Table Of Content2000. The Journal of Arachnology 28:241-242
RESEARCH NOTE
ESTIMATING FORAGING INTAKE:
A COMMENT ON TSO AND SEVERINGHAUS
(1998)
Keywords: Energy intake, allometry, digestible biomass
Tso & Severinghaus (1998) have recently verge with increasing body length. The con-
drawn attention to the problems of using clusion is thatbecause many large spiderstake
Schooner’s (1980) length-weight equations for a great range of prey sizes “. the relative
. .
insects as a means of estimating foraging in- energy content of large prey would be greatly
take by spiders. They pointed out that a bio- underestimated if determined by dry weight
mass estimate calculated from Schooner’s alone.” They recommend that “future studies
equations includes all tissues irrespective of should consider using ingestible biomass of
whether they are digestible or not. As preda- prey in estimating the foraging intake of spi-
tors do not utilize largely indigestible material ders.”
— &
(e.g., the exoskeleton -but note that at least Tso Severinghaus (1998) argue thatusing
some spider species can produce chitinase dry weights will tend to underestimate the in-
(Collatz 1987)) this will inevitably lead to in- gested biomass, and disproportionally so with
accuracies in determining digestible biomass increasing prey sizes. However, the ingested
acquisition by a foraging spider. Tso & Sev- biomass they suggest measuring is still not a
eringhaus (1998) therefore measured the ac- good estimate of the energy derived from the
tual biomass removed from a variety of prey prey because a large proportion of this bio-
items of different sizes and taxonomic groups mass will be the water responsible for the di-
by caged female Argiope trifasciata (Forskal vergence of the plotted curves; water is not a
source of metabolic energy. In very dry hab-
1775). They did this by simply subtracting the
weight of the discarded exoskeleton from the itats, the water content of the biomass ingest-
ed from a prey item may indeed be of great
initial wet weight of the prey. Dry weights of
importance, and the total volume of liquified
the prey items used were estimated by insert-
food ingested will certainly be a factor in de-
ing prey lengths into Schoener’s (1980) equa-
termining satiation level in situations where
tions for each of the various taxonomic
prey is not limiting. Wetbiomass ingestedwill
groups. Plots of ingested biomass and of dry
only be proportional to energy intake if the
weights of prey against prey length yielded
separate components of water and digestible
curves that increasingly&diverged towards biomass are in constant proportions (as as-
higher prey lengths. Tso Severinghaus as- sumed by Tso & Severinghaus) in different
sume dry weight is composed of digestible sized prey. This will only be the case ifwater
biomass -f exoskeleton; ingested biomass is content and digestible biomass both scale with
composed of digestible biomass + water; the size in exactly the same way. One might ex-
relative proportions ofthese three components pect both to be approximately proportional to
are constant and the absolute contribution of volume (i.e., oc length^) but the exact expo-
each is a function of size. This being so, be- nents would have to be determined empirical-
cause water comprises a large proportion of ly (see Schoener 1980), and their coefficients
the wet weight (and therefore, ingested bio- (0.7 for water and 0.1 for digestible macro-
mass) the absolute difference between ingest- molecules in the equation of Tso & Severin-
ed biomass and—dry weight will be a positive ghaus) checked for constancy across prey size
function of size the plotted curves will di- range.
241
242 THE JOURNAL OF ARACHNOLOGY
An appropriate measure that is likely to be mass) increases with total beetle mass (size).
a direct function of energy intake from a prey This is in direct contrast to the con—clusions of
&
item is total dry weight (= digestible dry Tso Severinghaus quoted above the use of
weight + exoskeleton) less the weight of the total dry weight as a surrogate for energy
dry exoskeleton rejected after feeding. The availability will produce an underestimate that
absolute intake of digestible dry weight must, decreases with increasing prey size. If energy
of necessity, always be less than the total dry intake is the currency of interest when inves-
weight of the prey and will therefore fall be- tigating spider foraging, ingested dry weight
low the lower curves in Tso & Severinghaus’s is the appropriate, and direct, measure to use.
fig. 1. Within this constraint, the shape of the I would like to thank Peter Mayhew and
digestible dry weight curve will depend on its Chris Rees for discussion.
allometric relationship with absolute size. AL LITERATURE CITED
though digestible dry weight probably scales
Collatz, K.-G. 1987. Structure and function ofthe
approximately oc length^ (but, again, see digestive tract. Pp. 229-238, In Ecophysiology
Schoener 1980) exoskeleton probably reflects of Spiders (W. Nentwig, ed.). Springer-Verlag,
more closely surface area (i.e., oc length^). As Berlin.
total dry weight increases with size, one Rees, C.J.C. 1986. Skeletal economy in certain
would therefore expect a greater proportion to herbivorous beetles as an adaptation to a poor
be represented by digestible material in larger dietary supply of nitrogen. Ecol. Entomol., 11:
221-228.
prey items. Some evidence forthis is provided Schoener, TW. 1980. Length-weight regression in
by Rees (1986) who investigated the relation-
tropical and temperate forest-understory insects.
ship between the fraction of total (wet) mass Ann. Entomol. Soc. America, 73:106-109.
attributable to dry skeletal mass and total wet Tso, I-M. & L.L. Severinghaus. 1998. Ingested
mass across taxa within six beetle families. biomass of prey as a more accurate estimator of
The slopes ofall six plots were negative (two- foraging intake by spiderpredators. J. ArachnoL,
tailed sign test, P = 0.03), although only one 26:405-407.
was individually significant. Total mass was
G.S. Oxford: Department of Biology, Uni-
measured as wet rather than dry weight, but versity of York, P.O. Box 373, York YOlO
ifthe degree oftissue hydration is constant or, 5YW, U.K.
if variable, not a function ofbeetle size, these
data suggest that skeletal mass decreases and, Manuscript received 6 March 1999, revised 3 No-
as a consequence, the remainder (digestible vember 1999.
