competition for food among spotted hyenas may favor highly aggressive individuals

7. 9.5 Dominance greatly advances female reproductive success in the spotted hyena

8. 9.6 Ultrasonic communication

9. 9.7 Evolution of a sensory system

10. 9.8 Arthropod gills have evolved into many different structures with different functions (Part 1)

11. 9.8 Arthropod gills have evolved into many different structures with different functions (Part 2)

12. 9.9 Evolutionary precursors of insect wings?

13. 9.10 A surface-skimming stonefly

14. 9.11 A possible evolutionary pathway from swimming to full flight in the stoneflies

15. 9.12 An ancestral signal has been co-opted in some bowerbirds

16. 9.13 Sensory exploitation and the evolution of a courtship signal in Neumania papillator

17. 9.14 Two evolutionary scenarios for the evolution of male courtship trembling in Koenikea (Part 1)

18. 9.14 Two evolutionary scenarios for the evolution of male courtship trembling in Koenikea (Part 2)

19. 9.15 A female cichlid fish (left) is attracted to the anal fin of a male by the orange spots on the fin

20. 9.16 Food, carotenoids, and female mate preferences in the guppy

21. 9.17 Sexual preferences for orange spots match foraging preferences by female guppies

22. 9.18 The response of least auklets to three novel artificial signals

23. 9.19 Receivers can respond to an ancestral signal not present in their species

24. 9.20 Sensory exploitation and swordtail phylogeny

25. 9.21 Mate preferences for a novel ornament

26. 9.22 The panda principle is evident in the sexual behavior of a parthenogenetic whiptail lizard

27. 9.23 A group of ravens feeding on a carcass to which they were attracted by a yelling companion

28. 9.24 Yelling is a recruitment signal

29. 9.25 Predation risk has affected the evolution of begging calls in warblers (Part 1)

30. 9.25 Predation risk has affected the evolution of begging calls in warblers (Part 2)

31. 9.26 Testosterone affects begging rate and feeding rate in black-headed gull chicks

32. 9.27 An honest signal of hunger?

33. 9.28 The European cuckoo chick’s begging call matches that of four baby reed warblers (Part 1)

34. 9.28 The European cuckoo chick’s begging call matches that of four baby reed warblers (Part 2)

35. 9.29 The cuckoo’s begging calls stimulate more frequent feeding by its host parents

36. 9.30 Illegitimate receivers can detect the signals of their prey (Part 1)

37. 9.30 Illegitimate receivers can detect the signals of their prey (Part 2)

38. 9.31 Great tit alarm calls

39. 9.32 Hearing abilities of a predator and its prey

40. 9.33 Convergent evolution in a signal

41. 9.34 Deep croaks deter rivals

42. 9.35 Threat displays are energetically demanding in the side-blotched lizard (Part 1)

43. 9.35 Threat displays are energetically demanding in the side-blotched lizard (Part 2)

44. 9.36 Convergent threat displays

45. 9.37 Antler span in two New Guinean fly species provides accurate information about body size

46. 9.38 An honest signal

47. 9.39 A firefly femme fatale

48. 9.40 A deceptive signaler