Behavioural and physiological studies of fighting in male Tilapia Zillii (Cichlidae)

Neat, Francis Columba (1996) Behavioural and physiological studies of fighting in male Tilapia Zillii (Cichlidae). PhD thesis, University of Glasgow.

Abstract

This thesis is a compilation of several behavioural and physiological studies of
fighting in a species of African cichlid fish, Tilapia zillii. The behavioural studies are
concerned with explaining the functional significance and behavioural organisation of
fighting. The physiological studies are concerned with attempting to elucidate the
mechanisms that underlie the expression of aggressive behaviour. My aim is show
how these two levels of study are related and why it is important to maintain a
balance between them in the study of animal behaviour.Chapter 2 reports a senes of laboratory experiments that investigated how
asymmetries in body size and gonadal state influence the fighting strategies of male T.
zillii, Fights between animals over limited resources often end in victory to the larger
contestant. Game theory predicts that relative body size is assessed during the fight
and thus also determines fight duration and intensity. However, if the contestants
differ in the value they place on the disputed resource, this can override the effects of
relative body size. In the experiment described here, relative gonad weight was a
much stronger predictor of fight outcome than relative body size. This suggests that
males with large gonads fight harder to defend their territory, implying that the value
of a territory is a function of the gonadal state of the individual. Despite this, a
detailed behavioural analysis suggested that relative body size is assessed through a
behaviour termed 'mouth wrestling'. In addition to persisting in the fight, smaller
contestants escalated to a higher degree (in terms of biting), especially if they went on
to win the tight. Winners and losers differed consistently with regard to a behaviour termed mouth locking, suggesting that through this behaviour the fish were assessing
an asymmetry related to the intention to persist with the fight, although I could not
demonstrate that this was related to relative gonad weight.Chapter 3 was undertaken with the aim of gaining information on the social structure
and occurrence of aggressive behaviours in a semi-natural environment. To this end, a
study was made of the social interactions of a group of male fish allowed free range
over approximately ten metres of artificial 'river bed' during a four week period.
Under these conditions, the social structure of T. zillii may be described as a loose
group with an unstable dominance hierarchy. Aggression is a major component of this
species' time budget, with different acts and degrees of escalation probably
functioning to establish, challenge and maintain dominance relationships within the
hierarchy. The expression of territorial behaviour appeared to be conditional upon
social status (only the two top ranked males were seen to dig nests and consistently
court females). One escalated fight was observed between the two highest ranked fish,
but it was not clear whether this was a territorial dispute, a challenge for the top
position, or a dispute over both. Thus, while the immediate reasons for escalated
fighting between male T. zillii depends upon the social and physical environment, it is
probably ultimately caused by competition for mating opportunities.
Chapter 4 deals with the issue of how body size and gonadal state relate to life-history
aspects of the animal by means of a morphometric analysis. The analysis provided
evidence to suggest that there was a trade off between gonad size and the storage of
fat It is not clear whether this relationship arises from a direct energetic trade-off or is a result of adopting different behavioural strategies. I also explored the relationship
between behaviour, morphology and somatic and reproductive condition. It was
possible to predict gonadal state on the basis of mouth morphology which may
explain why so much fighting involves use of the mouth in this species.In Chapter 5 I studied the proximate costs of fighting in terms of physical injury and
the metabolic consequences of engaging in a energetically demanding activity. In
relation to injuries incurred during fights, losers suffered greater scale loss than
winners, especially if the loser was larger than its opponent. In relation to energy
metabolism, fighting resulted in significant depletion of total sugar reserves from the
muscle and the liver (compared to unfought controls). It appears that the muscle
energy reserves are respired anaerobically, as was evident from the accumulation of
lactate in the muscle. Interestingly, losers had significantly higher levels of lactate
than winners. Together, the data on injury and metabolic state suggest that fighting is
costly for both winners and losers, but that this is especially marked for losers. These
data are discussed in relation to models of animal decision-making and it is concluded
that the summation of different proximate costs incurred during fighting is likely to
underlie the making of decisions such as continuing, giving up or escalating the fight.In Chapter 6 I report a study of plasma concentrations of gonadal steroids in T. zillii
in relation to fighting and gonadal state. The gonadal steroids, particularly the
androgens, have been shown to be associated with aggression in a wide range of
species and seemed like obvious candidates for the mechanism by which gonadal state
influences behaviour Blood samples were taken from fish immediately after fighting and the plasma concentrations of the following gonadal steroids were then determined
by radioimmunoassay: testosterone (T), l l-ketotestosterone (1IkT) and 17a,20~-
Dihydroxy-4-pregnen-3-one (17,20-P) and estradiol (Ez). T concentrations did not
correlate with GSI and no differences were detected between winners, losers and
unfought controls. II kT concentrations were on average twice those of T and
negatively correlated to GSI, although winners, losers and controls were not
significantly different. T and II kT concentrations were positively correlated with
each other but 17,20-P and Ez levels were too low to be accurately measured. It
appears that in T. zillii, II kT is the major androgen as it is in most other mature male
teleosts.While the present results suggest that these sex steroids may playa role in
physiological regulation of testicular maturation, they do not support the idea that
these steroids are the mechanism by which GSI influences aggressive behaviour.
In Chapter 7 I review the main conclusions of the previous chapters. I then offer my
personal opinion on how the different levels and approaches taken throughout the
study interrelate and collectively reinforce each other and why integration between
disciplines is important in the study of animal behaviour.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: Q Science > QL Zoology
• Colleges/Schools: College of Medical Veterinary and Life Sciences > Institute of Biodiversity Animal Health and Comparative Medicine
• Supervisor's Name: Huntingford, Professor Felicity and Beveridge, Dr. Malcolm
• Date of Award: 1996
• Depositing User: Ms Dawn Pike
• Unique ID: glathesis:1996-5039
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Mar 2014 12:12
• Last Modified: 24 Mar 2014 13:25
• URI: https://theses.gla.ac.uk/id/eprint/5039