related to growth and development, age at maturity, fecundity, parental care
and longevity (Lowe-McConnell 1959; Berns et al. 1978; Gwahaba 1978;
Hodgkiss and Man 1978; Babiker and Ibrahim 1979; Dudley 1979; Marshall
1979a, 1979b; Vareschi 1979; and the more general works mentioned
previously). At times we have had to paint with a broad brush to complete/ our picture, as necessary data were not always available, but the fit to our
hypothesis has been gratifyingly close, as will be seen.
We wanted to see if life histories fell into categories in our consideration,
and whether they fit an evolutionary progression as proposed from our
model, i.e., an evolutionary sequence, or back and forth alteration from
altricial to precocial in life history styles. We propose that selection should
act through heterochronous shifts of character anlagen in early ontogeny, to
favor increasingly precocial forms (i.e., species adapt to conditions which
tend to become more uniform and stable over time). However, the mech-
anism exists for this trend to be reversed, so that species will not necessarily
become "trapped" in an evolutionary "dead end" of a highly specialized
(and therefore unadaptable) precocial form (Balon 1980,1981a).
We use the terms altricial (generalist) and precocial (specialist) in the
general ecological sense, i.e., altricial young being those that are relatively
small and incompletely developed, whereas precocial young are relatively
larger and more completely developed, at a particular time in ontogeny (the
term precocial is not to be confused with 'precocious' breeding at a small
size in tilapias which can occur in both bearers and guarders, i.e., in associa-
tion with either 'precocial' or 'altricial' young).
So, for example, if we compare young tilapias at the onset of exogenous
feeding (Figure I), young of mouthbrooding species are clearly more ad-
vanced and better developed, i.e., precocial. At the corresponding time in
development (onset of exogenous feeding), young of guardhg species are
smaller and less welldeveloped, i.e., altricial. For this reason, we refer to the
guatding species as having a more altricial life style, and the bearers a more
precocial style. The same kind of comparison can be made between species
within a guild, or between forms within a species, to determine which is
more altricial or precocial (since the terms are relative).
The relative position of a species (in terms of its life history style) will be
apparent in terms of the relative timing of the developmental intervals in
ontogeny. Precocial forms will include a truncated larval period and delayed
maturation, and a senescent period of some length. Altricial forms, on the
other hand, will have the early periods (embryonic and/or larval) prolonged,
an early maturation and a reduced senescent period.
The only study of ontogeny in a species similar enough to tilapia to serve
directly as a model (Balon 1977 on Labeotropheus species) will form the
basis for much of what we will say regarding bearers. There have been several
studies of guarding tilapias (e.g., Fishelson 1966a, 1966b; Peters 1963,1965,
1973) but most have focused on descriptions or compilations of stages of
development (not corresponding to our saltatory steps of ontogeny, Balon
1979a), and descriptions of certain important features (e.g., adhesive organs,
Ilg 1952) of interest in these species. Data on fecundity are available for a
number of tilapias (although still surprisingly few species) and are by no
means complete. Data on growth and development (including age at first